Exergy study of co-firing processes of low-grade coal with oil palm kernel shell (as received, torrefied, and pyrolysed) in a brick furnace, using Aspen Plus

Exergy analysis of substitution of low-grade coal by raw, pyrolysed, or torrefied oil palm kernel shell (PKS) was simulated. A comparison of coal combustion base cases with different co-firing cases was made. Exergy efficiencies of pyrolysis and torrefaction agree with reported data. PKS pyrolysis and torrefaction decrease the fuel consumption compared to the base case. An increase in air requirements for co-firing cases with pyrolysed and torrefied PKS shows that, compared to the base case, high thermal power can be obtained with lower flame temperatures, decreasing irreversibilities in combustion and burning processes. Considering the overall process, total irreversibilities in pyrolysis are 30% greater than the case base, contrasting with torrefied cases, where irreversibilities are 20% greater. The lower fuel consumption and increase in irreversibilities in torrefaction cases suggest that it may be an option to replace coal in these furnaces. However, environmental analysis is recommended for a more comprehensive evaluation.