Mitigation of CO2 emissions in thermochemical conversions of coal and biomass.

摘要

Carbonaceous materials can be converted to bio-oil using fast pyrolysis or produce synthetic gas (syngas) using gasification. Utilizing concentrated solar energy, instead of combusting fossil fuels to provide the heat required for these endothermic reactions, reduces CO2 emissions. Also, in situ CO2 capture can increase the production rate and yield of hydrogen in syngas and help mitigate CO2 emissions.;In the first part of this study, biomass fast pyrolysis and coal gasification using concentrated solar energy were numerically simulated. A two-dimensional single particle model was developed to predict the products composition from biomass fast pyrolysis, using solar energy under different solar heat fluxes. The results of this study emphasized the importance of secondary reactions in this process. The maximum bio-oil yields were in the range of 53-60% obtained between 460 and 500°C. Next, a one-dimensional two-phase model was developed to simulate coal gasification using solar energy. The present model considered gasification of coal with CO2 in a solar gasifier, which has not been numerically simulated previously. The findings of this study provided an insightful explanation on the existence of an optimal CO2 inlet velocity for maximum solar-to-chemical energy conversion. Also, the energy balance analysis showed 8% of the incoming solar energy was stored as enthalpy of produced CO, 52% was used in heating the coal particles and gases, and 40% was lost to the surroundings.;In the second part of this study, biomass gasification with in situ CO 2 capture in a single reactor, utilizing calcium oxide (CaO), has been studied. First, the experimental conversion data of CO2 carbonation using CaO were used to determine the capture capacity and to develop a kinetics model. Using the kinetics model, the activation energy of CaO carbonation using CaO was determined to be 23 and 16.5 kJ mol-1 for dry and moist (5% steam) streams, respectively. Next, the effects of temperature, reacting gas partial pressure, and CaO-to-biomass mass ratio on syngas composition have been investigated. In situ CO2 capture led to a significant increase in H2 concentrations (> 80%) compared to conventional biomass gasification with H2 concentrations in the range of 40-50%.