| Abstract | Global warming, climate change and energy security issues are the forces driving the fossil fuel based energy system towards renewable and sustainable energy. Hydrogen as a clean energy carrier is believed to be the most promising source to replace fossil fuel. Biomass gasification with the presence of steam offers a feasible, sustainable, and environment-friendly option as well as a favorable alternative for higher hydrogen yields and for large-scale hydrogen production which can satisfy the need of hydrogen in the future. However, the process suffers from the problem of undesirable CO2 and tar formation. Calcium oxide (CaO), when added to the process, could play the dual role of CO2 sorbent and tar reforming catalyst, and thereby produce more hydrogen. The deactivation of CaO after carbonation reaction is however challenging to the continuous hydrogen production and from economical perspective. The concept of CaO-based chemical looping gasification (CaO-CLG) plays a key role in overcoming such a challenge.Previous studies showed that use of CaO in gasification process mostly focused on its role in CO2 capture but little attention to tar reforming aspect. Even though a limited number of works studied the influence of CaO on tar reforming in such process, a detailed analysis which considers not only tar amount but also tar compounds is still lacking in literature, especially in the area of biomass steam gasification using a CaO-CLG system. The information on the operation of CaO-CLG systems is also rarely available in the literature. This study aimed primarily at investigating the CaO-based catalytic tar reforming and the gasification performance in steam gasification of biomass for hydrogen-rich gas production in the CaO-CLG system. The specific objectives of the study are to: (1) theoretically investigate the gasification performance of the CaO-CLG system through kinetic rate modeling, (2) experimentally investigate the CaO-based catalytic tar reforming of the CaO-CLG system, and (3) experimentally investigate the overall gasification performance of the CaO-CLG system.The kinetics model was developed based on one-dimensional two-phase (i.e., bubble and emulsion) concept of a bubbling fluidized-bed gasifier (BFBG) with in-bed use of CaO. Three sub-models consisting of reaction kinetics, hydrodynamics and balances were developed to form the kinetics model of the gasifier. The developed model was simulated through MATLAB and validated using three data groups from two experimental literatures.The results obtained from the model simulation showed good agreement with those obtained from the literatures. The root mean square error (RMSE) values obtained by comparingthe present model and the literatures were in the range of 4.80−8.05. This indicates good precision and reliability in prediction of the model. The study on CaO-based catalytic tar reforming was conducted through experimentation on the BFBG of CaO-CLG system. The optimization study to obtain optimum operating conditions of the BFBG was first conducted and was then followed by comparative study to investigate the influence of CaO on tar reforming. The production of H2, CO2 and tar influenced by those studies was mainly observed. The optimization study was carried out by varying gasification temperature (550−700°C) and steam-to-biomass (S/B) ratio(1.47−3.41). Results showed that the optimal temperature of 650−700°C and S/B ratio of 3.41 were obtained. At such optimum S/B ratio, the highest H2 concentration of 63.07vol.% and the lowest CO2 concentration of 18.68 vol.% were obtained at 650°C, while the highest H2 yield of 256.81 ml/g-biomass and the lowest tar content of 6.45 g/Nm3 were iv obtained at 700°C. At the optimal temperature of 650°C and S/B ratio of 3.41, the comparative study was carried out by comparing the results obtained from in-bed use of CaO with those obtained from in-bed use of sand as well as mixed-bed of CaO and sand. Results showed that 14 vol.% higher H2 concentration, 6 vol.% lower CO2 concentration, and 57% lower tar content were obtained for CaO bed alone as compared to the mixed bed. Compared to a bed of sand alone, 20 vol.% higher H2 concentration, almost double H2yield, 2 vol.% lower CO2 concentration, and 67% reduction in tar content were obtained when a bed of CaO was used. The presence of CaO as bed material also brought in the catalytic effect on shifting of tar species from higher (Class 1 and 4 tars) to fewer (Class 2 and 3 tars) ring structures, which resulted in the reduction of tar dew point by 11°C and tar carcinogenic potential by almost 60% as compared to a bed of sand alone. These were interpreted that use of CaO has major influence on both technical and environmental hazards of tar, apart from enriching H2 and reducing CO2. The study to investigate the overall gasification performance in the CaO-CLG system was conducted through experimentation by running the whole looping process of the system. The optimization study on the system was carried out by varying solid circulation rates (0.91−1.14 kg/m2s) and then observing their effect on the production of H2, CO2 and tar.Results showed that the optimum solid circulation rate of the CaO-CLG system was obtained at 1.04 kg/m2s. This optimum solid circulation rate allowed the CaO-CLG system, operated at the optimum temperature and S/B ratio of the BFBG obtained from previous study, to produce the product gas with the highest H2 concentration of 78 vol.%, the lowest CO2 concentration of 4.98 vol.%, the highest H2 yield of 451.11 ml/g-biomass, the highest total gas yield of 578.38 ml/g-biomass, and the lowest tar content of 2.48 g/Nm3. The cold gas efficiency of the system was also found to be 46.69%, while that of ideal scenarioobtained from thermodynamic analysis was found to be 72.79%. In comparative study, a 30 vol.% higher concentration of H2, a 16 vol.% lower concentration of CO2, a three-fold increase in yield of H2 and a 96% lower tar content were found in CaO-CLG as compared to sand-based chemical looping gasification (Sand-CLG) at identical operating conditions. Compared to calcium oxide based bubbling fluidized bed gasification (CaO-BFBG) studied in previous section, the CaO-CLG allowed 15 vol.% higher concentration of H2, 14 vol.% lower concentration of CO2, almost double yield of H2, and 91% lower tar content. The product gas composition (i.e., H2, CO, CO2 and CH4) obtained from all experimental studies (objectives 2 and 3) was validated with values obtained from the developed model(objective 1) and it was found that they have good agreement. The root mean square error (RMSE) and the linear correlation coefficient (r) were 0.66−8.43 (average = 3.31) and 0.908−1 (average = 0.984), respectively, for BFBG study (objective 2), and 0.46−5.50 (average = 2.84) and 0.968−1 (average = 0.993), respectively, for CLG study (objective 3). This confirms a good reliability of the results obtained from the experimental study.To show the superiority in hydrogen production with in-situ CO2 capture and tar reduction, the results obtained from present study were compared with past literatures. The present CaO-BFBG was competitive with past CaO-based gasification in fixed bed and bubbling fluidized bed but uncompetitive with that in CaO-based looping gasification systems. The present CaO-CLG was competitive with all of such CaO-based gasification systems in the literatures. The information obtained from the study can confirm that the present CaO-CLG system is one of the best gasification technologies for hydrogen-enriched gas production with in-situ CO2 capture and tar reduction. |