Investigation Of Promoting Effect Of Alkali Metals In Biomass During Coal Gasification Process

During the co-gasification process of rice straw and anthracite, thesignificant difference of thermal chemical characteristics between the twodifferent raw materials will lead to biomass ash and coal char coexistence, whichis an inevitable stage during co-gasification. This topic mainly focus on thecatalytic effect of alkali metals in biomass and biomass ash, besides, the influenceof biomass addition on the fusing characteristics of coal ash.In this paper, Jincheng anthracite and rice straw were selected as rawmaterials.20wt%of biomass were added into coal then gasified with CO2as thegasification agent in a fixed bed reactor at1100°C. Alkali metal content changein solid residues at different gasification time were analyzed, the influence ofalkali metals on mineral transformation, organic structure and gasificationreactivity were also studied. On this basis, reference with chemical fractionationmethod, alkali metals in biomass ash were classified, then studied the catalyticability of different chemical forms of alkali metals in biomass ash bythermogravimetric analyzer (TGA). Furthermore, effect of biomass addition oncoal ash fusion characteristics were studied and explored the fluxing mechanism.The conclusions are as follows:1. The rice straw will gasified completely in3min at1100°C, during co-gasification with anthracite, the alkali metal content in gasification residueincreased during1min to7min. The increased alkali metal existed in the formsof water soluble and organic forms, which has better catalytic ability, thus thegasification reactivity of residues changed better. 2. The degree of carbon structure ordering of coal char during gasificationprocess increased along with the extension of gasification time, while the biomassaddition slowed the ordering process.3. During the early gasification process, the formation of SiC due to thereaction of SiO2with C will generate gas phase SiO, the SiO could combined withalkali metals in biomass to generate unstable sanidine thus increase the alkalimetals content in the gasification residues. Then, unstable sanidine decomposedand disappeared. Part of the alkali metals release into gas phase continuously, therest transform into albite and anorthite substances remain in gasification ash.4. Thermogravimetric experiments show that when the addition ratio ofbiomass ash was less than40wt%, the coal char gasification conversion rateincreases along with the addition amount of biomass ash, while the gasificationresidue fused into a ball when50wt%of biomass ash were added. The watersoluble alkali metal salts and hydrochloric acid insoluble alkali metal oxides inbiomass ash has catalytic activity during coal gasification. The presence ofminerals in coal weakened the catalytic ability of biomass ash, CaCO3additioncan inhibit the inhibiting effect.5. In reducing atmosphere,20wt%of biomass addition reduced the ashfusion temperature to about1350°C. The alkaline component increased afterbiomass addition lead to feldspar minerals formation, which could easily reactedwith mullite, quartz and other refractory components to form low temperatureeutectic, then lower the ash fusion temperature of coal ash.