In situ carbon dioxide capture during biomass fluidized bed gasification

The necessity to contain CO2 emissions has been leading to a growing interest in renewable and CO2 free energy sources. So far, co-gasification of coal and biomass is appealing for the production of a valuable energy or chemical vector as syngas. However, depending on the specific end-use, further treatments of the producer gas are required in order to reduce impurities including tars, dust and inorganic substances and/or to adjust the H2/CO/CO2 ratio. In this respect, sorption enhanced water gas shift reaction process with in situ CO2 capture by solid sorbents represents an intensified option for producing a H2-rich product stream. In addition to enhanced H2 production, due to a shift in the key equilibrium reactions of gasification, this process has, in fact, several advantages including: a) the production of a concentrated stream of CO2, suitable for storage (sequestration), as a consequence of the reversibility that generally characterizes these processes.; b) the exothermic carbonation reaction can supply most of the heat demand of the endothermic gasification reactions; c) particles sorbents, such as limestone, dolomite, olivine and high-iron solids, show some catalytic activity for tar reforming and cracking. The paper reports on the mutual influence in the reactor between coal/biomass steam-oxygen gasification and CO2 separation by means of a chemical sorbent. Experimental tests in a BFB (Bubbling Fluidized Bed) gasifier demonstrate that using a CaO-alumina mixed bed the H2/CO ratio can be strongly enhanced, provided that a good control of the temperature is assured because of the high thermal character of the carbonation. The use of CaO-alumina mixed bed offers the advantage of reducing the tar concentration and prevent the bed agglomeration typically observed when olive husk and quartzite sand are used.