CFD modeling of reduced-lignin black liquor combustion

Computational fluid dynamics (CFD) modelling is carried out to investigate how in-furnace processes are affected by lignin removal and what changes are needed in boiler operation. For a modern recovery boiler (5300 tds/day) with spraying operated in the flashing regime, two scenarios are considered. In one case, the recovery boiler is unloaded by removing lignin, but production is not increased, and therefore the dry solids to the boiler decrease. In the second case, the total flow to the recovery boiler is increased after lignin removal, so that the total flow to the recovery boiler stays the same as before lignin removal.

The simulations show that if droplet size remains the same, less char carbon is delivered to the lower furnace with lignin removal, and changes to boiler operation (air and spray) are needed to compensate for this change. The needed decrease in liquor firing temperature generally becomes greater as more lignin is removed and is greater in the boiler unloading scenario than when boiler dry solids load is maintained.

The simulations suggest that up to 20% lignin removal can be implemented in an existing recovery boiler. Simulations show that with air distribution unchanged (primary 22%/ secondary 43%/ tertiary 35%), a decrease in liquor firing temperature of approximately 1.0°C is needed. If air distribution is changed to deliver more air to the lower furnace (primary 28%/ secondary 40%/ tertiary 32%), a decrease in liquor firing temperature of approximately 3.6°C is needed. The simulations also show that the volatile-C / char-C ratio is an important model input.