The heat content of the combustible gases is computed on a dry tar-free basis. The energy density [kJ/m3] of the gases is represented in Table 3.10 for several ER and S:F ratios. Increased ER or S:F tends to increase the energy density of the gases; this is due principally to the increase in the production of hydrocarbons (HC) and H2. At constant S:F, increasing the ER tends to increase the HHV, due to more H2 and HC, until a certain ER beyond which the HHV starts to decrease. The energy density of the gases is strongly affected by the production of hydrocarbons such as CH4 and C2H6, which have a high HHV as compared to the other gases (CO and H2). For example, the HHV or energy density of the CH4 is 36264 kJ/SATP m3 while the HHV of CO and H2 are 11550 and 11700 kJ/SATP m3 respectively. Although the HHV of the H2 (141800 kJ/kg) ona mass basis is very high, its energy density is almost comparable to that of CO (only 1.08% higher) due to its low density (~0.0857kg/m3). At constant ER, increased S:F increase the H2/CO of the species produced (Fig. 3.49 and Fig. 3.50), which implies increasing the energy density slightly. For the set of operating conditions investigated the HHV of the gases ranged between 3268 and 4285 kJ/SATP m3, which correspond to a range between 9 and 12.6% of the energy density of the CH4 on a volume basis. Even though the energy density of the gases gives an idea of the energy content of the gases produced, it does not give information about the degree of energy conversion from biomass gasified.

Variation of HHV with ER in the presence and absence of steam for the case of air (21% O2) and enriched air mixtures (28% O2) is shown in Table 3.11. The enriched-air medium results in gas with higher HHV. The amount of hydrogen produced increases in the presence of steam, but the HHV based on mass is less even with H2 due to lower molecular weight of H2. For both air gasification and enriched-air gasification, we observe a decrease in HHV with ER. Table 3.11 also gives the HHV of the gas mixture with inerts (N2 and CO2) and without inerts (N2 and CO2) and these values are expressed in terms of percentage HHV of natural gas (Thanapal et al., 2012).

Ultimate analyses of samples of tar collected in the sample unit were obtained and were used to derive an empirical formula (CH2O0 48N0.064S0.0017). Because it was impossible to measure the mass of tar and H2O produced during the experiments, the volumetric flow of gases, required to calculate the energy recovery was estimated by mass balance using tar and gas compositions and with the knowledge of the char produced and the flows of the air and steam. Table 3.12 presents the energy conversion efficiency (ECE) estimated by atom balance and assuming gas composition on a dry tar free basis. Although, the energy density of the gases tends to increase with increased ERs, the ECE decreases, because increased ERs produce more mass of tar and char but less mass of gases per kg of DB gasified.

For the range of the operating conditions studied, the ECE ranged from 0.24 to 0.69; the remaining fraction corresponds to the energy in char, tar, and sensible heat of gases leaving the gasifier. This agrees with the fact that in a fixed bed gasifier the gases leave the gasifier at a lower temperature as compared to that of gases leaving a fluidized bed gasifier. Lower sensible heat of gases leaving the reactor implies higher gasifier efficiency, and hence more energy recovered in the gases.

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