GENERATION

TECHNICAL FIELD

[0001] The present subject matter relates, in general, to a process for production of syngas and, in particular, to production of hydrogen-rich syngas by biomass gasification.

BACKGROUND

[0002] Hydrogen is typically used in ammonia production, petroleum refining, and methanol synthesis. As hydrogen is a clean and green energy source, it can be a promising fuel source for future energy needs. Unlike conventional and renewable energy sources, such as fossil fuels, hydro, wind, and solar energy, hydrogen cannot be mined or harvested. Hydrogen can only be synthesized or produced ^" . ^" Hydro ^' gen"is typically produced by techniques such as steam methane reforming, partial oxidation of methane, electrolysis, and the like. Most (>95%) of the hydrogen produced is from fossil fuel source. Production of hydrogen from fossil fuel source causes production of produce carbon dioxide as byproduct. Other production techniques which do not depend on fossil fuels, for example, electrolysis, have low thermal efficiency.

BRIEF DESCRIPTION OF DRAWINGS

[0003] The detailed description is presented with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.

[0004] Fig. 1 illustrates the. process for production of hydrogen-rich syngas, in accordance with an implementation of the present subject matter. [0006] Fig. 3 depicts variation of FPR with oxygen content in a gasification medium comprising oxygen and nitrogen, in accordance with an implementation of the present subject matter.

[0007] Fig. 4 depicts variation of FPR with variation in moisture content in wet biomass, in accordance with an implementation of the present subject matter.

[0008] Fig. 5 depicts variation of FPR with mass flux of biomass, in accordance with an implementation of the present subject matter.

[0009] Fig. 6 depicts temperature profile within the downdraft gasifier with varying heights beyond the ignition nozzle, in accordance with an implementation of the present subject matter.

[00010] Fig. 7(a) and Fig. 7(b) depict a graphical representation of syngas composition as obtained by the method, in accordance with an implementation of the present subject matter.

[00011] Fig. 8 depicts H ₂ O fraction in syngas and H ₂ yield at varying Steam-to- Biomass Ratios (SBRs), in accordance with an implementation of the present subject matter.

[00012] Fig. 9 depicts variation of average bed temperature and H ₂ yield with SBR, in accordance with an implementation of the present subject matter.

[00013] Fig. 10 depicts variation of rate kinetic parameters ki and ks with the average bed temperature and respective H ₂ yield, in accordance with an implementation of the present subject matter.

[00014] Fig. 11 depicts carbon conversion at different SBRs, in accordance with an implementation of the present subject matter.

[00015] Fig. 12 depicts variation of H ₂ /CO ratio with SBR, in accordance with an implementation of the present subject matter.

[00016] Fig. 13 depicts variation in energy yield of syngas per unit mass of fuel and LHV of syngas with SBR, in accordance with an implementation of the present subject matter. [00017] Fig. 14 is a schematic representation of exergy analysis for oxy-steam gasification, in accordance with an implementation of the present subject matter.

[00018] Fig. 15 depicts variation in fraction of exergy input in 0 ₂ and steam generation at various SBRs, in accordance with an implementation of the present subject matter.

[00019] Fig. 16 depicts variation in energy and exergy efficiency with SBR, in accordance with an implementation of the present subject matter.

[00020] Fig. 17 depicts variation in product gas exergy components with SBR, in accordance with an implementation of the present subject matter.

DETAILED DESCRIPTION

[00021] The present subject matter relates to a process for production of hydrogen-rich syngas by biomass gasification eventually leading to produce pure hydrogen.

[00022] Hydrogen is likely to be used in transport sector and the distributed power generation sector as it is a clean energy source. Hydrogen can be produced but cannot be mined or harvested like fossil fuels, solar energy, wind energy, and the like. Some techniques for production of hydrogen are steam methane reforming, electrolysis, auto thermal reforming of methane, etc. These techniques, typically, produce carbon dioxide as by-product during production of hydrogen. Moreover, these are typically large scale processes which require high capital investment. In addition to these, biomass gasification is another technique that can be used for production of hydrogen.

[00023] Biomass gasification is a sub-stoichiometric combustion process. It includes stages of pyrolysis, oxidation, and reduction. These stages are typically carried out in a gasifier. Biomass gasification reaction is as shown below:

Biomass + heat + 0 ₂ →H ₂ +CO+C0 ₂ +CH ₄ +Higher Hydrocarbons (HHC) + char [00024] Gasification techniques such as coal gasification, direct biomass gasification, biomass pyrolysis have been used for production of gaseous products. The gaseous product, typically, comprises hydrogen, carbon monoxide, carbon- dioxide, methane and nitrogen. However, these techniques are typically not used for commercial scale hydrogen production as hydrogen content in biomass is about 6% by weight. Also, production of pure hydrogen from biomass is limited by char formation in the air gasification process and amount of hydrogen present in the gaseous product. Also, presence of nitrogen due to air gasification of biomass further leads to dilution of hydrogen in the gaseous product.

[00025] It was postulated that oxygen can be used instead of air as gasification medium. However, it was found that using oxygen critically increased flame propagation rate (FPR) in the gasifier, causing the gasifier operation to become unstable. Also, controlled FPR helps in establishing a thermal profile in the gasifier. FPR also has a significant influence on the gas residence time, which further impacts the gas quality. Using oxygen as a gasification medium leads to significant increase in FPR which further leads to the reduction in the gas residence time in the gasifier. This causes low pyrolysis rate with only surface charring leading to low gas yields, especially hydrogen.

[00026] To overcome the above mentioned problems, the present subject matter provides a process for producing hydrogen-rich syngas by biomass gasification. The process comprises the step of determining a height above the ignition nozzle in a downdraft gasifier for introduction of one of oxygen and an oxy-steam mixture in the downdraft gasifier, an important design parameter to ascertain required residence time reactants. The height is based on mass-flux of biomass, diameter of biomass particles, relevant ambient temperature for water gas shift reaction to proceed in the forward direction, residence time, and thermal diffusivity. The process further comprises the step of charging biomass through a lock-hopper into the downdraft gasifier and introducing the one of oxygen and oxy-steam mixture in the downdraft gasifier at the height determined. It further comprises igniting the biomass and collecting the hydrogen-rich syngas from a bottom of the gasifier. Mass-flux of the reactant (oxy-steam) is varied to control the biomass mass flux in a range of 0.05- 0.11 kg/m s in order to moderate the flame propagation rate (FPR) in the downdraft gasifier.

[00027] The process of the present subject matter uses oxy-steam as the gasification medium for biomass gasification. The steam provides endothermicity and enhances hydrogen yield. The use of oxy-steam as gasification medium increases production of hydrogen by causing water gas reaction (WGR) and water gas shift reaction (WGSR) to occur in the gasifier. This mitigates the need for an additional catalytic cracking system to implement WGR and WGSR. The height at which the oxygen or oxy-steam mixture is introduced is based on mass-flux of biomass, diameter of biomass particles, relevant ambient temperature for water gas shift reaction to proceed in the forward direction, residence time, and thermal diffusivity. These factors affect the FPR in the gasifier, which further affects the gas resident time, biomass conversion level and gas quality. The process further comprises introducing superheated steam through at least one of a plurality of elevations in addition to the oxy-steam mixture. The introduction of superheated steam can help in controlling the FPR throughout the gasifier. The process of the present subject matter also helps in generating clean gas with almost negligible HHCs, especially tars. The present subject matter also describes the downdraft gasifier in which the process can be implemented.

[00028] The above and other features, aspects, and advantages of the subject matter will be better explained with regard to the following description, appended claims, and accompanying figures. It should be noted that the description and figures merely illustrate the principles of the present subject matter along with examples described herein and, should not be construed as a limitation to the present subject matter. It is thus understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and examples thereof, are intended to encompass equivalents thereof. Further, for the sake of simplicity, and without limitation, the same numbers are used throughout the drawings to reference like features and components.

[00029] Fig. 1 illustrates the process 100 for production of hydrogen -rich syngas, in accordance with an implementation of the present subject matter. The process 100, at step 102, comprises determining a height above a fixed bed in a downdraft gasifier for introduction of one of oxygen and an oxy-steam mixture in the downdraft gasifier. The height is based on mass-flux of biomass, diameter of biomass particles, temperature for water gas shift reaction to proceed in the forward direction, residence time, and thermal diffusivity. The height is determined by equation 1 as given below.

The equation 1 was derived by empirical analysis of gasification.

where, K _c is a constant having value 0.012 m ⁴ k ^1'8 s ^L6 kg ^"1 ;

m _w is mass flux of biomass in kg/(m .h);

d _w is diameter of biomass in millimeters;

T'is temperature of one of oxygen or oxy-steam mixture injected in reactor in K;

d is diameter of the downdraft gasifier in meters; and

a is the thermal diffusivity in mm /s.

[00030] The diameter of the wood pieces is also related to the diameter of the reactor. In one implementation, d/d _w varies between 10-15 for d up to 0.25 m; 15-20 for d between 0.25 to 0.5 m; 20-25 for d 0.5 to 1 m; and beyond 1 m the d _w of maximum of 125 mm.

[00031] In an implementation, the process comprises pre-mixing oxygen and superheated steam prior to introducing the oxy-steam mixture in the downdraft gasifier at the determined height. In an implementation, a ratio of oxygen to super- heated steam in the oxy-steam mixture is in a range of 1:50 to 1: 150 (on mass basis). In another implementation, the ratio of oxygen to super-heated steam varies based on mass-flux and moisture content in the biomass. For example, the ratio of oxygen to super-heated steam increases with increase in mass flux. Similarly, ratio of oxygen to super-heated steam increases with increase in moisture content in the biomass. In an implementation, the moisture content in the biomass is in a range of 0-50% on mass basis.

[00032] Having determined the height, the step 104 comprises charging biomass through a lock-hopper into the downdraft gasifier. In an implementation, the biomass is pre-treated by techniques, such as torrefaction and pelletized. At step 106, the process 100 comprises introducing one of oxygen and oxy-steam mixture at the height determined in step 102. After charging of the biomass and introducing the one of oxygen and oxy-steam mixture, the process 100 at step 108 comprises igniting the biomass. The biomass is ignited by a heater or a burner at an ignition nozzle at a bottom end of the downdraft gasifier. In an implementation, the one of oxygen and oxy-steam mixture is continuously introduced after igniting the biomass. In another implementation, introduction of one of oxygen and oxy-steam mixture is halted until a pre-determined interval of time till the complete ignition of the biomass in the downdraft gasifier.

[00033] Ignition of the biomass causes the biomass to first dry. At temperatures less than 573 K, cellulose dehydrates to a more stable compound an hydrocellulose, which gives higher char yield with low porosity. Above 573 K, cellulose depolymerizes, producing volatiles. These volatiles move upwards through the downdraft gasifier and mix with oxygen and get oxidized. In an implementation, the process 100 comprises introducing superheated steam at at least one of a plurality of elevations in addition to the oxy-steam mixture. Introducing superheated steam provides endothermicity due to high latent heat of vaporization of water. Endothermicity reduces rate of pyrolysis and thereby the FPR. In addition to providing endothermicity, superheated steam increases hydrogen in the syngas produced by reacting with char formed, i.e., by water gas reaction (WGR) and water gas shift reaction (WGSR) as given below:

C+H ₂ O→CO+H ₂ ... (WGR)

CO+H ₂ O~C0 ₂ +H ₂ ... (WGSR)

Therefore, there is no further requirement to transfer char formed to a catalytic cracking system to crack the char comprising higher molecular weight compounds.

[00034] At step 110, the process 100 comprises collecting the hydrogen-rich syngas. In an implementation, the collected hydrogen-rich syngas is scrubbed to remove particulates and any water washable components. It is to be understood that other techniques can be used for purifying the hydrogen-rich syngas as will be obvious to a person skilled in the art. In another implementation, the hydrogen-rich syngas is routed for use in combustion processes. As mentioned previously, the process 100 is carried out in a downdraft gasifier as illustrated in Fig. 2.

[00035] Fig. 2 illustrates schematic of the downdraft gasifier 200, in accordance with an implementation of the present subject matter. The downdraft gasifier 200 comprises a gasification chamber 202. The gasification chamber 202 comprises an inlet 204 at a top end of the gasification chamber 202 for charging of the biomass. The inlet 204 is coupled to a lock hopper 206. The lock hopper 206 helps in operating the downdraft gasifier 200 at a pressure upto 30 bar. The lock hopper 206 also ensures a leak proof operation during charging of biomass into the downdraft gasifier 200. The downdraft gasifier 200 comprises a plurality of inlets 208a, 208b at the plurality of elevations along a wall 210 of the gasification chamber for introduction of the oxy-steam mixture and the superheated steam into the gasification chamber 202. In an implementation, the plurality of inlets 208a, 208b are coupled to an oxygen supply and a steam boiler. The plurality of inlets 208a, 208b can be coupled to a mixer, an atomizer, or a combination thereof. These can be further coupled to the oxygen supply and steam boiler to receive and pre-mix the oxygen and steam to provide the oxy-steam mixture. The oxy-steam mixture is then introduced via the plurality of inlets 208a, 208b in the wall 210.

[00036] In an implementation, the wall 210 is internally lined with ceramic to insulate the gasification chamber 202. The gasification chamber 202 also comprises an ignition nozzle 212 at a bottom end of the gasification chamber 202 for igniting the biomass. The gasification chamber 202 also comprises an outlet 214 at the bottom end for collecting the hydrogen-rich syngas generated in the downdraft gasifier 200. The outlet 214 is substantially below the ignition nozzle 212. The outlet 214 is coupled to a scrubber 216. The scrubber 216 helps in separating particulates and moisture and helps in cooling the hydrogen-rich syngas. In an implementation, for experimental purposes the scrubber 216 is further coupled downstream to a gas analyser 218 and flow measuring device 220. The gas analyser 218 analyses and provides composition of the hydrogen-rich syngas. The flow measuring device 220 measures flow of the hydrogen-rich syngas. The downdraft gasifier 200 has a turn- down ratio in a range of 1 :3 to 1 :4. The gasification chamber 202 also comprises a plurality of thermocouples 222 along its wall at different elevation to measure temperatures at different elevation. The temperatures measured can be used to generate a thermal profile in the gasification chamber 202, as will be explained later.

[00037] In an implementation, the gasification chamber 202 is coupled to an oxygen supply 226 and steam boiler 224. The oxygen supply 226 is coupled to the plurality of inlets 208a. The oxygen supply 226 supplies oxygen to the gasification chamber 202. The steam boiler 224 provides superheated steam to the gasification chamber 202. The steam boiler 224 is coupled to the plurality of inlets 208b. In an implementation, the oxygen from the oxygen supply 226 and steam from the steam boiler 224 at pre-mixed at a plurality of junctions 230. The opening and closing of the plurality of junctions 230 can be controlled by valves as will be understood by a person skilled in the art. For example, the valves can be a unidirectional valve for preventing backflow of oxygen and steam into the oxygen supply 226 and steam boiler 224. The valves can be controlled to cause mixing or prevent mixing of oxygen with steam as will needed for the process 100. It is also to be understood that the plurality of junctions 230 can be provided at any point in the oxygen and steam supply lines as will be understood by a person skilled in the art.

[00038] In an implementation, the gasification chamber 202 is coupled to a burner 228. The burner 228 is used for providing burning syngas generated for studying thermal properties of syngas. The burner 228 can also be used for providing heat for thermal applications by using the syngas as fuel. It is to be understood that the burner 228 can also be replaced by an engine for power generation, and the like, as will be understood by a person skilled in the art.

[00039] The following discussion is directed to various examples of the present subject matter. Although certain methods and compositions have been described herein as examples, the scope of coverage of this patent application is not limited thereto. On the contrary, the present subject matter covers all methods and compositions fairly falling within the scope of the claims either literally or under the doctrine of equivalents.

[00040] Certain terms are used throughout the description to refer to certain components and are to be construed as being mentioned by way of example and for purposes of explanation and not as limiting.

[00041] The term "steam to biomass ratio (SBR)" as used in the examples is defined as the ratio of the amount of steam passed to the amounted biomass consumed in the given time, on a molar basis

[00042] The term "Equivalence ratio (ER)" as used in the examples is defined as the actual oxygen to fuel ratio divided by the stoichiometric oxygen to fuel ratio.

[00043] The term "Lower Heating Value (LHV)" as used in the examples is defined as the amount of heat evolved when a unit weight (or volume in the case of gaseous fuels) of the fuel is completely burnt and water vapor leaves with the combustion products without being condensed. EXAMPLES

EXAMPLE 1 : VARIATION OF FPR WITH VARIATION IN GASIFICATION MEDIUM

[00044] Fig. 3 depicts variation of FPR with oxygen content in a gasification medium comprising oxygen and nitrogen, in accordance with an implementation of the present subject matter. As can be seen, with increase in oxygen content the FPR increases. This reduces residence time of the biomass at elevated temperatures during gasification. Reduction in residence time reduces heat penetration inside the particle, pyrolysis rate, and amount of gases generated (including hydrogen) with incomplete conversion of biomass further leading to unstable operation. Fig. 3 indicates that the gasification medium comprising oxygen should include a diluent. The diluent is to induce endothermicity during gasification. Endothermicity can be induced by using wet biomass with oxygen as gasification medium or by using the oxy-steam mixture as gasification medium with dry biomass.

[00045] Fig. 4 depicts variation of FPR with variation in moisture content in wet biomass, in accordance with an implementation of the present subject matter. Gasification medium used was oxygen. As can be seen, the FPR decreases with increase in moisture content as drying induce necessary endothermicity.

[00046] Fig. 5 depicts variation of FPR where gasification medium used is the oxy-steam mixture and biomass used is dry biomass, in accordance with an implementation of the present subject matter. The oxy-steam mixture, in this example, comprised 21% oxygen and 79% steam. Fig. 5 also depicts comparison of FPR with different gasification medium at different bed temperatures. As can be seen, the FPR for the oxy-steam mixture indicated by curve 502 is similar to that of air preheated to 600K indicated by curve 504. The curve 502 also indicates that an operating window for gasification based on mass flux of biomass for oxy-steam mixture exists. As the mass flux increases, FPR increases and reaches a maximum of 0.15 mm/s and then reduces. This indicates that mass flux can be varied in a range of 0.05-0.11 kg/m to maintain the FPR in a range of 0.05 mm/s-0.15 mm/s.

[00047] Fig. 6 depicts temperature profile within the downdraft gasifier 200 with varying heights beyond the ignition nozzle. Temperature at the varying heights was measured using a plurality of temperature sensors, such as thermocouples, at the varying heights. The temperature data was acquired using the IO tech PDQ2. Fig. 6 indicates temperature within the downdraft gasifier varying with height above the ignition nozzle and gasifier operation time. From Fig. 6 it can be concluded that introducing gasification medium at varying heights can modify or vary the FPR which could affect residence time and temperature profile in the downdraft gasifier 200. Equation 1, as disclosed previously, was derived by empirical analysis of gasification. Therefore, height at which the gasification medium is to be introduced in the downdraft gasifier 200 can be calculated in equation 1 to establish the required thermal profile as indicated in Fig. 6.

EXAMPLE 2: OPERATING CONDITIONS WITH VARIABLE SBR AND ER

[00048] In this example, effect of SBR and ER was observed on different gasification parameters, such as yield of H ₂ , CO, LHV, and the like. Dry biomass, in the form of wood chips, was used along with the oxy-steam mixture as gasification medium. The results of various operating conditions with variable SBR and ER are tabulated in the following Table 1.

Table 1 : Operating conditions with varying SBR and ER

[00049] To evaluate the influence of SBR on hydrogen yield in syngas, hydrogen balance was performed. The total hydrogen content in the syngas was calculated as the sum of hydrogen in H ₂ and CH ₄ . Hydrogen content in tar and residual char was ignored. With increase in SBR, the fraction of hydrogen in syngas contributed by steam was observed to be increasing from 20.2% (by volume) at SBR of 1 to 48.1% at SBR of 2.7. However, this was at the cost of extra energy input for steam generation. Also, as can be seen from Table 1 , at higher SBR, CO yield obtained was low and extra heat was needed for steam generation. This further resulted in loss of efficiency (Equation 2).

[00050] Table 1 also shows the variation of H ₂ to CO ratio and LHV with SBR. H ₂ to CO ratio is observed to be increasing with increase in SBR. LHV is observed to be decreasing with increase in SBR. This is due to the higher rate of reduction in CO mole fraction compared to increment in H ₂ yield. The lower heating value (LHV) of syngas was found to be varying from7.4 to 8.8 MJ Nm . 66 g of H ₂ per kg of biomass was obtained at SBR of 0.75 and ER of 0.21. Stable operation of oxy-steam gasification of wood chips was also achieved with hydrogen yield as high as 104 g per kg of biomass at SBR of 2.7 and ER of 0.3.

EXAMPLE 3: COMPARATIVE DATA OF WET BIOMASS GASIFICATION WITH OXYGEN AS GASIFICATION MEDIUM, AND DRY BIOMASS GASIFICATION WITH OXY-STEAM AS GASIFICATION MEDIUM

[00051] In this example, operating condition of gasification of wet biomass (oxygen as gasification medium) and dry biomass (with oxy-steam mixture as gasification medium) were compared. The values of the operating conditions are as shown in Table 2. When dry biomass was used, a higher yield of H ₂ of 71 g per kg of biomass was obtained as compared to 63 g H ₂ per kg of biomass with wet biomass at similar H ₂ O to biomass ratio of 1.4. The gasification efficiency was also found to be higher for oxy-steam gasification with dry biomass as compared to gasification with wet biomass and oxygen. At the H ₂ O to biomass ratio of 1.4, the gasification efficiency was found to be 55.8% with wet biomass in comparison with 67% with dry biomass and the oxy-steam mixture. [00052] It was also observed that syngas had higher LHV while using dry wood with oxy-steam mixture as gasification medium than wet biomass with oxygen as gasification medium. LHV of 8.8 MJ Nm ^" was obtained for dry biomass compared to 7 MJ Nm ^" with wet biomass at H ₂ O to biomass ratio of 1.4. Residual char was also found to be higher while using wet biomass compared to dry biomass. This further affects the conversion and energy efficiencies. Residual char obtained, at different H ₂ O to biomass ratio, was also found to be between 8-12% (of input biomass by weight) in the case of wet biomass. This was observed to be substantially higher compared to 2-5% while using dry biomass and the oxy-steam mixture at different SBR.

Table 2: Comparison of gasification results of wet biomass with 0 ₂ and dry biomass with oxy-steam mixture

[00053] The difference between hydrogen yield using wet biomass and dry biomass can be attributed to combustion of gaseous species. These gaseous species include volatiles generated within the packed bed. Combustion of the gaseous species maintains the bed temperature of 1200 K for the sustenance of operation. High ER was maintained in the case of wet biomass compared to dry biomass as the 0 ₂ requirement was more to maintain the required bed temperature. It can also be observed from Table 2 that the C0 ₂ fraction in syngas while using wet biomass was substantially higher than the C0 ₂ yield when the oxy-steam mixture was used with dry biomass. Conversion of moisture in the wet biomass needs extra energy within the reactor for phase change of water to steam and temperature rise. The extra heat is supplied through combustion of fuel gas that leads to lower H ₂ yield. Results thus obtained by adopting varying moisture and SBR values demonstrate the positive trend in hydrogen yield with increase in SBR. The hydrogen yield with oxy-steam mixture was also found to be substantially high than using wet biomass and oxygen.

[00054] From Table 2, it can also be observed that with increase in SBR from 0.75 to 2.7, the volume fraction of the reactant H ₂ O also varied from 78% to 89% in the oxy-steam mixture. The H ₂ O content in the hot gas outlet was found to be over 3 times higher at 46.9% at SBR of 2.7 compared tol6.6% at SBR of 0.75. Fig. 8 depicts variation of P _H2O at outlet against SBR with the respective hydrogen yield. As can be seen in Fig. 8, P _H2O changes continuously across the reacting bed. Change in the mole fraction of H ₂ O at outlet provides an insight into the overall condition in the reaction bed.

[00055] Further, Fig. 8 depicts an increase in volume fraction of H ₂ O in syngas and H ₂ yield with SBR. This suggests significant enhancement in char-steam reaction with increase in SBR. Also, with increase in SBR, bed temperature is also found to be decreasing. 0 ₂ fraction in the reactant reflecting the ER can be increased to maintain the desired bed temperature.

[00056] Fig. 9 depicts variation of average bed temperature at different SBRs, with respect to the H ₂ yield and ER. From Fig. 9 it can be observed that high bed temperature can be maintained by increasing the ER. This results in enhanced char conversion and H ₂ yield at higher SBRs.

[00057] To further evaluate cumulative effect of SBR and temperature on the reaction rate kinetics, the rate constants were calculated for the measured bed temperature for all the individual set of experiments. Fig. 10 depicts rate constants using data labels of SBR to indicate the respective proportional value of pH ₂ O. Ki and K5 vary exponentially with temperature, as shown in Fig. 10. The set of data points on the top right hand side in Fig. 10 indicate cumulative impact of temperature and pH ₂ O on the H ₂ yield. [00058] Fig. 11 indicates carbon conversion inside the downdraft gasifier 200, in accordance with an implementation of the present subject matter. From Fig. 11, it can be observed that carbon conversion inside the reactor increases with SBR and reached carbon boundary point (CBP) at SBR of 1.5, i.e., residual carbon approached zero. Further, mass and elemental balance technique was used to evaluate carbon boundary. It was observed that only 87% of carbon was converted at SBR of 0.75 and conversion rate was found to be increasing with increase in SBR. Complete carbon conversion is observed at SBR of 1.5 as shown in Fig. 11. Beyond SBR of 1.5, as no carbon is left to react with steam, any additional steam present reacts with CO to yield H ₂ and C0 ₂ . As a result, significant enhancement in H ₂ yield is observed with proportional loss in CO output. This also helps in establishing process conditions for WGR and WGSR.

[00059] Fig. 12 depicts variation of H ₂ /CO ratio with SBR. From Fig. 12 it can be observed that there was substantial increase in H ₂ /CO ratio beyond SBR of 1.5. Table 3 presents gas composition data at different SBR. It can be observed from Table 3 that C0 ₂ volume fraction remains roughly constant between 25-26% till SBR of 1.5 and later increases substantially with SBR. The H ₂ /CO ratio reduces after SBR of 1.5, from 1.63 to 1.5.

Table 3: Variation of H ₂ /CO ratio with SBR

[00060] From Table 3, importance of water gas shift reaction (WGSR) at higher SBR can be observed. Increase in H ₂ yield with WGSR can also be explained as follows. High partial pressure of steam shifts the WGSR equilibrium in favor of H ₂ while consuming CO leading to increased H ₂ and C0 ₂ yield, and thereby an increase in H ₂ /CO ratio. The char gasification is the rate -limiting step during gasification of biomass and higher residence time is essential for completion of heterogeneous char reactions. However, homogeneous water gas shift reaction approaches equilibrium in the shorter residence time.

[00061] WGSR is mildly exothermic. Calorific value of H ₂ is less than CO (14.5% lower on molar basis). This implies that once the carbon boundary is reached, no extra fuel gas is generated. However, chemical energy is transferred from CO to H ₂ . This further contributes to a higher yield of H ₂ but at the cost of efficiency.

[00062] In addition to increased H ₂ /CO ratio, total energy yields in syngas per unit biomass increased with SBR due to more carbon getting converted to fuel gas. However, calorific value of syngas (per unit volume or mass) reduces at higher SBR. As shown in Fig. 13, syngas energy saturates at around 18 MJ/kg of biomass once the carbon boundary is reached. On the other hand, LHV of syngas decreases gradually with an increase in SBR after CBP is reached (at SBR of 1.5). A sudden increase in H ₂ yield and energy fraction in the form of H ₂ with SBR, can also be seen in Fig. 13.

[00063] Next, equilibrium analysis results were compared with experimental results of H ₂ yield. It was observed that beyond CBP or SBR of 1.5, the experimental value of H ₂ yield approaches equilibrium. The equilibrium results showed complete carbon conversion beyond 1000 K. In the real scenario, the slow heterogeneous reactions of char with steam and C0 ₂ seldom reaches equilibrium in the given residence time and hence shows difference with equilibrium results. But, beyond CBP or SBR of 1.5, homogenous WGSR plays an important role to achieve equilibrium comparatively faster. Equilibrium analysis also shows complete carbon conversion yielding higher H ₂ output. However, this is not practical till CBP is reached. Table 4 depicts equilibrium and experimental results.

Table 4: Comparison between equilibrium and experimental results

EXAMPLE 4: STUDY OF INEFFICIENCIES

[00064] Thermodynamic analysis was conducted to study various inefficiencies that may occur in the process 100 and downdraft gasifier 200. Product gas composition, mass flow rates of the fuel, and reactants, along temperature, were used to arrive at energy input and efficiency for varying SBR and ER. It was observed that second law exergy efficiency was higher than first law energy efficiency. Difference between the second law exergy efficiency and first law energy efficiency can be attributed to the loss of sensible heat in the hot syngas. Sensible heat lost during syngas cooling can play a substantial role in deviation of energy efficiency from exergy efficiency.

[00065] Experiments with air as a gasifying medium were performed and, energy and exergy analysis was performed to compare with oxy-steam gasification inefficiencies. Air gasification experiments were performed at optimum operating conditions with air mass flux of 0.1 kg/m s, in the downdraft gasifier 200 with air supplied from the top. Tables 5 & 6 depict the exergy and energy efficiency for different operating conditions. Exergy and energy efficiencies of air gasification were found to be comparable to the oxy-steam gasification process at lower SBR 0.75.

[00066] It was observed that exergy efficiency reduced with increase in SBR. Maximum exergy efficiency of 85% was obtained at SBR of 0.75. This further reduced to 69.2% at SBR of 2.7. Similar trend was observed for energy efficiency. Energy efficiency was observed to be lower than the exergy efficiency at any given SBR. Further, from Table 5 it can also be observed that exergy input in steam generation increases significantly from 6.1% of total exergy input at SBR of 0.75 to8.4% at SBR of 2.7. For the same operating conditions, exergy input fraction in 0 ₂ generation showed an increment from 5.1% to 6.3%. Physical exergy fraction in output gas for air gasification was found to be 3.2% compared to oxy-steam gasification. Oxy-steam gasification showed an increase in physical exergy fraction from 5.9 to 11.3% with the increase in SBR from 0.75 to 2.7. However, chemical mixture exergy fraction during air gasification was found to be 3.2% compared to oxy-steam gasification, which showed an increment 1.7% to 2.8% in same range of SBR.

Table 6: Thermodynamics study (Operating conditions)

[00067] From Table 6, it can be observed that, for air gasification, energy yield (in output gas per kg of biomass) and energy density is significantly lower than that obtained during oxy-steam gasification. Calorific value of gas during oxy-steam gasification process was found to be decreased from 8.91 MJ/Nm 3 to 7.43 MJ/Nm 3 with increase in SBRfrom0.75to2.7. [00068] Fig. 14 provides a schematic representation of exergy flow, in accordance with an implementation of the present subject matter. Total energy input is split in to various parts where 62-81% of energy input is in fuel form, i.e., biomass. 6- 19% energy input is corresponding to heat for providing steam which, further, is a function of SBR. 5-6% of energy input is used for separating oxygen from air. 1- 1.5% of energy is for running auxiliary system of the downdraft gasifier 200. Fig. 15 presents the exergy input fraction in 0 ₂ and steam generation with a change in SBR. Exergy input for 0 ₂ separation and steam generation were roughly same for low SBR, but with the increase in SBR, the steam exergy increased significantly from 6% to over 18%. The marginal increase in oxygen level to maintain bed temperature has no significant impact on the exergy input.

[00069] Going back again to Table 5, in addition to exergy efficiency, energy efficiency is also found to be decreasing with increase in SBR. Efficiency is reduced from 80% to about 66% with increase in SBR from 0.75 to 2.7. Air gasification efficiency was found to be 80% which was comparable with oxy-steam gasification at SBR of 0.75. Fraction of energy input in oxygen separation accounts for 5% to 8% for the range of SBR used. In absolute terms input energy spent is almost double to a little over 2 MJ per kg of biomass at higher SBR from around 1 MJ at lower SBR of 0.85. Energy input for steam generation and superheating varies substantially with the SBR. Energy input fraction for steam generation increased almost three times with 10.3% at SBR of 0.75 to about 28.5% at SBR of 2.7.

[00070] Fig. 16 represents exergy and energy efficiency for air gasification and oxy-steam gasification with varying SBR, in accordance with an implementation of the present subject matter. Data for air gasification is plotted at SBR=0. Decrease in energy and exergy efficiency with SBR can be observed in Fig. 16. Physical exergy in syngas was found to be increasing from about 6% to 12% with increase in SBR from 0.75 to 2.7. However, physical exergy was found to below with a value of 3.2% in case of air gasification. Presence of unreacted steam in syngas can be considered to contribute to the physical exergy loss, which showed an increase from 13% to76% in the product gas. Reduction in energy efficiency at higher SBR was found. This could be due to the loss in sensible heat or physical exergy in the syngas.

[00071] From Fig. 16, it can be observed that variation in SBR has a significant effect on the overall efficiency. Once the CBP is reached (at SBR=1.5), efficiency gradually decreases with increase in SBR. Apart from extra energy input in steam generation, conversion of CO to H ₂ can be considered to be responsible for reduction in efficiency at higher SBR. Variation of different components of exergy losses with SBR is shown in Fig. 17. Steam in hot syngas was found to increase substantially with increase in SBR. Exergy in H ₂ O alone was found to account for 13.7-76% of the total physical exergy or 0.8-8.6% of the total exergy of syngas. Also, physical exergy in the formulation (equation 3 accounts for enthalpy rise and increase in entropy at elevated temperatures. This constitutes a loss during energy efficiency evaluation. With increase in SBR, physical exergy fraction in syngas increases from 5.9 - 11.3%.

[00072] Gasification provides gaseous fuels. Exergy loss due to the mixing of gases in case of air gasification was found to be about 3.2% of total hot gas exergy value as compared to 1.7% in case of oxy-steam gasification at lower SBR of 0.75. Also, total exergy (chemical and physical) in syngas due to N ₂ accounts for 3.3% which translates to a loss in energy efficiency. The loss in efficiency due to H ₂ O accounts for 0.8%, which is comparable to that in the case of oxy-steam gasification at lower SBR of 0.75. It is also to be noted that the presence of N ₂ in the product gas contributes significantly towards efficiency losses. However, in case of oxy-steam gasification, the steam or water fraction in syngas leads to similar losses at lower SBR, thereby making air gasification a less efficient process.

[00073] Gasification or energy efficiency (η) of 85.8% was achieved with SBR of 0.75. Syngas with LHV of 8.9 MJ Nm ^"3 was obtained at SBR of 0.75. This reduced by 17% to 7.4 MJ Nm ^" at SBR of 2.7. This is almost twice the energy content in producer gas obtained through air gasification. At SBR of 0.75, H ₂ yield of 66 g per kg of biomass was achieved with high gasification efficiency of 85.8% and high LHV of 8.9MJ Nm . With increase in SBR, H ₂ yield increased to 104 g per kg of biomass but with decrease in efficiency at 71.5% and LHV at 7.4 MJ Nm ^"3 for SBR of 2.7.

[00074] Therefore, the present invention provides a method for hydrogen-rich syngas generation where the syngas does not comprise nitrogen. Mass flux of reactant (oxy-steam) can be varied for stable operation. Flame propagation rate can be controlled to increase residence time and to increase hydrogen yield. The reactor and process steps of the present invention are used to generate clean hydrogen rich syngas of medium calorific value and low tar content. The reactor of the present invention possesses the capability to generate hydrogen rich syngas at pressures marginally higher than ambient. H ₂ /CO ratio in the generation of syngas varies from 1 : 1 to 3.9: 1. ER for a given SBR is maintained for a stable and sustained operation. By adopting the reactor and process steps of the present invention, H ₂ yield close to thermodynamic limit of 100 g of hydrogen per kg of biomass was achieved. Carbon boundary at SBR of 1.5 was achieved. In the present invention, wet biomass along with 0 ₂ as reactant with moisture to biomass (H ₂ O/Biomass) ratio between 0.6 to 1.1, is used on mass basis with H ₂ yield reaching upto 66 g/kg of dry biomass and 33% volume fraction.

[00075] The gasification process with oxy-steam was also conducted using coconut shells whose particle density is ~ 1000 kg/m3, higher than the wood density. The gas composition obtained during the testing is CO 15.6 ± 0.1, C0 ₂ 33.3± 0.2 CH ₄ 3.5 ± 0.25 and H ₂ 47 ± 0.5. These results clearly suggest the capability of the reactor to operate using various biomass with different densities. Therefore, the process 100 and the downdraft gasifier 200 can be used for gasification of various types of biomass, thereby providing fuel flexibility.

[00076] Although the subject matter has been described in considerable detail with reference to certain examples and implementations thereof, other implementations are possible. As such, the appended claims should not be limited to the description of the preferred examples and implementations contained therein.