Deposit formation in a 150 MWe utility PF-boiler during co-combustion of coal and straw

A conventional pc-fired boiler at the Danish energy company I/S Midtkraft h as been converted to coal-straw co-combustion, and a 2 year demonstration p rogram was initiated in January 1996, addressing several aspects of coal-st raw co-combustion. Deposition trials were performed as part of the demonstr ation program. A maximum straw share of approximately 20% (energy base) was used in the experiments. For the deposit samples collected, a visual analy sis procedure was developed and each sample evaluated according to this. In addition, a number of samples were analyzed by scanning electron microscop y (SEM) combined with energy dispersive X-ray analyses (SEM-EDX) and bulk c hemical analyses. In the visual analysis, a significant increase in the amo unt and tenacity of the upstream deposits was observed as a function of inc reased straw share, exposure time, and boiler load. The chemical analyses o f the deposits show increased amounts of K and S during co-combustion, and the Fe-dominated upstream deposits formed during coal combustion are shifte d toward more Ca- and Si-rich deposits during coal-straw cocombustion. Howe ver, the major part of K is observed to form K-Al silicates, which do not f orm problematic deposits. Go-firing straw also caused a change in the struc ture of the upstream deposits. During coal combustion an ordered, "finger" structure of the larger particles with small particles between was observed , whereas during co-combustion a more random deposition of the larger parti cles among the small ones was observed. No chlorine species was observed in the deposits collected, and selective chlorine corrosion is therefore not expected to constitute a problem in co-combustion of coal and straw up to 2 0% straw share, for the coal types utilized in the tests. However, depositi on problems could arise when burning other coals, particularly coals with a high S or alkali metal content or a low content of ash. The behavior of K, Ca, S, and Cl was evaluated by use of thermodynamic calculations. The ther modynamically stable species agree with the observed behavior in the experi ments, i.e. formation of stable K-Al silicate species as well as KxO4 is pr edicted. The calculations also emphasize that the mixing between the coal a nd straw species is essential for the deposition behavior, primarily by aff ecting the split between K-Al silicates and K2SO4.