The Migration And Catalytic Characteristics Of Alkali Metals During Coal And Biomass Co-pyrolysis Process

Catalyzed coal gasification has the advantages of lower operating temperatures, lower investment and gas selectivity. But there exists some problems such as high cost and reactor corrosion when alkali metal salts used as the catalyst in coal gasification. And biomass as one kind of renewable energy absorbs alkali metals during its growing process, and then some alkali metals will release to the gas phase during its thermal utilization process. It is important to study the release, migration rule and catalyzed effect of the alkali metals in biomass to coal gasification process.In this paper, Jincheng anthracite and rice straw were selected as raw materials. The experiments were carried out in a two-stage/single-stage fluidized reactors, including coal and biomass co-pyrolysis, acid-wash coal(AW) and biomass co-pyrolysis and single raw sample pyrolysis experiments, Microwave digestion method was used to digest the chars. The alkali metals were analyzed by the atomic absorption spectrometry (AAS) and the alkalis K/Na migration direction and patterns in the coal and biomass co-pyrolysis process was discovered. The effect of pyrolysis temperature, coal particle size and coal minerals to the alkalis migration rule was studied. And then the gasification reactivity of different chars was studied by the TGA analysis. The conclusions were as follows:(1) During the pyrolysis process of rice straw, Jincheng anthracite and AW-Jincheng anthracite, alkalis K/Na released to the gas of rice straw increases with the pyrolysis temperature. And the releasing amount of K/Na at1000℃is65%. From700to900℃, due to the lower pyrolysis degree, alkalis released to the gas of Jincheng anthracite is very low. The releasing amount of K/Na at1000℃is15%and10%. The release amount of K in AW-Jincheng anthracite increased obviously from700to800℃and remains stable from800℃to1000℃; the Na released amount to the gas increases with increasing the pyrolysis temperature.(2) The release amount of alkalis K/Na during the co-pyrolysis experiments was less than the calculation amount of alkalis based on the ratio of raw material in700-1000℃. Coal minerals and small coal particles can enhance the alkalis retaining in mixture char.(3) The experiments conducted in two-stage fluidized reactor indicate that the transformation of the alkali metal species in biomass to coal char is by gas-solid transfer.(4) The release rule of K and Na is almost the same; the difference is just at the quantity of the alkalis released.(5) The TGA analysis indicates that the gasification reactivity of co-pyrolysis char in700℃is better than that in1000℃, and the reactivity of char in800and900℃is in the middle; the co-pyrolysis chars of different particle size coal and biomass has the similar gasification reactivity. The gasification reactivity of co-pyrolysis char is proportional to the alkali metals content, which indicates that the alkali metals released from biomass in pyrolysis process could catalyze coal gasification.