ECN publication
Biobrandstof van verontreinigde grond: kwaliteit vliegas bij vergassing en meestoken in poedercentrales
Published by: Publication date:
ECN 1997
ECN report number: Document type:
ECN-C--97-099 ECN publication
Number of pages: Full text:
78 Download PDF  

The cultivation of energy crops, e.g. fast growing willow on cheapcontaminated sludge soil, could reduce the cost of energy crops as biomass fuel. The uptake of heavy metals from the soil could, however, endanger potential of the crops as a fuel in co-firing, because of the expected impact on the leaching behaviour of the fly ash. Increased leaching of heavy metals from the fly ash could limit its use in the cement and concrete industry. To avoid the inorganics (including heavy metals) of biomass from entering the boiler, an option would be to gasify it first and to co-fire the fuel gas in the boiler after a gas cleaning step. This, however, creates a separate biomass ash stream with high landfilling cost, when it is not used as building material, neutraliser or fertiliser. The literature on the quality and leaching behaviour of fly ash from gasification and co-firing biomass is very limited. For that reason the leaching behaviour was measured for a mixture of fly ash and willow ash and for willow ash alone. The willow was from clean soil and the leaching data are extrapolated to higher concentrations of heavy metals, based on the extensive knowledge and experience at ECN on leaching behaviour of solid waste streams. Willow from contaminated soil likely contains higher concentrations of cadmium and zinc compared to willow from clean soil. The co-firing ratio is probably limited by the high expected concentration of calcium in the fly ash, which limits its use in concrete and cement. The leaching behaviour and level of heavy metals from fly ash is not greatly affected when co-firing clean willow. Co-firing willow with higher concentrations of cadmium and zinc will give increased leaching only when the fly ash is used under extreme pH conditions or when it contains a significant amount of chlorine. Both are not very likely. Furthermore, not the leaching behaviour of the pure ash, but of the end product is important. The largest ash stream from gasification is separated in the cyclone after the gasifier and contains high concentrations of calcium, potassium and phosphor. It could therefore be used as fertiliser or neutraliser. There is almost no regulation for heavy metals in fertilisers, except that it can be used when concentrations do not exceed the limits for the soil. The cyclone ash from gasification of clean willow already shows high concentrations of heavy metals, exceeding the limits for the soil. This could make it difficult to obtain permission to use it as fertiliser or neutraliser, especially when the cadmium and zinc concentrations are even higher as expected in the case of willow from contaminated soil. The leaching of heavy metals from the fertiliser likely determines its impact on the environment. The leaching of molybdenum from ash from clean willow at pH>8 exceeds the limits stated for building materials (very strict limits). At very low pH also elements like zinc, cadmium and copper exceed the limits. With increased concentrations of cadmium and zinc, extreme pH conditions and high amounts of chlorine in the ash could give very high leaching levels. From this respect, it seems that the ash is not suitable for use as fertilizer on soil with pH 6-8. Because the ash is very fine it could also be very suitable for use as a filler in concrete, when the high carbon content is decreased by better conversion in the gasifier. 62 refs.

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