The Migration And Catalytic Characteristics Of Biomass Alkalis During Co-gasification Of Coal And Biomass

During the coal/biomass co-gasification process, the rich alkalis K and Na in biomass are likely the excellent catalysts of coal gasification reactions. But meanwhile the biomass alkalis can easily volatilize into gas phase, which may cause many issues of corrosion and deposition. In order to regulate the biomass alkalis migration and reduce the negative influence on co-gasification system, the migration characteristics of biomass alkalis during co-gasification should be studied. In this paper, the co-gasification process was logically divided into co-pyrolysis stage and char gasification stage. The char and ash produced from the two stages were collected; the contents and combined forms of biomass alkalis migrated to char and ash were measured. Then the quantity changes and form transformation of biomass alkalis were summarized and analyzed. In addition, in order to utilize the biomass alkalis K and Na to catalyze co-gasification reactions to promote the energy efficiency of system, the catalytic characteristics of the alkalis migrated into coal char were also studied. Summarizing all the research work, the main conclusions are listed as follow:1、The alkalis K and Na contained in biomass are mainly H2O soluble form. The migration routes of biomass alkalis can be described as follow:In the pyrolysis environment, the H2O soluble alkalis K and Na firstly volatilized and migrated to the coal char, meanwhile, these alkalis transformed into H2SO4 soluble and H2SO4 insoluble forms; During the subsequent gasification of coal char, the biomass alkalis reacted further with minerals of coal and then retained in coal ash as muscovite (KAl2[Si3AlO10](OH,F)2) and nepheline (NaAlSiO4). Due to this migration characteristic, co-gasification may promote more biomass alkalis to retain in ash to compare with biomass gasification. Thus, co-gasification is beneficial to reduce the release of biomass alkalis. The adsorption and chemical reaction between alkalis and coal char may be the main reason of biomass alkalis migration. The H2O soluble alkalis K and Na could be adsorbed onto the char surface and then reacted with minerals and organic structure of coal char.2、During the pyrolysis stage of co-gasification, the reaction conditions have significant effect on the migration of biomass alkalis K and Na. Raising temperature, adding a spot of gasification agent and increasing the blending ratio of coal and biomass could promote the migration of biomass alkalis. The temperature has a great influence on migration quantity and form transformation of biomass alkalis. Biomass alkalis could migrate to coal char only when the temperature is higher than 773 K. And the high temperature may accelerate the transformation of H2O soluble alkalis into H2SO4 soluble and H2SO4 insoluble forms. In addition, the minerals in coal are beneficial to improve the migration quantity of biomass alkalis, and may promote most of the migrated alkalis to retain in ash finally.3、Increasing the migration quantity of K and Na by raising co-pyrolysis temperature cannot improve the reactivity of coal char. In this circumstance, the catalytic characteristics of biomass alkalis are suppressed. However, adding a spot of H2O/O2 into co-pyrolysis process can effectively improve the gasification reactivity of coal char. It is mainly because of the abundant pore structure of coal char produced in H2O/O2 atmosphere. The abundant pore structure may lead more biomass alkalis to widely retain and disperse in coal char, and more gasification agent could contact with the organic structure of char. Therefore, more active sites may form in coal char, the biomass alkalis K and Na could catalyze the gasification of coal char. When the contents of minerals in coal are very low, the migrated biomass alkalis mainly combine with organic structure of coal. Thus, most alkalis can exist as active forms and catalyze the gasification of coal char. In this case, increasing the migration quantity of K and Na by regulating co-pyrolysis conditions could improve the catalytic effect further. In addition, there is close relation between the catalytic activity and existing forms of biomass alkalis. The H2O soluble K/Na existing in coal char can present strong catalytic effect.