coal selection criteria for industrial pfbc firing project 3.2 - CCSD
coal selection criteria for industrial pfbc firing project 3.2 - CCSD coal selection criteria for industrial pfbc firing project 3.2 - CCSD
“Coal Selection Criteria for Industrial PFBC Firing” Deposit formations on heat exchanger surfaces had been reported at Vartan, Tidd and Escatron (Steenari, Lindqvist et al. 1998). Furthermore, deposit formation and fouling were also found on cyclone surfaces and other parts of the flue gas ducts. The key element in fouling is iron. FeS2, which is the most common iron mineral pyrite, decomposed and oxidized during combustion (Steenari, Lindqvist et al. 1998). In reducing conditions, mixtures of FeS and FeO are formed. FeO and other iron-rich oxides react with kaolinite and quartz to form molten products at temperatures between 900 – 1000 o C (Steenari, Lindqvist et al. 1998). To reduce ash deposition and fouling, it is advised to use coals with low iron content. 3.4 Cyclone Plugging Cyclones play a significant role in ensuring the survival of the gas turbine, especially when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be sufficiently clean to minimize the turbine blade erosion. Osaki had encountered cyclone plugging, causing them to suspend their operation for inspection and it was found that the plugging was due to the properties of the coal which produced sticky ash material. The cyclone plugging in Tidd was due to the high coal and sorbent elutriation rates, maldistribution of ash loading to individual cyclones and undersized ash removal system (M.Marrocco and al. 1991). In addition, Tidd had experienced cyclone fires. The majority of the fires occurred in the lower part of the cyclones. They happened because of carbon carryover to the cyclones, which was due to operation at low loads where combustion efficiency and low bed particle residence time had significant impacts (M.Marrocco and al. 1991). The same problem was reported at the Wakamatsu plant in Japan and was solved by improving the coal particle size distribution (Sakanishi 1995). The Ca:S molar ratio was increased way above the requirements for SO2 control to reduce the fly ash stickiness and to maintain bed inventory. Page 12
“Coal Selection Criteria for Industrial PFBC Firing” Furthermore, the major reason causing several operational shutdowns of the Escatrón plant in Spain was cyclone ash extraction system blockage (Scott and Carpenter 1996). Sintered material (agglomerates) deposited on the cyclone walls and in the ash extraction system. Increasing the coal feed rate to increase the production of steam increased the bed height and the flow of particulates to the cyclone. This led to more agglomeration which blocked the cyclone. Moreover, the complex design of cyclones with many ducts and flow direction changes further intensified the plugging. Modifications to the cyclone ash removal system have reduced the problem (Martinez and Menendez 1995), (Martinez and Menendez 1994). 3.5 Filter Blockage This problem is only faced by PFBC plants which depend on the ceramic filter for secondary hot gas clean-up prior to the gas turbine inlet, an example of such plants is Tomatoh-Atsuma in Japan. This problem involved filter blockage, filter breakage, gas leakage and fires, attributed to temperature effects, hydrodynamic effects, mechanical effects, filter material effects, sorbent properties/reactions, ash composition effects and volatilisation / condensation of alkalis (Stubington 1997). Most of them have been solved but the problem is being investigated further to improve the understanding of ash chemistry. Finer ash particles penetrate into the filter, causing filter blockage. This ash was described as sticky due to its tendency to stick on the filter surface and it could not be removed by cleaning. It led to unstable pressure drop across the filter cake (Stubington, Wang et al. 1998). Excessive deposits could lead to filter breakage. Larger ash particles in the exhaust gas flow to the filter reduced the blockage, thus easing the cleaning of filter cake. This solution was demonstrated at Wakamatsu. Elutriated material from the attrition of limestone bed particles contained calcium compounds that could form low melting point eutectics which decreased the ash fusion temperature of material accumulated in the filter cake. A higher Ca:S ratio was necessary to maintain the bed height for low-sulfur Australian coals. This neutralized the high ash Page 13
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Deposit <strong>for</strong>mations on heat exchanger surfaces had been reported at Vartan, Tidd and<br />
Escatron (Steenari, Lindqvist et al. 1998). Furthermore, deposit <strong>for</strong>mation and fouling<br />
were also found on cyclone surfaces and other parts of the flue gas ducts.<br />
The key element in fouling is iron. FeS2, which is the most common iron mineral pyrite,<br />
decomposed and oxidized during combustion (Steenari, Lindqvist et al. 1998). In<br />
reducing conditions, mixtures of FeS and FeO are <strong>for</strong>med. FeO and other iron-rich oxides<br />
react with kaolinite and quartz to <strong>for</strong>m molten products at temperatures between 900 –<br />
1000 o C (Steenari, Lindqvist et al. 1998). To reduce ash deposition and fouling, it is<br />
advised to use <strong>coal</strong>s with low iron content.<br />
3.4 Cyclone Plugging<br />
Cyclones play a significant role in ensuring the survival of the gas turbine, especially<br />
when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be<br />
sufficiently clean to minimize the turbine blade erosion. Osaki had encountered cyclone<br />
plugging, causing them to suspend their operation <strong>for</strong> inspection and it was found that the<br />
plugging was due to the properties of the <strong>coal</strong> which produced sticky ash material.<br />
The cyclone plugging in Tidd was due to the high <strong>coal</strong> and sorbent elutriation rates,<br />
maldistribution of ash loading to individual cyclones and undersized ash removal system<br />
(M.Marrocco and al. 1991). In addition, Tidd had experienced cyclone fires. The majority<br />
of the fires occurred in the lower part of the cyclones. They happened because of carbon<br />
carryover to the cyclones, which was due to operation at low loads where combustion<br />
efficiency and low bed particle residence time had significant impacts (M.Marrocco and<br />
al. 1991).<br />
The same problem was reported at the Wakamatsu plant in Japan and was solved by<br />
improving the <strong>coal</strong> particle size distribution (Sakanishi 1995). The Ca:S molar ratio was<br />
increased way above the requirements <strong>for</strong> SO2 control to reduce the fly ash stickiness and<br />
to maintain bed inventory.<br />
Page 12
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