Some implications of co-combustion of biomass and coal in a fluidized bed boiler

Laboratory combustion experiments were conducted to clarify some implications of co-firing coal with hog fuel and sludge in a power boiler. Combustion tests in a fixed bed stainless steel reactor at four temperatures (675, 725, 775, and 825 °C) under conditions simulating different moisture contents of hog-sludge blends indicated no problems with ignition. Burn-out of the coal reached 88–99%. However, the burn-out was very sensitive to the excess air, especially when co-firing wet hog fuel. Co-firing with coal will lead to higher sulfur dioxide emissions. Sulfation experiments were conducted in a fixed bed quartz reactor for five limestone particle size ranges (90–150 μm, 212–300 μm, 425–595 μm, 850–1000 μm, 2.0–3.4 mm) at the same temperatures as the combustion tests, with steam added to simulate the variation in the moisture content of the fuel mixture. The tests showed that the capacity of limestone to capture sulfur depends on temperature and particle size. The highest Ca utilization of the limestone was 51% (for the smallest particle size, at 825 °C). At 825 and 775 °C, the sulfur capture capacity of the limestone decreased significantly with increasing particle size, whereas at lower temperatures (725 and 675 °C), the Ca utilization was much less dependent on particle size. The sulfur capture capacity of the limestone was unaffected by the moisture content of the hog-sludge fuel. Calcium contained in the sand used as an inert in the power boiler may be capable of capturing small amounts of sulfur.