CFD analysis on bioliquid co-firing with heavy fuel oil in a 400 MWe power plant with a wall-firing boiler

Liquid biomass derived from food processing, biodiesel production, or fast pyrolysis has good potential for use as fuel to displace diesel or heavy fuel oil (HFO) for heat and power, and a major demonstration was successfully conducted in a 400 MWe HFO-fired power plant with a wall-firing boiler configuration using a BL blend of palm oil, its residue, and animal fat at a 20% co-firing ratio. When compared to a condition with HFO as fuel, the heat transfer rate on the membrane wall significantly decreased, and that of the convective heat exchangers in the upper furnace increased. This trend was different from what was observed at two smaller plants of 100 and 75 MWe, both with a tangential-firing boiler. This study uses computational fluid dynamics (CFD) to investigate the reason for the difference in the heat transfer characteristics. In both boiler types, bioliquid (BL) combustion led to lower soot concentrations and a corresponding decrease in radiation. This directly reduced the heat absorption on the furnace wall by approximately 5% at 20%-BL co-firing in the wall-firing boiler, and it was predicted further decrease linearly by 14% at 100%-BL firing. However, the tangential-firing boilers had less than a 1% decrease in measured heat absorption on the furnace wall at 100%-BL firing. Such differences were attributable to the gas flow pattern of the two boiler configurations. The burner tilting and flue gas recirculation of the tangential-firing boilers also contributed to successful operation at 100%-BL firing. The conclusions indicate that without effective measures to control the heat absorption distribution, the capability of a wall-firing boiler for BL combustion is limited to low co-firing ratios. (C) 2017 Elsevier Ltd. All rights reserved.