Anita Pettersson

The Impact of Zeolites During Co-Combustion of Municipal Sewage Sludge with Alkali and Chlorine Rich Fuels

Municipal sewage sludge has proven to eliminate alkali metals and chlorine related problems during combustion of straw and refuse derived fuels (RDF). However, the mechanisms involved have not been clarified. The aim of this work was to gain more knowledge about the behaviour of sewage sludge and detergent zeolites in combustion and about their effects on alkali metal chemistry. Co-combustion tests with combinations of municipal sewage sludge, wood and straw were carried out in a 12 MW fluidised bed (FB) boiler. In addition, a detergent zeolite, Doucil A24, was used as additive during co-combustion of wood and straw. The chemical characteristics of fuels and fly ashes were studied using several methods, such as chemical fractionation and scanning electron microscopy with element analysis by energy dispersive fluorescence detection (SEM-EDX) and X-ray diffraction (XRD). In the co-combustion tests involving sewage sludge no KCl was found in the flue gas prior to the convection pass. The zeolite addition was less effective but the KCl concentration was reduced to some degree in favour of HCl compared to the reference case. Both SEM-EDX and XRD confirmed the presence of potassium-aluminium-silicates in the fly ash fraction in all cases. In addition, the laboratory study showed that Doucil A24 had the ability to capture potassium and KCl at temperatures in the range of 700-900°C.

Investigating the morphology and reactivity of chars from Triplochiton scleroxylon pyrolysed under varied conditions

A gasifier may be optimised via a good understanding of the char formation, morphology and reactivity. The effects of varying the pyrolysis pressure and heating rate on the morphology of the char were investigated using a thermogravimetric analyser (TGA), scanning electron microscope (SEM) and micrograph spot analyser. The gasified chars were produced at heating rates of 5, 10 and 20°C/min and pressures of 0.1, 0.4 and 0.6 MPa. All the chars have different degrees of apparent gasification reactivity. The random pore model (RPM) provided a better description of the experiment, with low average error values, θ, in all of the cases considered. The alkaline and alkaline earth metals (AAEM) in the tropical wood biomass Arere (Triplochiton scleroxylon) consist predominantly of calcium and could altogether be partly responsible for the noticeably high reactivity nature of the tropical Arere chars.

Leaching of phosphorous from ashes of co-combustion of sewage sludge and wood

Municipal sewage sludge has a high concentration of phosphorus, which should be recovered because phosphorus is a limited natural resource. In this work, sewage sludge was co-fired with wood in a FBC boiler. The aim of the investigating was to study the solubility of phosphorus in the ashes, by leaching as an alternative to the phosphorous recovery method of using the ashes directly on farmlands. The fly ashes from the boilers secondary cyclone and bag filter were leached at various pH-values and the release of phosphorus was measured. Only acidic leaching was applied. The ashes precipitated with Al2(SO4)3 released nearly all phosphorus at a pH-value of 1, whereas the ashes precipitated with Fe2(SO4)3 did not release all phosphorus even at the very low pH of 0.5. The concentrations of phosphorous in the leachate must be compared with liquid phosphorus sources such as human urine or liquid animal manure used as fertilisers. This may result in that the leachate has to be processed further. A continuation of the work to investigate to what extent the leachate is contami-nated with toxic trace elements is necessary.