Flue poisoning

Use of some biomass feedstocks can increase potential environmental risks. Municipal solid waste can contain toxic materials that can produce dioxins and other poisons in the flue gas, and these should not be burned without special emission controls. Demolition wood can contain lead from paint, other heavy metals, creosote, and halides used in presen a-tive treatments. Sewage sludge has a high amount of sulfur, and sulfur dioxide emission can increase if sewage sludge is used as a feedstock. 

To prevent damage to downstream equipments and poisoning of downstream catalysts, the following contaminants have to been removed from the flue gas particulate (ash, char, and fluid bed material) causing erosion, alkali metals (sodium and potassium compounds) responsible for hot corrosion, tars (high molecular weight hydrocarbons and refractory aromatics), and catalyst poisoning species (H2S, HC1, NH3, and HCN). 

In the case of several applications in the chemical industry, the presence of contaminants such as 02, SO, or NOr which may be present in the flue gases of power plants, might be deleterious by causing negative (poisoning) effects on the catalysts. 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

When the catalyst is exposed to the flue gas in a biofuel-fu ed boiler, it starts to deactivate immediately. The conversion (figure 3) declines exponentially from 80% down to about 40 %, where it seems to levels out, after about 17 combustion cycles. The reason for the deactivation is a loss of specific surface area. Washing in a water solution of citric acid can restore this area and the activity (ftgure 3 and 4). The catalyst cannot be restored by thermal treatment, at least not by heating to 700-800°C. This means that the poison is non-organic and easily soluble in slightly acidic solution compound, probably a salt. 

Poisoning of De NO SCR Calalyst by Flue Gases from a Waste Incineration Plant... 

POISONING OF DE-NOx SCR CATALYST BY FLUE GASES FROM A WASTE INCINERATION PLANT... 

Up to now a lot of attention has been paid to improving the performance of SCR catalysts with respect to making them more resistant to the poisonous compounds present in the flue gases, mainly SOx and As, which are results of burning with coal, oil and gas (ref. 5 and 6). At the same time relatively little information has been presented on the poisoning effect of flue gases from municipal waste incinerators, on the SCR-type de-NOx catalysts. 

Sulfur-, phosphorus- and arsenic- contents are undesirable, since they form poisonous flue gases. High Al203-contents lead to the formation of sticky slags, which can contaminate the end product. 

Many of these techniques are fairly sophisticated and are not trivial to operate and maintain in industrial furnace environments. For example, the catalytic reduction techniques require a catalyst which may become plugged or poisoned fairly quickly by dirty flue gases. Posttreatment methods are often capital intensive. They usually require halting production if there is a malfunction of the treatment equipment. Also, posttreatment does not normally benefit the combustion process in any way. For example, it does not increase production or energy efficiency. It is strictly an addon cost. 

The major concern in applying selective catalytic reduction is deactivation or poisoning of the catalyst. One cause of deactivation may be catalyst poisons present in the flue gases. 

The main cause of deactivation are elements or compounds which chemically attack the catalytically active material or its support. Also, structural changes and pore blocking are important issues of deactivation. A variety of poison compounds containing elements such as halogens, alkah metals, alkaline earth metals, arsenic, lead, phosphorus, and sulfur are mentioned in the hterature. AS2O3 is the most severe poison in coal-fired power plant operation in Germany. In power plants equipped with wet-bottom boilers alkah metal oxides mostly remain in the molten ash, whereas AS2O3 tends to escape into the flue gas and deposits on the catalyst. 

Two types of deactivation studies may be distinguished those in which catalysts are doped with the catalyst poison and those in which the catalyst is deactivated by compounds present in flue gases. 

From the preliminary results of the bench scale pilot plant of the U.S. Environmental Protection Agency, Tseng et al. reported that a commercial SCR catalyst, extruded V20sA i02 based with some W, gave reduced NO conversion when 95 ppm SO2 was introduced into their flue gas stream from natural gas combustion. They also stated that this catalyst was less active at 440 °C than at 353 °C when 95 ppm SO2 was present in the flue gas. No further comments regarding this SO2 poisoning were offered. 

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