Reduction of Ash-related Problems in Large-scale Biomass Combustion Systems via Resource Efficient Low-cost Fuel Additives

The incineration of waste wood is very often associated with ash-related problems (deposits, slagging and corrosion). This leads to short maintenance intervals, which result in significant power generation losses and high downtime costs. To avoid these problems, additives can be used, with particularly cost-effective additives being of great interest. Based on pre-evaluations, the addition of 2% gypsum and 3% coal fly ash was recommended, since an improved ash melting behaviour and reduced risk for high-temperature corrosion can be expected with addition of gypsum and coal fly ash. These additives with the recommended mixing rates were then investigated in a large-scale plant. Extensive investigations were carried out without additive (as a reference), and with the additives focusing on dust formation (aerosols and total dust), deposit formation and the corrosion behaviour of superheaters. These investigations were accompanied by fuel and ash analyses (grate, cyclone and filter). The addition of additives increased the amount of total dust in the flue gas up to 195% and 262% for gypsum and coal fly ash respectively. The chemical analysis of the total dust showed an enrichment of refectory species like Al for coal fly ash and Ca and Mg for gypsum which can positively influence the slagging behaviour. Aerosol measurements showed that the addition of coal fly ash minimised the amount of fine particulate matter, as less alkali metals (K and Na) were released into the gas phase. Gypsum addition increases the SO2 concentrations in the gas phase due to the decomposition of gypsum, as in the combustion chamber about 900°C are present. Due to the preferred sulphation reactions (binding of S to alkali metals) less Cl is bound to alkali metals and therefore the Cl concentrations in the aerosols were lower compared to the reference case. This effect was also found in the deposits sampled at the position of the superheater. Based on the chemical composition of deposits the molar 2S/Cl ratios were determined, which can be used to predict the risk for high temperature corrosion. The analysis data showed that an improvement concerning the high temperature corrosion risk is possible by adding coal fly ash, whereas a significant improvement in case of gypsum additions seems very likely. The measurements carried out so far showed the influence (built-up rate, chemical composition etc.) of the additive application on ash fractions, deposits and dusts. By taking a closer look at the change in chemical compositions of dusts and deposits, additives with an appropriate additivation ratio can be suggested. In case of coal fly ash 3% and in case of gypsum 1% additive related to dry fuel seems to be adequate additive ratios to positively influence the risk of high temperature corrosion and reduce the slagging behaviour