(60b) Ash Formation in Co-Firing Torrefied Wood with Coal on a Pilot-Scale Air/Oxy-Fuel Combustor
Torrefaction treatment improves biomass properties in terms of energy density, hydrophobic behavior and grindability. Torrefied wood as an abundant biomass fuel can be co-fired with coal in power generation. In this work, pure torrefied wood, Utah Sufco coal and their blend fuel mixed at ratio of 50/50 wt. % are burned in a 100 kW (rated) pilot-scale combustor. Both air-combustion (denoted as Air) and oxy-fuel combustion (as OXY70 for the 70 vol. % O2 in oxidant gas) are performed and compared. The minerals from wood have high contents in Ca and Mg while from coal are Si and Al. This work focuses on formation of ash aerosol and deposit through mineral transformation in air- and oxy-fuel combustion of the three fuels. Results show that the aerosol particle size distribution (PSD) in Air case of wood has a nucleation mode in sub-micron size range and a coarser coagulation mode appears in the OXY70. PSDs of co-firing case are shown to be a result of combined ash partitioning in both wood and coal combustion. The Size-segregated compositions indicate that the enrichment of aluminosilicate minerals which are brought by coal facilitates consuming vaporized alkali from the blend fuel. This causes the sub-micron aerosols of OXY70 contain much less alkali metals than those of Air in co-firing. The growth rate for the inside layer deposit initially formed on pipe surface in the blend Air case is found to increase compared with the two pure-fuel cases, but not for the subsequent outside layer deposition. With such fuel mixing ratio, notable effects from co-firing woody biomass with coal on ash formation are observed. These mechanisms under air- and oxy-fuel combustion conditions will be carefully studied in this work.