Understanding Biomass Gasification Techniques

[Music] good morning everyone in the previous lecture so in that we initiated our discussion on the refraction and the combustion processes so in this lecture we will continue our discussion in a similar line so we discuss another thermo chemical conversion process here that is gasification so the content of this particular modular thermo chemical conversion of biomass that is a gasification process so as discussed earlier a wide range of technological options are available for the conversion of biomass to a suitable kind of product for example the energy so most of these technologies are suitable for either one or few biomass resources and have limitation in terms of total available energy and the form of energy like if you consider the example of use of fire root as a direct fuel for production of the heat whereas in case if the fire root can be converted into a denser pellet and can subsequently be utilized as a fuel for the heating purpose can give more energy efficiency rather than utilizing the firewood as such because the fire road has high moisture content whereas the denser pellet has relatively low moisture content in the range of around like four to five percent only so as a result the energy efficiency of utilization of the denser pellet will be relatively high compared to that of utilizing fire root as such for the heating purposes so apart from that if the material can be stirred into slurry and then digested to yield biogas as a product so this is also one of the technological option which is available for the biodiversation of the raw material to produce the gas or alternatively the feed material can be gasified using a thermochemical conversion technique that is a gasification or it may be incinerated to produce the heat and the produce heat can be used for the steam generation so these are some processes which are available for the conversion of biomass to energy especially electrical energy or the heat energy so we already discussed about these techniques in the earlier lecture however in this lecture the main focus is on the gaseous field from the biomass so there are two most widely used technologies are available for the conversion of biomass to a gaseous field that is a biochemical method and the thermochemical method the biochemical method involve the anaerobic digestion of the material to produce the biogas as a product whereas the thermo chemical method it involves the gasification of the material in a gasifier to produce syngas as a product from the comparative analysis of these two techniques that is a biochemical method and the thermo chemical method it appears that the renewable energy resources like derived from the byproduct of the timber industry agriculture residues raw materials of the forest household waste can be gasified using following techniques that is a biochemical methyl and thermochemical method so if you look at the anaerobic digestion process as we discussed earlier about this particular technology as well because of the heterogeneous nature of the feedstock material these particular materials first need to be converted into a slurry and they need to be digested in the digester to produce the gas and the gas obtained from the gasifier is mainly consists of the co2 and the ch4 in its composition along with the traces of other gases it also contains the h2s as a gas and the caloric value of the gas produced from the anaerobic digester is around 20 megajoule per meter cube and the gas produced from the gasifier can be used for the cooking purpose as well as for street lighting purpose but for engine application further processing is required whereas in case of thermo chemical conversion process so the feedstock material is thermo chemically converted into a gasified product that is called as a syn gas and the main constituents of the syngas are co h2 ch4 and co2 and the balance is nitrogen and the gas produced from the thermochemical conversion processes has the heating value of around four to five mega joule per meter cube and the produce gas is relatively a clean gas can be used for the burning purpose so for ic engine use further processing is required for this kind of gas as well so based on these two particular technology it appears that still the gasification is the best suitable alternative for the conversion of biomass to a gaseous fuel because of its following importance the gasification it offers high flexibility in terms of various biomass material as the feedstock so in case of gasification it is not limited to any specific kind of fish stock the material which contain high moisture can also be utilized for the gasification purpose in the thermochemical gasification process apart from that the msw can also be classified to produce the syngas so this is what is the meaning of this sentence here that it offers a high flexibility in terms of various biomass materials as the feedstock and the second important point about this technology is the thermochemical conversion efficiency of a gasification it is in the range of 70 to 90 percent and which is highest among various alternative technologies which i have discussed in this lecture as well as in the previous lecture and the output capacity of this particular process especially in the high output range it is controlled by the availability of the adequate feed materials rather than other technical consideration apart from that the output obtained from the gasifiers are suitable as a fuel to all types of internal combustion engine with capacity durating in the range of around like 15 to 30 percent so because of these advantages this is considered as the best suitable technology among the available technology for the conversion of raw material to a gaseous product now if you just look at the history of gasification process so the gasification is a well-established technology and the first commercial application of which dates back to year 1830 so this is a very well established technology and it is being used for the production of the steam gas from date backs to even 1830 and in the beginning years the coal and the pit were used as a feedstock material in the 18th century the nascent idea of the process was to produce town gas for the lighting and the cooking purpose around 1920 the major shift came in this area where it was used to produce synthetic material and this is what is the main shift happen in this particular technology where the produced syngas was used to produce the synthetic chemical as well and during world war two the biomass gasification system it played an important role and when due to shortage of petroleum feedstock the wood gas generator called gasogens we are used to power the motor vehicle and even by 1945 heavy vehicles were powered by the gasification system so if you see the development happen in the gasification technology so it is evident from this particular slide that how the technological development happens in the gasification process and yes because of this advantages and the technological importance of this process it is widely being used for the production of the gas from the solid material so this all talks about the history of the gasification and how this particular technology is evolved for the production of the syn gas from the waste material now apart from the advantages which are discussed in the previous slide the gasification is essentially converts the potential fail from one form to the another and apart from that there are other three motivation for such transformation of waste material to a high value or high quality product so the first is it increases the heating value of the fuel by rejecting the non combustible compound in the form of water and the nitrogen so this is what is the advantage of this particular technology where the low quality materials can be converted into a high quality fuel so it reduces the carbon to hydrogen ratio in the final product as well and the biomass it contains more oxygen with respect to carbon than coal and this includes the gasification process as the gasification is a partial oxidation process hence the oxygen which is required for the process can be supplied by the oxygen containing in material itself apart from that the fuel with high oxygen to carbon ratio have a smaller heating value and then those with the low oxygen to carbon ratio which can be seen from the graph which is shown here if you look at this particular graph this indicates the oxygen to carbon ratio whereas the y axis it indicates the hydrogen to carbon ratio so in this case the biomass has oxygen to carbon ratio it varies from even like 0.4 to 0.8 so it has a vast range of oxygen to carbon ratio and if you talk about even the hydrogen to carbon ratio it is relatively high in case of biomass as a material whereas in case of coal if you see here so the coal has relatively low oxygen to carbon ratio whereas low hydrogen to carbon ratio does the material which contain higher oxygen to carbon ratio which has a relatively smaller heating value can be converted into a high density material by converting this particular material using a thermochemical conversion process that is the gasification so let us discuss about this gasification technology in more detail so what is gasification the word gasification it implies converting a solid or liquid feedstock into gaseous fuel by thermo chemical method without leaving any solid carbonyl residue inside the reactor and the thermal conversion of organic material at elevated temperature and control environment it leads to virtually all the raw material being converted to gas and this particular process it takes place in the two stages so in the first stage what happens is the biomass is partially combusted to form the producer gas and charcoal and this is called as a primary zone whereas in the second stage the co2 and the h2o produced in the first stage is chemically reduced by the charcoal forming co and the h2 and the co and s2 is the major constituents of the gas produced in the gasifier the composition of the gas it mainly consists of 18 to twenty percent hydrogen nineteen to twenty one percent carbon monoxide two to three percent is methane and eight to ten percent is co2 and the rest is nitrogen these stages are specially separated in the gasifier and thus gasifier design is very much dependent on the feedstock characteristics what kind of stock is being used for the gasification purpose based on that the gasifier need to be designed to achieve a high process efficiency after understanding the gasification let us see how the process of gasification it takes place in the gasifier so if you see this particular schematic here the gasifier is divided into a four zone that is dry zone pyrolysis zone this is the oxidation zone and this is the reduction zone so the producer gas in the gasifier is formed by partial combustion of the solid biomass in a vertical flow pack bed so this is a vertical flow pack bed reactor in the conventional producer gas theory following processes takes place in sequence in the gasifier that is drying zone then followed by the paralysis zone where the material is get pyrolyzed inside the gasifier and then the produce material is oxidized in the oxidation zone followed by the reduction of the produced material in the reduction zone so that's why as mentioned the gasifier is sequentially divided in the four zone in the gasifier so if you see in this particular process the function of the drying zone in the gasifier is to remove the moisture from the incoming feed material so if you see here this is the biomass which is fed inside the gasifier and the function of this drying zone here is to remove the moisture from the incoming feed material below the drying zone is the paralysis zone where the draft historic material is converted into a residual carbon as a char so if you see here this is a residual carbon as hr which is getting converted into a pyrolysis zone followed by it also produces some combustible gases along with the tar as the product so the heat for the pyrolysis and the drying zone is supplied by the partial combustion of this product in the oxidation zone so in the oxidation zone the oxygen in the air as well as in the steam it reduces the carbon which is coming from the pyrolysis zone to hydrogen and the carbon monoxide and the co2 form in the oxidation zone it gets reduced to carbon monoxide in the reduction zone therefore the final composition of the producer gas it mainly depends on the water gas shift reaction which approaches the chemical equilibrium corresponding to the exit gas temperature and because of that this particular composition of the producer gas it also varies in the gasifier apart from that if you see in this case the heat of combustion in the oxidation zone it also transforms to a non-combustible material and it gets converted into a molten slag the molten slag along with the ash is drained out continuously from the silk slag tap into the water quench water quench tank so as a result the all the ash along with this particular slag is drain out continuously from this particular end which is called as a sil slag tap so this is how the gasification takes place in the gasifier so now as we discussed the gasification of the biomass is a very complex process and it mainly consists of consecutive heterogeneous and homogeneous reaction so if you look at the type of reaction which are taking place in the gasifier if you see here the first is carbon reaction followed by the oxidation reaction and then shift reaction methylation reaction and steam reforming reaction where the entire methane is getting converted into co and the h2 so if you see in this case the first four reactions are mainly the heterogeneous reaction where the carbon in the char is getting reacted with the gases like co2 h2o hydrogen and the oxygen and producing this gas composition so in oxidation reaction the first reaction also is a heterogeneous reaction where the carbon in the charge is getting oxidized with the oxygen in the air or the steam to produce co2 as a gas and all the remaining reactions are in the gaseous phase that's what is called as a homogeneous gas phase reaction because here the co gas is reacting with the oxygen here the methane is reacting with the oxygen and hydrogen is reacting with the oxygen and producing corresponding gases as the product in the oxidation reaction followed by the shift reaction here also the carbon monoxide is reacting with the water which is in the gas phase producing co2 and the hydrogen as the gas so that's why this is also homogeneous reaction in the gasifier followed by the methylation reaction where the methane formation is taking place by the reaction of co with the hydrogen and forming methane as the product followed by the steam reforming reaction where these methane which are formed in the methanol reactions are getting converted into a co and the h2 this all talks about the conjugate to heterogeneous and the homogeneous reaction which are taking place in a gassy fire here in this case the plus q it represents the amount of heat absorbed in the reaction so if you see here where in all these reactions the plus q it represents the amount of heat absorbed during the reaction whereas the minus q it represents the amount of heat which is released during the reaction now after understanding the process of gasification as well as understanding the reaction which are taking place in the gasifier let us try to understand the equivalence ratio and its importance in the gasifier with the help of the above reaction scheme and the equilibrium constant it is possible to predict the equilibrium composition of the gaseous product in the gasifier and the equilibrium composition for a given solid fuel it depends on the air supply per unit weight of the biomass or the feedstock fed into the gasifier and the dimensionless parameter which is known as a equivalence ratio is usually defined to characterize the air supply condition as follows so this is the concept which i already discussed in the previous lecture that is in the combustion the same concept we'll be discussing here again so the equivalence ratio as we know it is the ratio of air to phase ratio which is in the statutory amount to the air to fuel ratio which is actually required for the gastritis reaction to test place in the gasifier because the stationary amount is nothing but the exact amount of the oxygen which is required for the solid material to get gasified whereas in the practical it may not be so and hence excess air or the oxygen need to be supplied so that the complete oxidation of the material will takes place in the gasifier and will achieve a high process efficiency so if you just rearrange these steps here you'll get fit to air ratio which is actual divided by fit to air ratio in the statiometric amount and the denominator in the above equation this particular term here in the equation it represents the statutory quantity of oxygen on the air is required for the process and it varies from fail to fuel it is one of the important parameter in the gasification process as i mentioned because of the diverse composition of the biomass the air to fuel ratio in the gasifier also varies and all depends on the fail to fill basis it is generally observed that for the effective gasification and optimum equivalence ratio of around 0.2 to 0.4 is desired if it is less than or equal to 0.2 so in that case it results in incomplete gasification because of insufficient supply of oxygen in the gasifier as a result the incomplete gasification will takes place in the gasifier and then it produces more char and low calorific value product gas and it is termed as a pyrolysis dominant process because in this case because of partial supply of oxygen and that too is an incomplete gasification process so the pyrolysis will dominate the entire process if the value of this equivalent ratio is higher that is 0.4 or higher than 0.4 or equal to 0.4 then in that case it will alter the gasification into the combustion and the predominant process in this case is a combustion process because of increase in the oxygen supply in the gasifier so what happens is like the complete oxidation will takes place and because of the excess supply of the oxygen in the gasifier there will be a combustion which will be happening with the fish stock material so rather than producing the producer gas it will produce the flue gas as the product so this is all about the equivalence ratio and how it is imported in the gasifier so once you understand the process of gasification as well as the importance of the equivalence ratio in the gasifier so next point is like how to classify the gasifier so the gasifiers are classified based on the scheme of interaction of gasifying agent and the solid fuel the gasifiers are classified as the fixed bed gasifier fluidized bed gasifier and then entrant bed gasifier based on the scheme of interaction of the gasifying agent and the solid fuel so if you first talk about the fixed bed gasifier the fixed gate gasifiers are further classified as up drought gasifier downdraft gasifier and the cross draft gasifier and the basis of classification is direction of the flow of the gasifying agent in the gasifier but the issues involved in this particular gasifier are the slagging and the bed agglomeration this is one issue in the fixed bed gasifier syngas exit blockage is another issue in the fixed bed gasifier and the slag mobility followed by the refractory degradation is another issue in the fixed weight gasifier as we mentioned in the earlier slide that the heat of combustion in the oxidation zone it transforms to the non-combustible material in the gasifier which results in the formation of the molten slag and the molten slag which produced in the gasifier it often blocks the path of the producer gas in the gasifier as well as the mobility of this slag is an issue apart from that it also degrades the refractory in the gasifier so the another class of gasifier is a fluidized bed gasifier and the fluorescent gasifier further sub classified into a bubbling fluidized weight gasifier this is a fluid aspect gasifier and circulating fluidized bed gasifier even in this case the basis of classification is velocity of flow of the gasifying agent and the operational issue involved in the fluidized bed gasifiers are the bed agglomeration and the defluidization of the material and the ash deposition and the falling so these are the operational issue in the fluid gasifier and the last classification is entrained bed gasifier and the operational issue involved in the entrained bed gasifier is slag mobility again the same issue of that of the fixed bed gasifier the refractory degradation in the gasifier and syn gas cooler falling and the plugging so these are the operational issue in the entrance gasifier so after understanding the classification of the gasifier now let us discuss about this gasifier in detail so first is fixed weight gasifier so the fixed bed gasifier here is classified based on the flow of the gasifying medium inside the gasifier and it is sub classified as downdraft gasifier updraft gasifier and the cross draft gasifier so in the updraft gasifier it is further subdivided into a closed stop gasifier and the open top gasifier so in this case the biomass is fed from the top and the produced gas is forced to pass through an high temperature zone where the unburned paralytic material can be cracked to produce the gaseous hydrocarbon as a fuel and mostly a clean gas is obtained from the downdraft gasifier and it find its use in thermal and the engine application but there are certain issues also with the open and the closed top gasifier so in case of the closed top this is relatively a old design whereas open top is a recent design for the gasification of the biomass material similarly reasonably dry wood is required in case of the closed top gasifier whereas in case of the open top reasonably moist wood is also sufficient so in case of the open top even the reasonably moist wood can be gasified to produce the gas better quality of the gas can be achieved in the closed stop whereas in case of the open top superior quality gas can be achieved as i mentioned because the gas is forced to pass through this high temperature zone here and as a result the unburned paralytic material undergoes the cracking to produce the gaseous hydrocarbon as a fuel so as a result this particular material while passing through the gasification chamber it also get clean and then produce more or less a clean gas from this kind of gasifier similarly if we just talk about the up drop gasifier so in case of abduct gasifier the biomass is also fed from the top and the product gas is drawn off from the top of the gasifier as well so as a result in this case because of the movement of the gas from high temperature zone to a low temperature zone along with that it also carries significant amount of the tar and the moisture because of the passing of the gas from the high temperature zone to the low temperature zone where the outgoing gas it also carries away the tar and the moisture present in the feed material and relatively low temperature gas is obtained in the outer gasifier but the issue with this particular gasifier is it produce high tar as i mentioned because it carried away the tar in the incoming material along with that even the moisture content in the producer gas is relatively high although better quality of the gas can be also obtained from the updraft gasifier now if you look at the cross drop gasifier in case of cross drop gasifier the biomass is also fed from the top here whereas in case of the cross draft gasifier the oxidation of the feed material it takes place just opposite to the exit of the producer gas as a result because of the short path of the producer gas inside the gasifier it produces a poor quality gas and even high torque quality gas also getting produced in case of cross drop cassifier as a result this kind of gasifiers are not in common use now once you understand the mechanism as well as the process of the down draft of draft and the cross dot cassifier let us discuss about this gasifier in more detail so first is the downdraft gasifier so if you see here this is the schematic of the down drop gasifier where the biomass is fed inside the gasifier from the top and if you see here this particular gasifier is divided into four zone that is the drying zone pyrolysis zone this is the oxidation zone and the reduction zone so as i said the gas produced in this particular gasifier is forced to pass through the high temperature zone and then as a result the unburned pyrolysis products are cracked in this particular zone here to produce gaseous hydrocarbon as a fuel so in this case the temperature in the drying zone is around 350 degrees c whereas the temperature in the paralysis zone if you see here it is in the range of 350 to 600 degrees c and in the oxidation zone it reaches to around 1000 to 1200 degrees c where the oxidation of the parallel product is taking place to form the co hydrogen and the co2 as a product the co2 which is produced in this particular oxidation zone is further reduced to carbon monoxide in the reduction zone where the temperature is in the range of 700 to 1000 degree c so if you see the operation of this particular gasifier here so exit gas from the gasifier is allowed to pass through the annular jacket so that the efficient transfer of the heat will takes place to the gasifier and heat will also be transferred to the incoming solid material as a result the drying of the solid material will takes place in the gasifier so that to achieve a high thermal efficiency so the roundup type gasifier is best suited for varieties of the fish stock material ranging from msw to high moisture containing material as well so dry wood materials can also be gasified in the downdraft gasifier due to its design the raw products are forced to pass through the high temperature so as i discussed all the product gas are forced to pass through the high temperature zones if you see here these are the high temperature zone and all the product gases has to pass through this high temperature zone so as a result what happens is like in this case the unburned parallax products are getting cracked in this particular zone to produce a relatively clean gas with a more hydrocarbon contained and that is what is the advantage of the downdraft gasifier so in the steady state operation the heat of combustion cause wood chips to paralyze and lose around 70 to 80 percent of their weight the product of the paralysis chamber which is mainly consists of the co hydrogen co2 and fraction of the methane it get oxidized in the oxidation zone here to produce mainly co and hydrogen as a product and the co2 produced in the oxidation zone it gets reduced in the reduction zone to produce carbon monoxide as a result it produces relatively clean gas in the downdraft gasifier the downtrodden gasifier also called the concurrent moving bed gasifier and the ash as well as the slag material which is produced during the gasification process is collected in the water quenching tank here this is a water quenching tank so after understanding the downdraft gasifier let us discuss the importance of the up drought gasifier as well so the updraft gasifier is also called as a counter flow gasifier because in this case the biomass is also fed from the top and the producer gas is drawn off from the top of the gasifier here so in this type of gasifier the air enters at the combustion zone so this is the entry of the air in the combustion zone here and produce gas leaves from the top of the gasifier as shown here so the produce gas is coming out from the top of the gasifier here and this type of gasifier is easy to build and operate the drought gasifier it achieves higher process efficiency as the hot gas pass through the fuel bed and it loose at relatively low temperature from the gasifier as a result the gas produced from the gasifier practically has no ash but it contains tar and water vapor because of passing of gas through the unburnt fill because when this gas is passing through the unburned fuel so along with the gas it also takes away the tar and the water vapor which are or you can say the moisture present in the unburned fill as a result it contains more tar and water vapor in the exit gas the up drop gasifier are suitable for tar free feedstock material and are not suitable for highly volatile material because the gas is allowed to pass through the top of the gasifier so the incoming feed material if it contains the high volatile material so most of the volatile materials will be lost in the exit gas and will not take part in the reaction that's why this kind of gasifier are not suitable for the high volatile material so after understanding the drought gasifier now let us discuss about the cross drop gasifier and how the operation of the cross dot classifier takes place so this is the schematic of the cross dot gasifier in this case the biomass is allowed to enter inside the gasifier from the top and this is the air inlet to the gasifier and this particular portion is the gas outlet from the gasifier so in case of cross dot gasifier air enters the gasifier through a water cooled nozzle which is mounted on one side of the firebox so the gasifier which is operating at a higher temperature it confines its oxidation and the reduction zone just next to the air nozzle and due to the short path length for the gasification reaction this type of gasifier respond most rapidly for change in the gas production because there is hardly any residence time for the produce gas in the gasifier so as a result because of the short path length of the gasification reaction this type of gasifier responds most rapidly for change in the gas production because there is no sufficient time for the gas to remain inside the gasifier so after oxidation it will undergo reduction reaction here and immediately the gas will be exit out from the gasifier the high exit temperature of the gas and low co2 reduction it results in poor quality gas and low efficiency as well because the gas produced from this particular gasifier will have relatively a high temperature and it will not undergo the co2 conversion as well as a result this particular gasifier and the product produced from this particular gasifier will have poor quality as well as low efficiency and therefore this type of gasifier are not in common use as i said earlier that's the reason the cross dot gasifiers are not in common use for the gasification process so now after understanding the fixed weight gasifier let us discuss about the fluidized bed gasifier so this is the schematic of the fluidized bed gasifier so the fluidized bed gasifier is considered as the most versatile and it can operate on any biomass form and that is what is the advantage of the fluidized bed gasifier it takes the advantage of excellent mixing characteristics and high reaction rates of gas solid mixture in the gasifier a fluidized bed gasifier is a chamber with bed of inner sand particles which are supported by the distributor plate the velocity of the air is progressively increased till the upper drag of air on the bed particle support the entire weight of the bed here and this moving mass of solid in this bed is called a fluidized bed and the turbulence of the bed in this particular gasifier it can be increased with velocity which is above the minimum fluidization velocity and the availability of the high area in the feudalized bed and constantly moving mass it results in the good conversion efficiency and lower operating temperature compared to that of the fixed bed gasifier and this results in the good conversion efficiency in the fluidized bed gasifier moreover it also gives clean producer gas as a output from the fluidized bed gasifier now once you understand the fluidized bed gasifier let us discuss about the circulating fluidized bed gasifier as well so this is the schematic of the circulating fluidized bed gasifier where if you see here the circulating fluid design is a very compact and it achieves a high throughput as well and inferior quality fails can also be gasified to generate the fuel in the circulating bed few days gasifier high energy conversion ratio because of the intense gas solid mixing which is taking place in the circulating fluidized bed gasifier recirculation of the unburned fuel back into the gasifier it also results in the high conversion efficiency because of the circulating nature of the raw material the unburned material will be collected in the cyclone will be recirculated back into the gasifier for the oxidation purpose so as a result it gives relatively high conversion efficiency the less pollution in terms of the nox sox and the co2 due to the low temperature operation in the circulating fluidized bed gasifier but there are also certain challenges in the circulating fluidized bed gasifier in terms of complex gas solid flow phenomena another is like maintaining pressure balance in the loop and overall solid circulation is another challenge here the solid movement through the loop seal for the continuous operation is one of the challenge of the circulating fluid gasifier and maintaining the uniform temperature along the height of the fast bed reactor is also a challenge so the fuel gas cleaning as well as the ash disposal is also an issue in the cycloning fluidized bed gasifier so after understanding the circulating feed aspect gasifier let us discuss about the bubbling fluid albeit gasifier this represents the bubbling fluidized bed gasifier and this particular part here it represents the bubbles which are formed in the gasifier so in this case if you see here this kind of gasifier also achieve a high throughput and can operate on any kind of biomass material and tolerate reasonably high moisture material as well the less pollution again in terms of nox sox and co2 because of the low temperature operation in the bubbling fluid aspect gasifier more or less in this case also there are certain challenges while operating this kind of gasifier because of the complex process of gas solid multiphase flow and the chemical reaction and the heat and mass transfer this kind of gasifier need to be operated very effectively and the complex operation in terms of the precision of control of the operating parameter is essentially needed in the bubbling fluidized bed gasifier this is also one of the challenge in the bubbling feudalized bed gasifier an unequal pressure distribution across the bed moreover the hot spot and the agglomeration and the slugging is taking place in the bed which is another challenge in the bubbling feudalized bed gasifier and the illustration of the solid files and the loss of unburnt fuel particles from the bed is another major challenge of the bubbling feudalized bed gasifier so this all talks about the different types of gasifier which are used for the gasification purpose so after understanding the fluidized bed gasifier let us see what are the advantage of this fluidized bed gasifier so field flexibility is one of the advantage of the few dashboard gasifier and the type of fuel with the calorific value in the range of say 800 to 8000 kilocalorie per kilogram can also be used to produce the syn gas in the fluidized bed gasifier apart from that the good heat storage capacity to always ensure the combustion inside the gasifier is the major advantage of the fluidized bed gasifier quick startup and high combustion efficiency is another advantage of the fluidized bed gasifier it also has high output rate the consistent rate of combustion is another important advantage of the fluid gasifier as i mentioned because of the uniform distribution of the solid biomass as well as the gasifying medium in the fluidized bed gasifier it ensures the consistent rate of combustion in the gasifier the uses of fuel with high moisture content can also be preferred in the fluidized bed gasifier the rapid response to fuel input changes it is another importance of the feudal bed gasifier and because of low temperature combustion corrosion caused by alkali compound in ash significantly reduced in case of the fluidized blade gasifier because this kind of gasifiers it relatively operate at a lower temperature than that of the fixed blade gasifier the uniform temperature throughout the furnace volume provides the high process efficiency and reduced emission in the form of harmful nitrous oxide because the process is carried out at a lower temperature range as a result it reduces the emission of harmful gases from the gasifier in the form of nitrous oxide sulphur dioxide emission can be reduced to acceptable level with less expense as well and this is one of the important advantage of the fluidized bed gasifier so after understanding the different classification of the gasifier as well as the advantage of the fluid gasifier if we just compare the gasification process with that of the combustion so if you see here one small table has been tabulated which shows the comparative analysis of the combustion with the gasification process in the terms of amount of the flue gas so in case of combustion the amount of the fluid gas obtained is more whereas in case of the gasification it is less concentration of co2 in the combustion produced gas is lower whereas in case of the gasification it is higher the so2 emission in the form of sox is higher in the combustion process whereas in case of gasification it is lower as we already discussed just now the nox emission is also higher in case of combustion whereas in gasification it is lower because the temperature in the gasification is relatively lower than that of the combustion process and the generation of the solid waste it is higher in this case whereas in case of the gasifier it is lower because most of the carbon in the solid field is undergoes the gasification that is oxidation and the reduction process as a result it does not produce any unburned carbon in the gasifier or without leaving any carbonaceous residues material the entire material takes part in the reaction and produces into the gas as the product the mode of transportation in this case it is difficult here but it is very easy in case of the gasified product and the value added product cannot be generated from the combustion product because it is mostly a flue gas whereas the gasified product can be further processed to produce valuable chemical as well as convenient product the application of the gas produced from the combustion process as very narrow range it can be used either as a heat source or can be used to produce the steam whereas the gaseous product produced from the gasification process has wide range of application it can be used as a heat source apart from that it can be used in ic engine after further processing moreover it can be further converted into a valuable product or a valuable chemical as well so let me correct this point here because the carbon dioxide production in the combustion process is relatively higher than that in the gasification process whereas the carbon dioxide production in the gasification process is relatively lower now if you look at the gaseous product produced in the combustion as well as in the gasification process so the carbon in the solid feed material is getting converted into co in the gasification and co2 in the combustion process whereas the hydrogen is getting converted into a h2 as a stable product in the combustion process because of the complete oxidation of the hydrogen in the combustion process because of the reduction reaction the hydrogen in this case is getting reduced to h2 in the gasification process and the nitrogen is coming out as such without taking part in the reaction whereas in this case it is forming as a nox in this case it produces h2s if the sulphur is present in the sum of the biomass then it will result in the formation of the h2s here in this case it will form shocks because of the complete oxidation of the sulfur compound in the biomass if any and the oxygen if it is in the excess then it will come out as it is from the combustion chamber whereas it is a partial combustion process so as a result the unutilized oxygen will not come out from the gasification process because this entire process is carried out in the limited supply of the oxygen so this is how the composition of the gaseous product varies in the combustion and the gasification process so with this i guess it is clear now how the gasification operation takes place in the gasifier so with this we'll stop here in the next lecture we will discuss about another thermo chemical conversion process that is pyrolysis liquefaction and chemical conversion processes thank you regarding this lecture if you have any doubt feel free to contact me at vivi goud at that iitg dot ac dot in thank you [Music] you

[Music] good morning everyone in the previous lecture so in that we initiated our discussion on the refraction and the combustion processes so in this lecture we will continue our discussion in a similar line so we discuss another thermo chemical conversion process here that is gasification so the content of this particular modular thermo chemical conversion of biomass that is a gasification process so as discussed earlier a wide range of technological options are available for the conversion of biomass to a suitable kind of product for example the energy so most of these technologies are suitable for either one or few biomass resources and have limitation in terms of total available energy and the form of energy like if you consider the example of use of fire root as a direct fuel for production of the heat whereas in case if the fire root can be converted into a denser pellet and can subsequently be utilized as a fuel for the heating purpose can give more energy efficiency rather than utilizing the firewood as such because the fire road has high moisture content whereas the denser pellet has relatively low moisture content in the range of around like four to five percent only so as a result the energy efficiency of utilization of the denser pellet will be relatively high compared to that of utilizing fire root as such for the heating purposes so apart from that if the material can be stirred into slurry and then digested to yield biogas as a product so this is also one of the technological option which is available for the biodiversation of the raw material to produce the gas or alternatively the feed material can be gasified using a thermochemical conversion technique that is a gasification or it may be incinerated to produce the heat and the produce heat can be used for the steam generation so these are some processes which are available for the conversion of biomass to energy especially electrical energy or the heat energy so we already discussed about these techniques in the earlier lecture however in this lecture the main focus is on the gaseous field from the biomass so there are two most widely used technologies are available for the conversion of biomass to a gaseous field that is a biochemical method and the thermochemical method the biochemical method involve the anaerobic digestion of the material to produce the biogas as a product whereas the thermo chemical method it involves the gasification of the material in a gasifier to produce syngas as a product from the comparative analysis of these two techniques that is a biochemical method and the thermo chemical method it appears that the renewable energy resources like derived from the byproduct of the timber industry agriculture residues raw materials of the forest household waste can be gasified using following techniques that is a biochemical methyl and thermochemical method so if you look at the anaerobic digestion process as we discussed earlier about this particular technology as well because of the heterogeneous nature of the feedstock material these particular materials first need to be converted into a slurry and they need to be digested in the digester to produce the gas and the gas obtained from the gasifier is mainly consists of the co2 and the ch4 in its composition along with the traces of other gases it also contains the h2s as a gas and the caloric value of the gas produced from the anaerobic digester is around 20 megajoule per meter cube and the gas produced from the gasifier can be used for the cooking purpose as well as for street lighting purpose but for engine application further processing is required whereas in case of thermo chemical conversion process so the feedstock material is thermo chemically converted into a gasified product that is called as a syn gas and the main constituents of the syngas are co h2 ch4 and co2 and the balance is nitrogen and the gas produced from the thermochemical conversion processes has the heating value of around four to five mega joule per meter cube and the produce gas is relatively a clean gas can be used for the burning purpose so for ic engine use further processing is required for this kind of gas as well so based on these two particular technology it appears that still the gasification is the best suitable alternative for the conversion of biomass to a gaseous fuel because of its following importance the gasification it offers high flexibility in terms of various biomass material as the feedstock so in case of gasification it is not limited to any specific kind of fish stock the material which contain high moisture can also be utilized for the gasification purpose in the thermochemical gasification process apart from that the msw can also be classified to produce the syngas so this is what is the meaning of this sentence here that it offers a high flexibility in terms of various biomass materials as the feedstock and the second important point about this technology is the thermochemical conversion efficiency of a gasification it is in the range of 70 to 90 percent and which is highest among various alternative technologies which i have discussed in this lecture as well as in the previous lecture and the output capacity of this particular process especially in the high output range it is controlled by the availability of the adequate feed materials rather than other technical consideration apart from that the output obtained from the gasifiers are suitable as a fuel to all types of internal combustion engine with capacity durating in the range of around like 15 to 30 percent so because of these advantages this is considered as the best suitable technology among the available technology for the conversion of raw material to a gaseous product now if you just look at the history of gasification process so the gasification is a well-established technology and the first commercial application of which dates back to year 1830 so this is a very well established technology and it is being used for the production of the steam gas from date backs to even 1830 and in the beginning years the coal and the pit were used as a feedstock material in the 18th century the nascent idea of the process was to produce town gas for the lighting and the cooking purpose around 1920 the major shift came in this area where it was used to produce synthetic material and this is what is the main shift happen in this particular technology where the produced syngas was used to produce the synthetic chemical as well and during world war two the biomass gasification system it played an important role and when due to shortage of petroleum feedstock the wood gas generator called gasogens we are used to power the motor vehicle and even by 1945 heavy vehicles were powered by the gasification system so if you see the development happen in the gasification technology so it is evident from this particular slide that how the technological development happens in the gasification process and yes because of this advantages and the technological importance of this process it is widely being used for the production of the gas from the solid material so this all talks about the history of the gasification and how this particular technology is evolved for the production of the syn gas from the waste material now apart from the advantages which are discussed in the previous slide the gasification is essentially converts the potential fail from one form to the another and apart from that there are other three motivation for such transformation of waste material to a high value or high quality product so the first is it increases the heating value of the fuel by rejecting the non combustible compound in the form of water and the nitrogen so this is what is the advantage of this particular technology where the low quality materials can be converted into a high quality fuel so it reduces the carbon to hydrogen ratio in the final product as well and the biomass it contains more oxygen with respect to carbon than coal and this includes the gasification process as the gasification is a partial oxidation process hence the oxygen which is required for the process can be supplied by the oxygen containing in material itself apart from that the fuel with high oxygen to carbon ratio have a smaller heating value and then those with the low oxygen to carbon ratio which can be seen from the graph which is shown here if you look at this particular graph this indicates the oxygen to carbon ratio whereas the y axis it indicates the hydrogen to carbon ratio so in this case the biomass has oxygen to carbon ratio it varies from even like 0.4 to 0.8 so it has a vast range of oxygen to carbon ratio and if you talk about even the hydrogen to carbon ratio it is relatively high in case of biomass as a material whereas in case of coal if you see here so the coal has relatively low oxygen to carbon ratio whereas low hydrogen to carbon ratio does the material which contain higher oxygen to carbon ratio which has a relatively smaller heating value can be converted into a high density material by converting this particular material using a thermochemical conversion process that is the gasification so let us discuss about this gasification technology in more detail so what is gasification the word gasification it implies converting a solid or liquid feedstock into gaseous fuel by thermo chemical method without leaving any solid carbonyl residue inside the reactor and the thermal conversion of organic material at elevated temperature and control environment it leads to virtually all the raw material being converted to gas and this particular process it takes place in the two stages so in the first stage what happens is the biomass is partially combusted to form the producer gas and charcoal and this is called as a primary zone whereas in the second stage the co2 and the h2o produced in the first stage is chemically reduced by the charcoal forming co and the h2 and the co and s2 is the major constituents of the gas produced in the gasifier the composition of the gas it mainly consists of 18 to twenty percent hydrogen nineteen to twenty one percent carbon monoxide two to three percent is methane and eight to ten percent is co2 and the rest is nitrogen these stages are specially separated in the gasifier and thus gasifier design is very much dependent on the feedstock characteristics what kind of stock is being used for the gasification purpose based on that the gasifier need to be designed to achieve a high process efficiency after understanding the gasification let us see how the process of gasification it takes place in the gasifier so if you see this particular schematic here the gasifier is divided into a four zone that is dry zone pyrolysis zone this is the oxidation zone and this is the reduction zone so the producer gas in the gasifier is formed by partial combustion of the solid biomass in a vertical flow pack bed so this is a vertical flow pack bed reactor in the conventional producer gas theory following processes takes place in sequence in the gasifier that is drying zone then followed by the paralysis zone where the material is get pyrolyzed inside the gasifier and then the produce material is oxidized in the oxidation zone followed by the reduction of the produced material in the reduction zone so that&#39;s why as mentioned the gasifier is sequentially divided in the four zone in the gasifier so if you see in this particular process the function of the drying zone in the gasifier is to remove the moisture from the incoming feed material so if you see here this is the biomass which is fed inside the gasifier and the function of this drying zone here is to remove the moisture from the incoming feed material below the drying zone is the paralysis zone where the draft historic material is converted into a residual carbon as a char so if you see here this is a residual carbon as hr which is getting converted into a pyrolysis zone followed by it also produces some combustible gases along with the tar as the product so the heat for the pyrolysis and the drying zone is supplied by the partial combustion of this product in the oxidation zone so in the oxidation zone the oxygen in the air as well as in the steam it reduces the carbon which is coming from the pyrolysis zone to hydrogen and the carbon monoxide and the co2 form in the oxidation zone it gets reduced to carbon monoxide in the reduction zone therefore the final composition of the producer gas it mainly depends on the water gas shift reaction which approaches the chemical equilibrium corresponding to the exit gas temperature and because of that this particular composition of the producer gas it also varies in the gasifier apart from that if you see in this case the heat of combustion in the oxidation zone it also transforms to a non-combustible material and it gets converted into a molten slag the molten slag along with the ash is drained out continuously from the silk slag tap into the water quench water quench tank so as a result the all the ash along with this particular slag is drain out continuously from this particular end which is called as a sil slag tap so this is how the gasification takes place in the gasifier so now as we discussed the gasification of the biomass is a very complex process and it mainly consists of consecutive heterogeneous and homogeneous reaction so if you look at the type of reaction which are taking place in the gasifier if you see here the first is carbon reaction followed by the oxidation reaction and then shift reaction methylation reaction and steam reforming reaction where the entire methane is getting converted into co and the h2 so if you see in this case the first four reactions are mainly the heterogeneous reaction where the carbon in the char is getting reacted with the gases like co2 h2o hydrogen and the oxygen and producing this gas composition so in oxidation reaction the first reaction also is a heterogeneous reaction where the carbon in the charge is getting oxidized with the oxygen in the air or the steam to produce co2 as a gas and all the remaining reactions are in the gaseous phase that&#39;s what is called as a homogeneous gas phase reaction because here the co gas is reacting with the oxygen here the methane is reacting with the oxygen and hydrogen is reacting with the oxygen and producing corresponding gases as the product in the oxidation reaction followed by the shift reaction here also the carbon monoxide is reacting with the water which is in the gas phase producing co2 and the hydrogen as the gas so that&#39;s why this is also homogeneous reaction in the gasifier followed by the methylation reaction where the methane formation is taking place by the reaction of co with the hydrogen and forming methane as the product followed by the steam reforming reaction where these methane which are formed in the methanol reactions are getting converted into a co and the h2 this all talks about the conjugate to heterogeneous and the homogeneous reaction which are taking place in a gassy fire here in this case the plus q it represents the amount of heat absorbed in the reaction so if you see here where in all these reactions the plus q it represents the amount of heat absorbed during the reaction whereas the minus q it represents the amount of heat which is released during the reaction now after understanding the process of gasification as well as understanding the reaction which are taking place in the gasifier let us try to understand the equivalence ratio and its importance in the gasifier with the help of the above reaction scheme and the equilibrium constant it is possible to predict the equilibrium composition of the gaseous product in the gasifier and the equilibrium composition for a given solid fuel it depends on the air supply per unit weight of the biomass or the feedstock fed into the gasifier and the dimensionless parameter which is known as a equivalence ratio is usually defined to characterize the air supply condition as follows so this is the concept which i already discussed in the previous lecture that is in the combustion the same concept we&#39;ll be discussing here again so the equivalence ratio as we know it is the ratio of air to phase ratio which is in the statutory amount to the air to fuel ratio which is actually required for the gastritis reaction to test place in the gasifier because the stationary amount is nothing but the exact amount of the oxygen which is required for the solid material to get gasified whereas in the practical it may not be so and hence excess air or the oxygen need to be supplied so that the complete oxidation of the material will takes place in the gasifier and will achieve a high process efficiency so if you just rearrange these steps here you&#39;ll get fit to air ratio which is actual divided by fit to air ratio in the statiometric amount and the denominator in the above equation this particular term here in the equation it represents the statutory quantity of oxygen on the air is required for the process and it varies from fail to fuel it is one of the important parameter in the gasification process as i mentioned because of the diverse composition of the biomass the air to fuel ratio in the gasifier also varies and all depends on the fail to fill basis it is generally observed that for the effective gasification and optimum equivalence ratio of around 0.2 to 0.4 is desired if it is less than or equal to 0.2 so in that case it results in incomplete gasification because of insufficient supply of oxygen in the gasifier as a result the incomplete gasification will takes place in the gasifier and then it produces more char and low calorific value product gas and it is termed as a pyrolysis dominant process because in this case because of partial supply of oxygen and that too is an incomplete gasification process so the pyrolysis will dominate the entire process if the value of this equivalent ratio is higher that is 0.4 or higher than 0.4 or equal to 0.4 then in that case it will alter the gasification into the combustion and the predominant process in this case is a combustion process because of increase in the oxygen supply in the gasifier so what happens is like the complete oxidation will takes place and because of the excess supply of the oxygen in the gasifier there will be a combustion which will be happening with the fish stock material so rather than producing the producer gas it will produce the flue gas as the product so this is all about the equivalence ratio and how it is imported in the gasifier so once you understand the process of gasification as well as the importance of the equivalence ratio in the gasifier so next point is like how to classify the gasifier so the gasifiers are classified based on the scheme of interaction of gasifying agent and the solid fuel the gasifiers are classified as the fixed bed gasifier fluidized bed gasifier and then entrant bed gasifier based on the scheme of interaction of the gasifying agent and the solid fuel so if you first talk about the fixed bed gasifier the fixed gate gasifiers are further classified as up drought gasifier downdraft gasifier and the cross draft gasifier and the basis of classification is direction of the flow of the gasifying agent in the gasifier but the issues involved in this particular gasifier are the slagging and the bed agglomeration this is one issue in the fixed bed gasifier syngas exit blockage is another issue in the fixed bed gasifier and the slag mobility followed by the refractory degradation is another issue in the fixed weight gasifier as we mentioned in the earlier slide that the heat of combustion in the oxidation zone it transforms to the non-combustible material in the gasifier which results in the formation of the molten slag and the molten slag which produced in the gasifier it often blocks the path of the producer gas in the gasifier as well as the mobility of this slag is an issue apart from that it also degrades the refractory in the gasifier so the another class of gasifier is a fluidized bed gasifier and the fluorescent gasifier further sub classified into a bubbling fluidized weight gasifier this is a fluid aspect gasifier and circulating fluidized bed gasifier even in this case the basis of classification is velocity of flow of the gasifying agent and the operational issue involved in the fluidized bed gasifiers are the bed agglomeration and the defluidization of the material and the ash deposition and the falling so these are the operational issue in the fluid gasifier and the last classification is entrained bed gasifier and the operational issue involved in the entrained bed gasifier is slag mobility again the same issue of that of the fixed bed gasifier the refractory degradation in the gasifier and syn gas cooler falling and the plugging so these are the operational issue in the entrance gasifier so after understanding the classification of the gasifier now let us discuss about this gasifier in detail so first is fixed weight gasifier so the fixed bed gasifier here is classified based on the flow of the gasifying medium inside the gasifier and it is sub classified as downdraft gasifier updraft gasifier and the cross draft gasifier so in the updraft gasifier it is further subdivided into a closed stop gasifier and the open top gasifier so in this case the biomass is fed from the top and the produced gas is forced to pass through an high temperature zone where the unburned paralytic material can be cracked to produce the gaseous hydrocarbon as a fuel and mostly a clean gas is obtained from the downdraft gasifier and it find its use in thermal and the engine application but there are certain issues also with the open and the closed top gasifier so in case of the closed top this is relatively a old design whereas open top is a recent design for the gasification of the biomass material similarly reasonably dry wood is required in case of the closed top gasifier whereas in case of the open top reasonably moist wood is also sufficient so in case of the open top even the reasonably moist wood can be gasified to produce the gas better quality of the gas can be achieved in the closed stop whereas in case of the open top superior quality gas can be achieved as i mentioned because the gas is forced to pass through this high temperature zone here and as a result the unburned paralytic material undergoes the cracking to produce the gaseous hydrocarbon as a fuel so as a result this particular material while passing through the gasification chamber it also get clean and then produce more or less a clean gas from this kind of gasifier similarly if we just talk about the up drop gasifier so in case of abduct gasifier the biomass is also fed from the top and the product gas is drawn off from the top of the gasifier as well so as a result in this case because of the movement of the gas from high temperature zone to a low temperature zone along with that it also carries significant amount of the tar and the moisture because of the passing of the gas from the high temperature zone to the low temperature zone where the outgoing gas it also carries away the tar and the moisture present in the feed material and relatively low temperature gas is obtained in the outer gasifier but the issue with this particular gasifier is it produce high tar as i mentioned because it carried away the tar in the incoming material along with that even the moisture content in the producer gas is relatively high although better quality of the gas can be also obtained from the updraft gasifier now if you look at the cross drop gasifier in case of cross drop gasifier the biomass is also fed from the top here whereas in case of the cross draft gasifier the oxidation of the feed material it takes place just opposite to the exit of the producer gas as a result because of the short path of the producer gas inside the gasifier it produces a poor quality gas and even high torque quality gas also getting produced in case of cross drop cassifier as a result this kind of gasifiers are not in common use now once you understand the mechanism as well as the process of the down draft of draft and the cross dot cassifier let us discuss about this gasifier in more detail so first is the downdraft gasifier so if you see here this is the schematic of the down drop gasifier where the biomass is fed inside the gasifier from the top and if you see here this particular gasifier is divided into four zone that is the drying zone pyrolysis zone this is the oxidation zone and the reduction zone so as i said the gas produced in this particular gasifier is forced to pass through the high temperature zone and then as a result the unburned pyrolysis products are cracked in this particular zone here to produce gaseous hydrocarbon as a fuel so in this case the temperature in the drying zone is around 350 degrees c whereas the temperature in the paralysis zone if you see here it is in the range of 350 to 600 degrees c and in the oxidation zone it reaches to around 1000 to 1200 degrees c where the oxidation of the parallel product is taking place to form the co hydrogen and the co2 as a product the co2 which is produced in this particular oxidation zone is further reduced to carbon monoxide in the reduction zone where the temperature is in the range of 700 to 1000 degree c so if you see the operation of this particular gasifier here so exit gas from the gasifier is allowed to pass through the annular jacket so that the efficient transfer of the heat will takes place to the gasifier and heat will also be transferred to the incoming solid material as a result the drying of the solid material will takes place in the gasifier so that to achieve a high thermal efficiency so the roundup type gasifier is best suited for varieties of the fish stock material ranging from msw to high moisture containing material as well so dry wood materials can also be gasified in the downdraft gasifier due to its design the raw products are forced to pass through the high temperature so as i discussed all the product gas are forced to pass through the high temperature zones if you see here these are the high temperature zone and all the product gases has to pass through this high temperature zone so as a result what happens is like in this case the unburned parallax products are getting cracked in this particular zone to produce a relatively clean gas with a more hydrocarbon contained and that is what is the advantage of the downdraft gasifier so in the steady state operation the heat of combustion cause wood chips to paralyze and lose around 70 to 80 percent of their weight the product of the paralysis chamber which is mainly consists of the co hydrogen co2 and fraction of the methane it get oxidized in the oxidation zone here to produce mainly co and hydrogen as a product and the co2 produced in the oxidation zone it gets reduced in the reduction zone to produce carbon monoxide as a result it produces relatively clean gas in the downdraft gasifier the downtrodden gasifier also called the concurrent moving bed gasifier and the ash as well as the slag material which is produced during the gasification process is collected in the water quenching tank here this is a water quenching tank so after understanding the downdraft gasifier let us discuss the importance of the up drought gasifier as well so the updraft gasifier is also called as a counter flow gasifier because in this case the biomass is also fed from the top and the producer gas is drawn off from the top of the gasifier here so in this type of gasifier the air enters at the combustion zone so this is the entry of the air in the combustion zone here and produce gas leaves from the top of the gasifier as shown here so the produce gas is coming out from the top of the gasifier here and this type of gasifier is easy to build and operate the drought gasifier it achieves higher process efficiency as the hot gas pass through the fuel bed and it loose at relatively low temperature from the gasifier as a result the gas produced from the gasifier practically has no ash but it contains tar and water vapor because of passing of gas through the unburnt fill because when this gas is passing through the unburned fuel so along with the gas it also takes away the tar and the water vapor which are or you can say the moisture present in the unburned fill as a result it contains more tar and water vapor in the exit gas the up drop gasifier are suitable for tar free feedstock material and are not suitable for highly volatile material because the gas is allowed to pass through the top of the gasifier so the incoming feed material if it contains the high volatile material so most of the volatile materials will be lost in the exit gas and will not take part in the reaction that&#39;s why this kind of gasifier are not suitable for the high volatile material so after understanding the drought gasifier now let us discuss about the cross drop gasifier and how the operation of the cross dot classifier takes place so this is the schematic of the cross dot gasifier in this case the biomass is allowed to enter inside the gasifier from the top and this is the air inlet to the gasifier and this particular portion is the gas outlet from the gasifier so in case of cross dot gasifier air enters the gasifier through a water cooled nozzle which is mounted on one side of the firebox so the gasifier which is operating at a higher temperature it confines its oxidation and the reduction zone just next to the air nozzle and due to the short path length for the gasification reaction this type of gasifier respond most rapidly for change in the gas production because there is hardly any residence time for the produce gas in the gasifier so as a result because of the short path length of the gasification reaction this type of gasifier responds most rapidly for change in the gas production because there is no sufficient time for the gas to remain inside the gasifier so after oxidation it will undergo reduction reaction here and immediately the gas will be exit out from the gasifier the high exit temperature of the gas and low co2 reduction it results in poor quality gas and low efficiency as well because the gas produced from this particular gasifier will have relatively a high temperature and it will not undergo the co2 conversion as well as a result this particular gasifier and the product produced from this particular gasifier will have poor quality as well as low efficiency and therefore this type of gasifier are not in common use as i said earlier that&#39;s the reason the cross dot gasifiers are not in common use for the gasification process so now after understanding the fixed weight gasifier let us discuss about the fluidized bed gasifier so this is the schematic of the fluidized bed gasifier so the fluidized bed gasifier is considered as the most versatile and it can operate on any biomass form and that is what is the advantage of the fluidized bed gasifier it takes the advantage of excellent mixing characteristics and high reaction rates of gas solid mixture in the gasifier a fluidized bed gasifier is a chamber with bed of inner sand particles which are supported by the distributor plate the velocity of the air is progressively increased till the upper drag of air on the bed particle support the entire weight of the bed here and this moving mass of solid in this bed is called a fluidized bed and the turbulence of the bed in this particular gasifier it can be increased with velocity which is above the minimum fluidization velocity and the availability of the high area in the feudalized bed and constantly moving mass it results in the good conversion efficiency and lower operating temperature compared to that of the fixed bed gasifier and this results in the good conversion efficiency in the fluidized bed gasifier moreover it also gives clean producer gas as a output from the fluidized bed gasifier now once you understand the fluidized bed gasifier let us discuss about the circulating fluidized bed gasifier as well so this is the schematic of the circulating fluidized bed gasifier where if you see here the circulating fluid design is a very compact and it achieves a high throughput as well and inferior quality fails can also be gasified to generate the fuel in the circulating bed few days gasifier high energy conversion ratio because of the intense gas solid mixing which is taking place in the circulating fluidized bed gasifier recirculation of the unburned fuel back into the gasifier it also results in the high conversion efficiency because of the circulating nature of the raw material the unburned material will be collected in the cyclone will be recirculated back into the gasifier for the oxidation purpose so as a result it gives relatively high conversion efficiency the less pollution in terms of the nox sox and the co2 due to the low temperature operation in the circulating fluidized bed gasifier but there are also certain challenges in the circulating fluidized bed gasifier in terms of complex gas solid flow phenomena another is like maintaining pressure balance in the loop and overall solid circulation is another challenge here the solid movement through the loop seal for the continuous operation is one of the challenge of the circulating fluid gasifier and maintaining the uniform temperature along the height of the fast bed reactor is also a challenge so the fuel gas cleaning as well as the ash disposal is also an issue in the cycloning fluidized bed gasifier so after understanding the circulating feed aspect gasifier let us discuss about the bubbling fluid albeit gasifier this represents the bubbling fluidized bed gasifier and this particular part here it represents the bubbles which are formed in the gasifier so in this case if you see here this kind of gasifier also achieve a high throughput and can operate on any kind of biomass material and tolerate reasonably high moisture material as well the less pollution again in terms of nox sox and co2 because of the low temperature operation in the bubbling fluid aspect gasifier more or less in this case also there are certain challenges while operating this kind of gasifier because of the complex process of gas solid multiphase flow and the chemical reaction and the heat and mass transfer this kind of gasifier need to be operated very effectively and the complex operation in terms of the precision of control of the operating parameter is essentially needed in the bubbling fluidized bed gasifier this is also one of the challenge in the bubbling feudalized bed gasifier an unequal pressure distribution across the bed moreover the hot spot and the agglomeration and the slugging is taking place in the bed which is another challenge in the bubbling feudalized bed gasifier and the illustration of the solid files and the loss of unburnt fuel particles from the bed is another major challenge of the bubbling feudalized bed gasifier so this all talks about the different types of gasifier which are used for the gasification purpose so after understanding the fluidized bed gasifier let us see what are the advantage of this fluidized bed gasifier so field flexibility is one of the advantage of the few dashboard gasifier and the type of fuel with the calorific value in the range of say 800 to 8000 kilocalorie per kilogram can also be used to produce the syn gas in the fluidized bed gasifier apart from that the good heat storage capacity to always ensure the combustion inside the gasifier is the major advantage of the fluidized bed gasifier quick startup and high combustion efficiency is another advantage of the fluidized bed gasifier it also has high output rate the consistent rate of combustion is another important advantage of the fluid gasifier as i mentioned because of the uniform distribution of the solid biomass as well as the gasifying medium in the fluidized bed gasifier it ensures the consistent rate of combustion in the gasifier the uses of fuel with high moisture content can also be preferred in the fluidized bed gasifier the rapid response to fuel input changes it is another importance of the feudal bed gasifier and because of low temperature combustion corrosion caused by alkali compound in ash significantly reduced in case of the fluidized blade gasifier because this kind of gasifiers it relatively operate at a lower temperature than that of the fixed blade gasifier the uniform temperature throughout the furnace volume provides the high process efficiency and reduced emission in the form of harmful nitrous oxide because the process is carried out at a lower temperature range as a result it reduces the emission of harmful gases from the gasifier in the form of nitrous oxide sulphur dioxide emission can be reduced to acceptable level with less expense as well and this is one of the important advantage of the fluidized bed gasifier so after understanding the different classification of the gasifier as well as the advantage of the fluid gasifier if we just compare the gasification process with that of the combustion so if you see here one small table has been tabulated which shows the comparative analysis of the combustion with the gasification process in the terms of amount of the flue gas so in case of combustion the amount of the fluid gas obtained is more whereas in case of the gasification it is less concentration of co2 in the combustion produced gas is lower whereas in case of the gasification it is higher the so2 emission in the form of sox is higher in the combustion process whereas in case of gasification it is lower as we already discussed just now the nox emission is also higher in case of combustion whereas in gasification it is lower because the temperature in the gasification is relatively lower than that of the combustion process and the generation of the solid waste it is higher in this case whereas in case of the gasifier it is lower because most of the carbon in the solid field is undergoes the gasification that is oxidation and the reduction process as a result it does not produce any unburned carbon in the gasifier or without leaving any carbonaceous residues material the entire material takes part in the reaction and produces into the gas as the product the mode of transportation in this case it is difficult here but it is very easy in case of the gasified product and the value added product cannot be generated from the combustion product because it is mostly a flue gas whereas the gasified product can be further processed to produce valuable chemical as well as convenient product the application of the gas produced from the combustion process as very narrow range it can be used either as a heat source or can be used to produce the steam whereas the gaseous product produced from the gasification process has wide range of application it can be used as a heat source apart from that it can be used in ic engine after further processing moreover it can be further converted into a valuable product or a valuable chemical as well so let me correct this point here because the carbon dioxide production in the combustion process is relatively higher than that in the gasification process whereas the carbon dioxide production in the gasification process is relatively lower now if you look at the gaseous product produced in the combustion as well as in the gasification process so the carbon in the solid feed material is getting converted into co in the gasification and co2 in the combustion process whereas the hydrogen is getting converted into a h2 as a stable product in the combustion process because of the complete oxidation of the hydrogen in the combustion process because of the reduction reaction the hydrogen in this case is getting reduced to h2 in the gasification process and the nitrogen is coming out as such without taking part in the reaction whereas in this case it is forming as a nox in this case it produces h2s if the sulphur is present in the sum of the biomass then it will result in the formation of the h2s here in this case it will form shocks because of the complete oxidation of the sulfur compound in the biomass if any and the oxygen if it is in the excess then it will come out as it is from the combustion chamber whereas it is a partial combustion process so as a result the unutilized oxygen will not come out from the gasification process because this entire process is carried out in the limited supply of the oxygen so this is how the composition of the gaseous product varies in the combustion and the gasification process so with this i guess it is clear now how the gasification operation takes place in the gasifier so with this we&#39;ll stop here in the next lecture we will discuss about another thermo chemical conversion process that is pyrolysis liquefaction and chemical conversion processes thank you regarding this lecture if you have any doubt feel free to contact me at vivi goud at that iitg dot ac dot in thank you [Music] you

Transcript for:Understanding Biomass Gasification Techniques

Transcript for:
Understanding Biomass Gasification Techniques