Dead trees killed by bark beetles are abundantly available in western US forests. To reduce wildfire risks, it is proposed to collect and use this low-value biomass as a supplementary fuel in existing coal-fired power plants. Burning biomass-based fuel is considered to be carbon neutral and results in a lowering of net carbon dioxide emissions when it replaces fuels such as coal in the generation of electricity. However, potential impacts to boiler performance when co-firing wood with coal may be caused by changes in ash deposition on heat transfer surfaces. This paper presents results from the second phase of a two-phase project in which the effects of combustion scale on ash deposition from combustion of identical coals and coal/biomass blends were investigated in a 1.5 MW th pilot-scale furnace (part 1) and a 471 MW e operating full-scale boiler (this work). Results presented in this paper, however, can stand by themselves, with a focus on practical effects of biomass addition to coal fired in a full scale unit. The coal/biomass co-fired blend consisted of 15% of torrefied wood made from local dead spruce and 85% of pulverized bituminous coal. Of interest are the effects of addition of woody biomass feedstock to pulverized coal on the ash aerosol and ash deposition. Fly ash and ash deposits were alternatively sampled by an iso-kinetic sampling probe and a surface temperature-controlled deposition probe, respectively, which were inserted to the same boiler penetration in the vertical reheat tube bundles. Measurements include real-time particle size distributions and ash deposition rates during both coal combustion and co-firing cases. The size-segregated (0.0324−15.7 μm) particles and time-differentiated deposits were analyzed in terms of composition and microscopic morphology. Results show no significant changes in ash transformation when switching from coal combustion to co-firing with torrefied wood. The results of this full-scale demonstration are further compared with those obtained in the pilot-scale furnace (part 1) to investigate the scale effect on ash aerosol formation and deposition in coal and co-firing biomass combustion.
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 × 10 6 kW (471 MW e ) 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/(m 2 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.
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