Ash Deposition during Co-firing Biomass and Coal in a Fluidized-Bed Combustor

Abstract

The objective of this study was to examine the ash deposition tendencies of biomass fuels and the biomass-coal blended fuels against the base fuel (coal) during co-firing and how the operating parameters influence the ash deposition tendencies. In this study, ash deposition behaviors during combustion and cocombustion of white pine pellets (WPP) and lignite coal were investigated in a pilot-scale, fluidized-bed combustor. Employing a custom-designed, air-cooled probe installed in the freeboard zone of the reactor to simulate a heat-transfer surface, effects of various operating parameters on the ash deposition rate and compositions of the ash deposits were studied, including the fuel type, fuel blending ratio (0-100% biomass on a thermal basis), moisture content, and air/fuel ratio. A new parameter, "relative deposition rate" (RD A ), was proposed in this study to evaluate the relative deposition tendencies of biomass fuels and biomass-coal mixed fuels against the coal as the base fuel for co-firing. As expected, co-firing of the lignite and the wood pellets (with a much lower ash content than the lignite) resulted in a decreased superficial rate of ash deposition. However, co-firing of WPP and crushed lignite (CL) did not significantly increase the ash deposition tendency in terms of the values of RD A , and more interestingly, co-firing of the 50% CL-50% WPP fuel blend produced a lower RD A . Another new and interesting discovery of this study was that fluidized-bed combustion of an individual fuel or a mixture fuel with a higher moisture content produced not only a more uniform temperature profile along the fluidized-bed column but also a reduced ash deposition rate.

Ash Deposition in Co-firing Three-Fuel Blends Consisting of Woody Biomass, Peat, and Lignite in a Pilot-Scale Fluidized-Bed Reactor

“…As was discussed in our previous study, 29 conclusions were difficult to be drawn based on the D A results only, because the causes of a low ash deposition rate could be due to either a lower total ash content in the feed (e.g., 2 wt % ash content for the peat versus 22 wt % ash content for the lignite) or the positive interaction (to suppress the ash deposition) between the ash elements from the component fuels in the fuel blends. A relative ash deposition rate (RD A ) was proposed and used in our previous study.…”

“…A relative ash deposition rate (RD A ) was proposed (eq 4) and used in our previous study. 29 The value of RD A was obtained from that of D A by correction with the ash feeding rate of the base fuel (i.e., CL) in relation to the ash feeding rate of the co-firing fuel in the test. As such, a comparison of RD A values for the co-firing tests would help to rule out the effects of the total ash content in the feed on ash deposition.…”

Comparison of selected parameters of biomass and coal

“…In this study, the moisture content (wt% ) was between 2.9 and 8.8 wt%, which was significantly lower than in literature. Shao et al (2010Shao et al ( , 2011 conducted the first investigation of the effects of moisture content (<5 wt% and 30-35 wt%) in the feed on ash deposition during combustion of individual fuels (woody biomass/peat/coal) and some fuel blends. In their research, the as-received/air-dried feedstock that contained around 30 to 35% moisture performed better than the ovendried feedstock in retarding ash deposition and chlorine deposition.…”

Ash Deposition in Biomass Combustion or Co-Firing for Power/Heat Generation

“…Shao et al [30,31] conducted the first investigation of the effects of moisture content (<5 wt% and 30-35 wt%) in the feed on ash deposition during combustions of individual fuels (woody biomass/peat/coal) and some fuel blends. In their observations, the as-received/air-dried feedstock that contains around 30% to 35% moisture performed better than the oven-dried feedstock in retarding ash deposition and chlorine deposition.…”

“…The surface temperatures of deposition probe can thus be monitored or controlled to some extent via adjusting the flow rate of cooling air depending on flue gas temperature and the probe properties (i.e., size, metal conductivity). In most of deposit-related studies, the surface temperature of ash deposition probes was controlled at the steam-tube metal temperature in boilers, typically at 430-600 °C [10,12,30,31,[42][43][44][45]. On the other hand, ash deposit sampling was performed at the superheater zone in a small boiler furnace or multiple locations in a large scale unit using the air-cooled deposition probes.…”