Thermodynamic Optimization of Biomass Gasification
4.4 R ESULTS
4.4.2 Trends in Carbon Distribution
The biomass mainly comprises of 4 elements, viz. C, H, N and O. Out of these elements, C, H, and N undergo partial / total oxidation during gasification process. The major components of the producer gas resulting from gasification of biomass are CO, CO2, CH4, H2 and N2 along with water vapor (H2O). Moreover, depending on the supply of oxygen through gasification medium, not all of the carbon gets oxidized to CO or CO2 and some carbon may remain unconverted at certain conditions of air ratio and temperature. Among C, H and N, carbon and hydrogen undergo oxidation in preference to nitrogen, which relatively inert element. It would be worthwhile to see as how these elements get distributed among various products. In Tables 4.9 to 4.14, we give distribution of carbon and hydrogen in terms of mole fraction in various products.
4.4.2.1 Gasification with air alone: Table 4.9 gives the distribution of carbon in the gasification mixture among four product species, viz. CO, CO2, CH4 and unconverted carbon, for gasification with air alone. Some peculiar trends are as follows: (1) For low gasification temperatures and low air ratios, significant fraction of carbon remains unconverted. For any gasification temperature, the fraction of unconverted carbon reduces with increasing air ratio.
For AR = 0.2 and higher, no unconverted carbon is seen for gasification temperatures above 800
°C. For rice husk and bamboo dust, complete conversion of carbon is found to occur at even lower temperature of 700 °C. (2) As far as relative distribution of carbon among two oxides, viz.
CO and CO2, is concerned, an interesting trend is seen. For low air ratios (0.2–0.4), the fraction of carbon undergoing partial oxidation increases rapidly as the gasification temperature rises, with proportionate reduction in fraction in total carbon undergoing complete oxidation to CO2. On the other hand, as air ratio rises for any gasification temperature, the fraction of carbon getting converted to CO2 increases. Obviously, at AR = 1, when stoichiometric air is supplied for complete oxidation, entire carbon is converted to CO2. (3) Fraction of carbon getting converted to CH4 is relatively small. The highest production of CH4 is seen for truly pyrolytic conditions, i.e. AR = 0. With increasing air ratio any gasification temperature, the fraction of carbon getting unconverted to CH4 reduces. Same trend is seen with rising gasification temperature at any given air ratio. For high gasification temperature (800–1000 °C), total carbon is distributed only among CO and CO2, with other two products, viz. CH4 and unconverted carbon, reducing to zero.
4.4.2.2 Gasification with air–steam (10% mole/mole) mixture: Table 4.11 gives the distribution of carbon in the gasification mixture among four product species, viz. CO, CO2, CH4 and unconverted carbon, for gasification with air–steam (10% mole/mole) mixture. The overall trend in distribution of carbon among the four products is same as in case of gasification with air alone. However, some deviations occur as follows: (1) For saw dust, significant amount of carbon is seen unconverted even at high gasification temperature of 700–900 °C at air ratio of 0.2. As noted earlier, CO production shows sudden rise at 1000 °C. For other two biomasses, viz. bamboo dust and rice husk, significant reduction in unconverted carbon is seen to occur for all temperature and air ratios, as compared to gasification with air alone. (2) For higher gasification temperature of 800–1000 °C, the distribution of carbon towards CO2 increases slightly. This change is seen for all air ratios. (3) The fraction of carbon getting converted to
CH4 also increases slightly for all air ratios at low gasification temperature. For higher gasification temperature, however, no change is seen in production of CH4.
4.4.2.3 Gasification with air–steam (30% mole/mole) mixture: Table 4.13 gives the distribution of carbon in the gasification mixture among product species for gasification with air–steam (30% mole/mole) mixture. Once again, the overall trend is same as that for gasification with 10% mole/mole steam–air mixture. However, some aberrations are seen as follows: (1) The fraction of unconverted carbon shows marginal reduction at all temperatures and air ratios.
Typically, for all biomasses no unconverted carbon is seen at and above AR = 0.6 for all gasification temperatures. (2) The fraction carbon converted to CH4 increases slightly for low gasification temperatures (400–500 °C) at all air ratios. However, for higher gasification temperatures, (800–1000 °C), fractional conversion of carbon to CH4 reduces. (3) The fractional conversion of carbon to CO2 increases further, as compared to gasification with air alone or 10%
mole/mole steam–air mixture. This is, of course, accompanied by proportionate reduction in carbon conversion to CO – especially at high gasification temperatures of 800–1000 °C, where distribution of carbon to other two products, viz. CH4 and unconverted carbon, drops to zero.
4.4.2.4 The effect of O/C and H/C ratio on carbon distribution: It could be seen from Table 4.2 that O/C ratio in the gasification increases with AR (due to oxygen supplied by air), and it further increases with presence of steam (due to oxygen contribution by water molecules). On the other hand, H/C ratio does not change with AR for gasification with air alone (as air does not contribute any hydrogen), but it does increase with presence of steam in gasification medium.
The oxygen and hydrogen in gasification mixture compete for reaction with carbon. The products of the reaction between C and O are CO and CO2, while reaction between H and C gives CH4. It is interesting to note the trend in production of CO, CO2 and CH4 and correlate them with O/C and H/C ratios in gasification mixture. If we assess the H/C and O/C ratios for three biomasses, we see that for any gasification medium, temperature and air ratio, O/C and
H/C ratios show the following trend: Rice husk > Bamboo dust > Saw dust. We now examine the variation in carbon distribution among CO, CO2 and CH4 for various gasification conditions and try to justify it in terms O/C and H/C ratios for these conditions.
(1) Higher H/C (than O/C) ratio for AR = 0 makes CH4 dominant product for carbon in gasification mixture for any biomass, especially at lower temperatures. At higher temperatures, however, CO and CO2 dominate the product distribution and we attribute this to higher energy of C–O bond (–1096.38 kJ/mole) than C–O bond (–338.4 kJ/mole), due to which compound having C–O bond are favored at higher temperature.
(2) As AR increases, O/C ratio increases for all three gasification media. However, H/C ratio shows increase only for air–steam mixtures. As a result, fraction of carbon getting converted to CH4 drops rapidly with increasing air ratio for air gasification than gasification with air–steam mixtures.
(3) Comparison of variation in carbon distribution for any biomass with different gasification media (for same air ratio and temperature) reveals that fraction of C converting to CH4 increases with increasing steam content. This is clearly attributed to higher H/C ratio at higher steam content of gasification media. O/C ratio also increases with steam content of the gasification medium. As a result rise in both H/C and O/C ratios, the fraction of unconverted carbon seen for certain combinations of AR and temperature reduces.
(4) Comparison of carbon distribution among different biomasses for a given gasification medium, AR and temperature reveals interesting features. For any AR and gasification medium, both O/C and H/C ratios for rice husk are much higher than saw dust. On the other hand, the O/C and H/C ratios for rice husk and bamboo dust are the similar. This difference is reflected in terms of unconverted carbon. We see that fraction of unconverted carbon seen for certain combination of AR, temperature and gasification medium is much higher for saw dust than rice husk and bamboo dust.