Alkali chlorides 582 fossil

In the last few years, there has been a move away from burning fossil fuels through the co-utilisation of biomass and coal and finally to 100% biomass such as wood and waste wood products. Unfortunately, burning of biomass causes widespread fouling of superheater tubes and corrosion can occur rapidly under the sticky alkali chloride deposits. Even at today s maximum steam temperatures of 500 to 540°C there are some severe corrosion problems when burning 100% wood-based fuel [1]. It is also desirable to be able to burn other environmental fuels such as straw, demolition wood or other waste wood products, to reduce production costs and avoid dumping waste at landfill sites. This, however, makes the corrosion and fouling problems even more serious [2, 3]. 

Although the chemical analysis of biomass does not reveal directly any alarming concentrations of corrosive elements, there is one fundamental difference between fossil and biomass fuels that can be derived from it. This is the excess of alkali metals compared to the amount of sulfur plus chloride. Even if one assumes that all the sulfur in the fuel is reacted to alkali sulfates and all the chloride is reacted to alkali chlorides, there is still excess alkali left in biomass whereas with the fossil fuels this is not the case. The free alkali index has been used to predict the existence of alkali hydroxides in a biomass boiler [3] ... 

Human activities have resulted in the release of a wide variety of both inorganic and organic forms of mercury. The electrical industry, chloro-alkali industry, and the burning of fossil fuels (coal, petroleum, etc.) release elemental mercury into the atmosphere. Metallic mercury has also been released directly to fresh water by chloro-alkali plants, and both phenylmer-cuiy and methylmercury compounds have been released into fresh and sea water -phenylmercury by the wood paper-pulp industry, particularly in Sweden, and methyl-mercury by chemical manufacturers in Japan. Important mercury compounds which also may be released into the environment include mercury(II) oxide, mercury(II) sulfide (cinnabar), mercury chlorides, mer-cury(II) bromide, mercury(II) iodine, mer-cury(II) cyanide, mercury(II) thiocyanate, mercury(II) acetate, mercury nitrates, mercury sulfates, mercury(II) amidochloride monoalkyl- and monoarylmercury(II) halides, borates and nitrates dialkylmercury compounds like dimethylmercury, alkoxyal-kylmercury compounds or diphenylmercury (Simon and Wiihl-Couturier 2002) (for quantities involved, see Section 17.4). 

Most biomass fuels have high contents of alkali metals and chlorine, but they contain very little sulphur compared to fossil fuels. The alkali metal of major concern in wood is potassium. The majority of potassium is released into the gas phase during combustion and is present mainly as potassium chloride, KCl, and potassium hydroxide, KOH. The alkali metals form compounds with low melting temperatures and can condense as chlorides causing widespread fouling of superheater tubes and other operational problems during combustion [3]. 

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