Gasification of biochars: Evolution of pore structure, effects of alkalis and alkali release

Sammanfattning: Renewable energy sources are indispensable to meet the rising demand of energy usage  while  reducing  the  negative  environmental  impact  of utilising fossil  fuels. Gasification  is  an  efficient  technology  to  convert  biogenic waste  into  valuable gaseous products. The rate of conversion of char, produced in an intermediate step in the conversion, plays an essential role in the conversion of biogenic materials. The conversion of char is significantly affected by properties such as the structure of  the  char  and  its  alkali content.  This  thesis  presents  findings  related  to  the influence  of  char pore  structure  development  and  alkalis  content  on  char gasification, as well as the alkali release during gasification and co-gasification. Experimental  results  show  that  the  generation  of  micropores  are  directly proportional to the observed reactivity up to 70% of char conversion, after which the catalytic  effects  of  potassium  become  the  dominating  factor.  Furthermore, investigations of the effect of different intrinsic potassium contents on woody char reactivity demonstrate that no alkali surface saturation point is reached, as is the case for high-ash chars. Application of a modified random pore model enabled a successful  capture  of  the  later  stages  of  char  conversion  in  comparison  to  other kinetic models applied.  Alkali release and sample mass changes were monitored simultaneously, using a thermogravimetric analyser together with a surface ionization detector (TGA-SID). The  studies  revealed  a  significant  release  of  alkali  as  woody  char conversion approaches completion during CO2  gasification. For straw char the release of alkali decreased  continuously  throughout  the  conversion  process. Similar  results  were obtained  for  biochar  gasification  under  steam conditions  in  a  fixed  bed  reactor. However,  in  this  case  the  process  is more  complex,  including  transfer  of  alkali between particles inside the fixed bed, which influences char conversion.  Co-gasification of different types of biomass can substantially affect char conversion efficiency. In comparison to pure wood, mixing wood and straw had positive effects on  the  char  conversion  for  rates  below  90%  of  conversion,  while  exceeding  this degree of conversion resulted in negative effects. The most significant positive effect was observed at a gasification temperature of 900 °C, particularly when using a wood-straw blend of 75 wt%:25 wt%.   The above findings are important for the understanding of the mechanisms of char conversion and are valuable in the design of gasifiers. The research provides with a deeper understanding of char structure development, alkali release, and migration during gasification of biogenic materials.