Analysis of a Wood-Fueled Trimburner System for Use in a Combined-Cycle, Wood-Fired Power Plant

Abstract

This paper investigates the use of wood to fuel a trimburner incorporated in a combined-cycle, wood-fired power plant. The trimburner is designed to boost the temperature of the air stream entering the gas turbine. Wood conversion processes capable of producing a clean synthetic fuel were investigated since direct wood combustion products are too “dirty” to be allowed to pass through the turbine blading. Of the three wood conversion processes considered (pyrolysis, gasification, methanol production), gasification was selected as the most applicable process for the trimburner concept. Three wood-fired trimburner systems employing an up-draft gasifier design were developed and simulated. These subsystems differ in the way the producer gas, formed in the gasifier, was compressed to the trimburner operating pressure. The effects of changing system variables, such as wood moisture content and gasifier air/fuel equivalence ratio, on the performance of the subsystems and the overall system were evaluated. It was determined that the most efficient operation of all the trimburner subsystems occurred at the lowest allowable operating gasifier equivalence ratio, about 0.275. Increasing the wood moisture content from 15 percent to 50 percent decreased the efficiency of the overall system about 3 percentage points, regardless of the specific trimburner system. At the usual wood moisture content of 50 percent, the best trimburner system, operating at the optimum equivalence ratio, increased the overall system performance about 8 percent (1.7 percentage points) relative to the equivalent metallic heat exchanger based system with no trimburner. The system that used air from the gas turbine compressor in a pressurized gasifier exhibited slightly superior performance (approximately 0.5 percentage points) relative to the system using the other trimburner designs. However, this performance superiority must be tempered since the pressurized gasifier system is more sensitive to the efficiency of the heat exchanger used to recover energy from the dirty producer gas.