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a b s t r a c tChemical fractionation, SEM-EDX and XRD was used for characterisation of fly ashes from different cocombustion tests in a 12 MW circulating fluidized bed boiler. The fuels combusted were wood pellets as base fuel and straw pellets as co-fuel in order to reach a fuel blend with high alkali and chlorine concentrations. This fuel blend causes severe problems with both agglomeration of bed material if silica sand is used and with deposits in the convection section of the boiler. Counter measures to handle this situation and avoiding expensive shut downs, tests with alternative bed materials and additives were performed. Three different bed materials were used; silica sand, Olivine sand and blast furnace slag (BFS) and different additives were introduced to the furnace of the boiler; Kaolin, Zeolites and Sulphur with silica sand as bed material. The results of the study are that BFS gives the lowest alkali load in the convection pass compared with Silica and Olivine sand. In addition less alkali and chlorine was found in the fly ashes in the BFS case. The Olivine sand however gave a higher alkali load in the convection section and the chemical fractionation showed that the main part of the alkali in the fly ashes was soluble, thus found as KCl which was confirmed by the SEM-EDX and XRD.The comparison of the different additives gave that addition of Kaolin and Zeolites containing aluminium-silicates captured 80% of the alkali in the fly ash as insoluble alkali-aluminium-silikates and reduced the KCl load on the convection section. Addition of sulphur reduced the KCl load in the flue gas even more but the K 2 SO 4 concentration was increased and KCl was found in the fly ashes anyhow. The chemical fractionation showed that 65% of the alkali in the fly ashes of the Sulphur case was soluble.

Combustion of sewage sludge with subsequent recovery of phosphorous is a relatively new sludge treatment option. In the work described in this paper, recovery of phosphorus by acid leaching of fly-ashes from co-combustion of sewage sludge with wood in a circulating fluidized bed combustor was investigated. The results showed that it is possible to find a pH range (0.5-1) in which it is possible to extract a significant fraction of the phosphorous from these ashes without precipitation of secondary phosphates. The type of flocculation agent used in the waste water treatment plant where the sludge is formed has a significant effect on the phosphorous recovery. Ashes from combustion of sewage sludge that was formed using aluminium sulphate as flocculating agent released nearly all phosphorus at a pH-value of 1. When iron sulphate was used as flocculating agent, this affected the chemistry of the resulting ashes, making phosphorous recovery more difficult. The yield of phosphorous from those ashes was 50-80%. In part II of the project [1] focus is on the trace element release to the leachate. This determines if the leachate can be used directly as a fertiliser or if further removal of trace elements is necessary.Sludge, biofuel, phosphorus, leaching, co-combustion and fluidized bed 2

Co-combustion of animal waste, in waste-to-energy boilers, is considered a method to produce both heat and power and to dispose of possibly infected animal wastes. This research conducted full-scale combustion tests to identify the impact of changed fuel composition on a fluidized-bed boiler. The impact was characterized by analyzing the deposit formation rate, deposit composition, ash composition, and emissions. Two combustion tests, denoted the reference case and animal waste case, were performed based on different fuel mixes. In the reference case, a normal solid waste fuel mix was combusted in the boiler, containing sorted industry and household waste. In the animal waste case, 20 wt% animal waste was added to the reference fuel mix. The collected samples, comprising sampling probe deposits, fuel mixes, bed ash, return sand, boiler ash, cyclone ash and filter ash, were analyzed using chemical fractionation, SEM-EDX and XRD. The results indicate decreased deposit formation due to animal waste co-combustion. SEM-EDX and chemical fractionation identified higher concentrations of P, Ca, S, and Cl in the bed materials in the animal waste case. Moreover, the risk of bed agglomeration was lower in the animal waste case and also a decreased rate of NO x and SO 2 emissions were observed.

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Sewage sludge and its ashes after combustion are contaminated with metals in various concentrations. In the work described in this paper, the mobility of metals during recovery of phosphorus by acid leaching of fly-ashes from co-combustion of sewage sludge with wood was investigated. The metal concentrations in two sewage sludges, fly ashes and leachates from acid phosphorus extraction were compared with phosphorus rock and different fertilisers used in the agriculture.The secondary cyclone ashes were found to be less contaminated with trace elements than the bag filter ashes. The largest problem is cadmium, which in all ashes studied has a too high level to meet the legislative limits for the maximum dosage in Sweden and the proposed limits in the European Union. The legislation includes limits for cadmium (Cd), mercury (Hg), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) calculated for an amendment of X kg of fertiliser per year and ha of land.The solubility of the trace metals in the cyclone ash, with Cd as an exception, is in general much lower than the solubility of phosphorus. A decrease in pH results in an increased release of Cd but has just a slight influence on the other metals analysed in this study. The Cd yield increases by 30% when pH is lowered from 2.5 to 1.0 whereas the Hg release is not affected at all. The trace element concentrations in the leachates are far below the European and Swedish limitations of metal concentrations in fertilisers. The leachates thus fit as fertilisers and also as raw material to the industry, if the Cd is removed. In part I of the project [1] focus was on the phosphorus recovery only.

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Steam-fluidized bed gasification of biomass, which produces combustible gases from which transportation fuels can be synthesized, is a promising option for replacing the use of fossil fuels in the transportation sector. Similar to other thermal conversion processes, the release of potassium (K), chlorine (Cl), and sulfur (S) from biomass fuels to the gas phase during this process may be conducive to ash-related problems. Catalytic tar and char conversion by K has also been observed. In addition to operational conditions, the extent to which these elements are released to the gas phase may be affected by fuel ash characteristics such as the ash composition and the speciation (or association) of ash-forming elements in the fuel matrix. In the present work, the influence of these fuel ash characteristics on the extent to which K, Cl, and S are released from biomass fuels to the gas phase was studied under steam-fluidized bed gasification. The aim was to assess whether these fuel ash characteristics provide information that could be useful in making a quick judgment as to what extent K, Cl, and S would be released to the gas phase. To this end, the release of K, Cl, and S from forest residues and wheat straw during devolatilization and steam gasification of the char was quantified in a laboratory-scale bubbling fluidized bed reactor. The speciation of these elements in the virgin fuels was studied with chemical fractionation. The results reveal that the extent to which S is released from biomass fuels to the gas phase mainly depends on its speciation in the fuel matrix. While both the ash composition (mainly the Cl/K molar ratio) of the fuel and the speciation of K in the fuel matrix are relevant for the release of K, they appear to be unimportant with respect to the release of Cl.