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The purpose of this work was to study different ways to mitigate alkali-related problems during combustion of biomass in circulating fluidized beds. Wood chips and wood pellets were fired together with straw pellets, while the tendency to agglomerate and form deposits was monitored. In addition to a reference case, a number of countermeasures were applied in related tests. Those were addition of elemental sulphur, ammonium sulphate and kaolin to a bed of silica sand, as well as use of olivine sand and blast-furnace slag as alternative bed materials. The agglomeration temperature, composition and structure of bed-ash samples were examined. The flue-gas composition, including gaseous alkali chlorides, was measured in the hot flue gases and in the stack. Particles in the flue gas were collected and analysed for size distribution and composition. Deposits were collected on a probe in hot flue gases and their amount and composition were analysed. Addition of kaolin was found to be the best method to counteract the agglomeration problem. The deposition problem is effectively counteracted with addition of ammonium sulphate, while kaolin is too expensive to be used commercially against deposits, and sulphur is less effective than ammonium sulphate.

The emission of potassium- and sodium-containing compounds during rapid birchwood pyrolysis was studied. Birchwood particles (2−130 mg) were inserted into a preheated furnace at constant temperature (350−850 °C) and the alkali emission was measured. Particle mass, furnace temperature, and moisture were varied. At temperatures ≤ 500 °C, the alkali emission from birchwood particles took place solely during the pyrolysis phase. At temperatures ≥ 600 °C, alkali evaporation from the ash increased. The total alkali release increased with temperature in the range studied and the release during the pyrolysis was larger or equal to the release from the ash phase. Small particles (2−10 mg) emitted more alkali per unit mass than large ones (60−130 mg) and this tendency increased with temperature. At 800 °C the emission per unit mass from small particles was 10 times the one for large particles. Wet particles went through a drying phase, which delays the heating, and thereby the alkali emission. The present findings are of importance for actions aimed at minimizing alkali related problems during large-scale biomass conversion.

This paper is part I in a series of two describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets in a 12 MW thermal CFB boiler. In paper I the results of using the mixture of wood chips and wood pellets as a base fuel are described. Up to 45% on energy basis of rapeseed cake was cocombusted during a 4 h test. Two approximately 12 h tests with energy fractions of rapeseed cake of 12 and 18% were performed with limestone as a varying parameter. Fuels were characterized by means of chemical fractionation and standard methods. Elemental mass balances were calculated for ingoing and outgoing streams of the boiler. In addition SEM/EDX analyses of ashes were performed. Gaseous (KCl þ NaCl) as well as HCl and SO 2 were measured upstream of the convection pass, where deposit samples were also collected with a deposit probe. The deposit samples were analyzed semiquantitatively by means of SEM/EDX. The elemental mass balances show accumulation of alkali metals and phosphorus in the boiler. Analyses of bed material particle cross sections show the presence of phosphorus compounds within a K-silicates matrix between the agglomerated sand particles, indicating a direct attack of gaseous potassium compounds on the bed surface followed by adhesion of ash particles rich in phosphorus. Build-up of deposit during the cocombustion tests mainly took place on the windward side of the probe; where an increase of K, Na, and P has been observed. Addition of limestone prevented formation of K-silicates and increased retention of phosphorus in the bed, most probably due to formation of high-melting calcium phosphates. During the tests with limestone, an increase of potassium chloride upstream of the convection pass and a decrease of phosphorus in the fly ash fraction could be noticed. Agglomeration and slagging/fouling when cofiring wood with rapeseed cake may be linked to its high content of organically bonded phosphorus;phytic acid salts;together with high contents of water-soluble alkali metals chlorides and sulfates in the fuel mixture.

The objective of this work is to survey the fate of potassium in the gas phase of a fluidised bed boiler and gain deeper understanding of the involved mechanisms during co-firing of municipal sewage sludge with biomass containing high amounts of potassium and chlorine. The results show that formation of alkali chlorides in the flue gas and corrosive deposits on heat transfer surfaces can be controlled by addition of municipal sewage sludge even though the fuel is highly contaminated with chlorine. The beneficial effects are partly due to the content of sulphur in the sludge, partly to the properties of the sludge ash. The sludge ash consists of both crystalline and amorphous phases. It contains silica, aluminium, calcium, iron and phosphorus which all are involved in the capture of potassium.

Kaolin addition to a 35 MW circulating fluidized-bed boiler, fired with forestry residues, was performed to study the effect on bottom and fly ash. Ash samples from a period of kaolin dosage were compared with ash samples without kaolin. The samples were analyzed for elemental composition, water solubility, and mineralogical composition. The cross-section of bed particles was examined for elements. The agglomeration temperature of the material from the particle seal was determined. The kaolin, in the form of a fine powder, was supplied to the particle seal. It was found to be carried by the flue gases to the electrostatic filter rather than staying in the bed. Despite this, alkali is removed from the furnace by the kaolin, and thereby, the agglomeration temperature of the bed material increases. The alkali ends up in the fly ash, in which the content of alkali increases and the solubility of alkali decreases, suggesting that alkali aluminum silicates have been formed. These compounds have high melting points and are therefore unlikely to become liquid and stick to the surface of superheaters. A small fraction of the kaolin stays in the furnace by forming a thin layer on bed particles. It is concluded that kaolin addition can significantly enhance the operation of a fluidized bed fired with biofuels both with respect to superheater deposition and corrosion and bed agglomeration. The results show that findings in small-scale reactors are also valid in large-scale combustors. However, under the present circumstances, most kaolin ends up as fly ash.

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An instrument for on-line measurements of alkali components in hot flue gas streams is presented. The instrument is based on surface ionization technique and is equipped with a hot sampling line used to extract flue gas for continuous alkali measurements at pressures up to 30 bar. The instrument, its support system and the calibration procedure is described. Field campaigns with the instrument have been performed at fluidized-bed combustion facilities operating under pressurized and atmospheric conditions and using several fuels, including coal, biomass, and demolition waste. The instrument performance has been satisfactory both during pressurized circulating fluidized-bed coal combustion with alkali concentrations downstream of a hot gas filter in the ppb range, and in particle-laden conditions with alkali levels up to about 10 ppm. The measured alkali concentration corresponds to the concentration of alkali components present as vapors and fine-mode particles. In systems with high levels of fluidized-bed material or fly ash, a contribution to the signal from alkali bound to coarse particles is also expected. Under these conditions, the instrument is suggested to be operated in a pulse counting mode where the concentration of coarse particles can be estimated. The instrument is concluded to provide a durable, sensitive, and reliable alkali measurement method with a high time resolution and with a lower detection limit of around 1 ppb, which should be suitable for a large range of applications.