Ash Aerosol and Deposit Formation from Combustion of Coal and Its Blend with Woody Biomass at Two Combustion Scales: Part 1─1.5 MWTH Pilot-Scale Combustor Tests

Abstract

There is great interest worldwide in repurposing electric utility boilers designed to fire pulverized coal to fire, instead woody biomass or blends of woody biomass with coal. In this investigation, two prepared biomass/coal blends and the pure parent coal were fired in a 1500 kW pulverized coal combustor with a primary objective of elucidating the mineral particle behavior. The results reported here, although complete in themselves, comprise the first part of a two-part systematic study to investigate the effects of combustion scale on ash deposition rates at scales of 1500 kW (this study), and 1.2 × 106 kW (471 MWe) using identical coal and coal/biomass fuels and similar analytical techniques. The woody biomass of interest was composed of materials collected in a Utah National Forest and was prepared using a torrefaction technique (for both combustion scales) and a separate steam explosion technique (for the 1500 kW pilot scale only, described here). Biomass samples were blended in 15 wt % biomass with a Utah bituminous coal and were pulverized along with samples of pure coal at a specification of 70 wt % passing through 200 mesh. To sample entrained and deposited mineral matter, a water-cooled extractive probe for ash aerosol and an air-cooled ash deposit probe were designed, constructed, and implemented. The probes were inserted into sample ports at gas temperatures in the range of 1200–1370 K with a deposit coupon surface temperature of 811 K, conditions representative of a utility boiler vertical reheater and of a fouling deposit mechanism. Aerosol size distributions were determined using a scanning mobility particle sizer (SMPS) and an aerodynamics particle sizer (APS), and a Berner low-pressure Impactor (BLPI) collected size-segregated aerosol samples for subsequent elemental analysis. A laser diffraction particle size analyzer (Beckman Coulter LS230) was used to determine the size distributions of ash deposit samples. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS) was used to determine the morphology and composition of ash deposit samples. For the pilot-scale tests reported here, the experimental results suggested little difference in mineral matter behavior between pure coal and biomass blends. The measured aerosol PSDs showed nearly identical behaviors, with modes at approximately 20 nm and 3 μm. The size-segregated aerosol particles were slightly enriched in Na, K, and Ca and deficient in Si and Al for biomass blends compared to the pure coal. The deposition rates were 60% greater in port 7 than in port 10 at 106 vs 65 g/(m2 h). Deposit samples collected for 90 min showed essentially the same deposition rate for coal and the two biomass blends, while short deposition times (30 min) suggested higher initial deposition rates for the biomass blends. The deposit composition and morphology were nearly identical for all three fuels.