In order to increase the share of renewables in future energy systems, biomass will replace coal in medium- to large-scale Combined Heat and Power (CHP) plants. Anyhow, since the fuel composition of biomass is quite different from that of coal, the composition and concentration of particulate matter PM2.5 in the flue gas, as well as deposit formation propensities, during biomass combustion is of special concern. High concentrations of alkali species, S and Cl in the fine particles, influence the deposit build-up, and, consequently, the efficiency and availability of the power station.
As part of the recent EU-project Biofficiency, fine particles were therefore sampled in the reheater section of at the 800 MWth Avedøre Power Station Unit 2 (AVV2) as well as at the 900 MWth Studstrup Power Station Unit 3 (SSV3), both using wood pellets as fuel. Coal fly ash (CFA) was applied in both power stations to act as an alkali capture additive. In addition, experiments in a pilot-scale pulverized-fuel biomass combustion test-rig were conducted, using the same fuel and additive, in order to compare the impact of the CFA-addition to that observed in the full-scale experiments. A comparison of particle concentrations revealed a higher quantity of submicron particles (PM1) at AVV2 compared to SSV3, while the PM1 values at the pilot-scale combustion unit were substantially higher, compared to fine particle concentrations at the full-scale CHPs.
Ash deposition behavior during firing of wood pellets with CFA-addition, was also quantified by an advanced deposit probe, at SSV3. The results showed that with a probe surface temperature of 550 °C, and 2 % or 3 % CFA-addition, no significant deposit was formed. When increasing the probe surface temperature to 650 °C, at 3 % CFA-addition, some deposit was formed on the upstream side of the probe. No Cl was detected in the deposit samples, indicating Cl-related corrosion can be effectively handled by 2,0 % or 3,0 % coal fly ash addition.
Finally, ash deposition behavior when firing different biomass (pulverized wheat straw, wood, bark and HTC leaves), with and without K-capturing additives, was also investigated through combustion experiments in an entrained flow reactor (EFR). In terms of deposit formation, the K- and Cl-rich Danish wheat straw proved to be the most problematic fuel, while milled wood pellets, HTC leaves and bark, had significantly lower deposition propensity. Kaolin and two types of CFA, rich in Al and Si, were applied as additives. The results showed that the deposition propensity of K- and Cl-rich Danish wheat straw decreased significantly, when applying a molar ratio of fuel-K/additive-Al of 1.0.
This paper contains a very brief introduction to the EU-project Biofficiency, a state-of-the-art outline of the experiments conducted, results obtained, as well as an outline of implications and impact on future applications of biomass for heat and power production units.
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In order to increase the share of renewables in future energy systems, biomass will replace coal in medium- to large-scale Combined Heat and Power (CHP) plants. Anyhow, since the fuel composition of biomass is quite different from that of coal, the composition and concentration of particulate matter PM2.5 in the flue gas, as well as deposit formation propensities, during biomass combustion is of special concern. High concentrations of alkali species, S and Cl in the fine particles, influence the...
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