Britt-Marie Steenari

High-temperature reactions of straw ash and the anti-sintering additives kaolin and dolomite

Abstract Straw of various types of rape, wheat and barley have been studied with respect to the formation of crystalline compounds and high-temperature reactions in ash, as well as sintering and melting behaviour. During the low-temperature ashing process simple, crystalline compounds such as carbonates, sulphates and chlorides were formed. A significant part of the ash from wheat and barley straw was amorphous whereas rape ash was found to be mainly crystalline. The large content of potassium compounds present in wheat and barley straw ash contributes to their low melting points. The ash components primarily formed are reactive. Solid state reactions at temperatures above 800°C lead to the formation of secondary products such as oxides and silicates. Minerals such as kaolin and dolomite have been suggested as fuel additives to give the ash a higher melting point. High-temperature reactions between straw ash and kaolin, Al 2 Si 2 O 5 (OH) 4 , or dolomite, CaMg(CO 3 ) 2 , respectively, were therefore investigated. Kaolin was found to be the more effective additive. The reaction between kaolin and potassium salts in straw ash gave KAlSiO 4 and KAlSi 2 O 6 . A laboratory study of reactions involving K 2 SO 4 or KCl and kaolin showed that several products are possible, one of which is KAlSiO 4 . The potassium capture by kaolin partly explains the higher melting point of the ash-additive mixture. Dolomite added to wheat and barley ash reacted with silica to form silicates. No reaction between dolomite and potassium compounds could be detected. The observed enhancement of the melting point caused by dolomite is probably an effect of dilution or adsorption.

Characterization of Ashes from Wood and Straw

Abstract Two bark/wood chips fuels and one pelletized straw fuel were studied with regard to elemental composition, ash composition and sintering/melting behaviour with an emphasis on the chemical reactions involved in sintering. Ashes were prepared by two different methods: low-temperature laboratory ashing and combustion in a laboratory fixed-bed furnace. X-ray diffraction, thermogravimetric analysis and scanning electron microscopy indicated the type of crystalline compound which can be found in ashes from biomass fuels with a high calcium, silicon and potassium content. The potassium content of the straw ash was 3 times higher than that in the bark/wood fuels. This potassium was present in crystalline compounds with low melting points and in the molten phase of sintered straw ash.

Evaluation of the leaching characteristics of wood ash and the influence of ash agglomeration

Abstract The release of mineral nutrients and other species from untreated and stabilised wood ash has been investigated. Stabilisation is applied with the aim to modify the solubility of ash components and the ash particle size, i.e. to form dense ash particle agglomerates. This process induces the formation of several secondary minerals. The most important reaction is the transformation of Ca(OH) 2 into CaCO 3 which lowers the calcium leaching rate significantly. A significant fraction of the alkali metals, K and Na, is present in salts which are rapidly released. The short-term release of these salts, as simulated in laboratory experiments, was not reduced by the stabilisation methods applied. Generally, low leaching rates were observed for the important plant nutrients P and Mg as well as for Fe and other metals from both untreated and agglomerated ashes. Thermodynamic equilibrium modelling of the hardening process showed that in addition to the transformation of Ca(OH) 2 to CaCO 3 , formation of the mineral ettringite is possible at a high pH. Experimental results have confirmed this. As the pH in the pore solution decreases during long-term leaching ettringite will be transformed into calcium carbonate and gypsum. In accordance with the experimental results, no formation of secondary solubility controlling potassium or sodium minerals was indicated by the modelling results.

Removal of hazardous metals from MSW fly ash-An evaluation of ash leaching methods

Incineration is a commonly applied management method for municipal solid waste (MSW). However, significant amounts of potentially hazardous metal species are present in the resulting ash, and these may be leached into the environment. A common idea for cleaning the ash is to use enhanced leaching with strong mineral acids. However, due to the alkalinity of the ash, large amounts of acid are needed and this is a drawback. Therefore, this work was undertaken in order to investigate some alternative leaching media (EDTA, ammonium nitrate, ammonium chloride and a number of organic acids) and to compare them with the usual mineral acids and water. All leaching methods gave a significant increase in ash specific surface area due to removal of soluble bulk (matrix) compounds, such as CaCO(3) and alkali metal chlorides. The use of mineral acids and EDTA mobilised many elements, especially Cu, Zn and Pb, whereas the organic acids generally were not very effective as leaching agents for metals. Leaching using NH(4)NO(3) was especially effective for the release of Cu. The results show that washing of MSW filter ash with alternative leaching agents is a possible way to remove hazardous metals from MSW fly ash.

Release of salts from municipal solid waste combustion residues

Residues from fluidized bed combustion of municipal solid waste were investigated with respect to their leaching behavior and possible extraction of salts. The total water extractable amounts of Na, K, Ca, Cl(-), Br(-), F(-) and SO(4)(2-) along with the total dissolved solids of bottom, hopper, cyclone and bag house filter ashes were determined. A simple multistage washing process (using water as the extraction medium) was tested in lab scale experiments. The effect of variations in parameters, such as water to ash weight ratio, contact time, temperature and number of extraction steps was investigated. The leaching behavior of untreated and washed cyclone and bag house filter ashes was evaluated by a two-step batch-leaching test, i.e. the CEN test. The ashes investigated in this study can be arranged according to their decreasing water extractable contents and total dissolved solids as follows: filter ash > cyclone ash > hopper ash > bottom ash. A triple extraction with water at liquid to solid ratio 2 and extraction time 5 min gave the best results for the extraction of Ca, Na, K, Cl(-) and SO(4)(2-) from the cyclone as well as from the filter ashes. The leached amounts of salts in the CEN test performed on the washed cyclone ash were considerably lower than the corresponding amounts released from the unwashed ash. Thus, the washed cyclone ash was made more stable with respect to salt leachability. On the other hand, large amounts of salts were leached from the washed filter ashes as well as from unwashed filter ashes. Therefore, it can be concluded that three stage water extraction is not a suitable stabilization method for this type of filter ashes.

Fly ash characteristics in co-combustion of wood with coal, oil or peat

Abstract Co-firing of bio fuels with fossil fuels is often applied for environmental, technical and economic reasons. The properties of the solid residues produced from such fuel combinations cannot be predicted based on the properties of the ash from each fuel. In this work, fly ash materials from combustion of wood/oil, wood/coal and wood/peat combinations were characterized. Elemental compositions, mineralogical speciation of major elements as well as leaching properties are reported. The fly ash materials from co-combustion had in many aspects similar properties to those from combustion of pure wood. The differences noted were lower calcium, potassium and chlorine contents and higher contents of aluminium, iron and sulphur when coal or peat was added. The speciation of alkali metals in the ash materials studied here is somewhat different from that of wood ash, which leads to lower salt levels in leachates. The pH of these leachates was not as high as in leachates from pure wood ash because of a lower content of calcium oxide in the ash. Trace metal levels and leaching rates were in the same range as for wood ash materials studied earlier.

A study of Cr(VI) in ashes from fluidized bed combustion of municipal solid waste: leaching, secondary reactions and the applicability of some speciation methods

The use of the fluidized bed technique for the combustion of municipal solid waste is a rather new concept. This type of combustor produces ash residues with somewhat different properties than the residues generated from the traditional mass burn techniques. Therefore, chemical characterization and the investigation of toxic metals behavior during ash water reactions are necessary for the safe disposal of these residues. In the present work, the total elemental composition, mineralogy and leaching behavior of ashes from the combustion of municipal solid waste in a fluidized bed combustion boiler have been investigated. The cyclone ash and, in particular, the filter ash contained considerable amounts of soluble substances, thus giving leachates with high levels of Cl-, Na+, K+, Ca2 + and Al(IIl). On the other hand, the two ash fractions taken in the boiler, the bottom and hopper ashes, were much more stable with respect to the release of salts and heavy metals. Since Cr(VI) is mobile and toxic its release from combustion residues can pose environmental problem. Even though the total Cr contents were similar in all ashes studied, the bottom ash gave about a thousand times higher levels of Cr(VI) in test leachates than the hopper, cyclone and filter ashes. However, it was found that the leached amount of Cr(VI) from the bottom ash decreased significantly when bottom ash was mixed with the hopper ash. The most probable cause for this decrease is the coupled oxidation of Al(0) to Al(III) and reduction of dissolved Cr(VI) to Cr(III). This finding that the mixing of two ash streams from the same boiler could result in the immobilization of Cr may point at a simple stabilization method. Selective extraction of water soluble, exchangeable and sparingly soluble forms of Cr(VI) was also investigated. Extraction methods were evaluated for their suitability for ash matrixes. It was found that interferences due to the presence of reducing substances in some ash materials may occur.

Leaching optimization of municipal solid waste incineration ash for resource recovery: A case study of Cu, Zn, Pb and Cd

Ash from municipal solid waste incineration (MSWI) may be quite cumbersome to handle. Some ash fractions contain organic pollutants, such as dioxins, as well as toxic metals. Additionally, some of the metals have a high value and are considered as critical to the industry. Recovery of copper, zinc and lead from MSWI ashes, for example, will not only provide valuable metals that would otherwise be landfilled but also give an ash residue with lower concentrations of toxic metals. In this work, fly ash and bottom ash from an MSWI facility was used for the study and optimization of metal leaching using different solutions (nitric acid, hydrochloric acid and sulfuric acid) and parameters (temperature, controlled pH value, leaching time, and liquid/solid ratio). It was found that hydrochloric acid is relatively efficient in solubilizing copper (68.2±6.3%) and zinc (80.8±5.3%) from the fly ash in less than 24h at 20°C. Efficient leaching of cadmium and lead (over 92% and 90% respectively) was also achieved. Bottom ash from the same combustion unit was also characterized and leached using acid. The metal yields were moderate and the leachates had a tendency to form a gelatinous precipitate, which indicates that the solutions were actually over-saturated with respect to some components. This gel formation will cause problems for further metal purification processes, e.g. solvent extraction.

Ash sintering and deposit formation in PFBC

Agglomerated bed material and deposits from various surfaces in the bed area and in the flue gas duct of pressurized fluidized bed combustion (PFBC) plants were examined by chemical and X-ray diffraction methods. Crystalline compounds present in the deposits were identified and possible mechanisms leading to the formation of bed material agglomerates, deposits and fouling on surfaces in the flue gas duct are discussed. The results show that all of the chemical systems CaS-CaSO4-alkali species, silicate-alkali-chlorine and silicate systems containing iron-rich oxides are important in the sintering and fouling process.

ALTERNATIVE MATERIALS FOR ADSORPTION OF HEAVY METALS AND PETROLEUM HYDROCARBONS FROM CONTAMINATED LEACHATES

ABSTRACT In the present work, waste products from forest industries (sawdust, pine bark and fibre sludge ash), as well as some biological materials (peat, shrimp shells and seaweed), have been investigated with respect to their capacities to adsorb metals and hydrocarbons from contaminated waters. Batch and column experiments were carried out with artificial metal ion solutions and contaminated leachates from an industrial landfill. The fibre sludge ash and the Sphagnum peat showed the highest sorption capacities for metals among the materials studied in batch experiments with single‐metal solutions. The uptake of metals by the fibre ash for the metals studied was: Cu and Pb 112 μg g−1, Zn 115 μg g−1 and Cr 97 μg g−1. For peat the uptake was: Pb 109 μg g−1, Cu 105 μg g−1, Zn 100 μg g−1 and Cr 99 μg g−1. These materials were also effective in adsorption of diesel oil, and the n‐alkanes C16 and C12. Peat and ash adsorbed respectively 36.6 and 36.4 mg g−1 of C12, 1.84 and 1.94 mg g−1 of C16 and for both 0.98 mg g−1 of diesel oil. Bark adsorbed diesel oil to 0.83 mg g−1. In the column experiments, the removal of metals from a contaminated landfill leachate by ash and peat was lower than from artificial solutions with only a few metals. The results suggest interference from other components in the leachates, such as competition of ions for the same active sites. It is quite clear that laboratory tests can overestimate the performance of adsorbents and that experiments should be specific for the intended application. For most of the metals studied in columns, peat appeared to be the best adsorbent, with respect to both sorption capacity and service time. The addition of 10 % by weight of fibre ash to the peat gave higher adsorption capacities for Cd, Ni and Pb but lower for the Cu and Zn.