Rainer Backman

The prediction of behaviour of ashes from five different solid fuels in fluidised bed combustion

Abstract The behaviour of different ashes was predicted by the combination of extended fuel analysis with advanced global thermodynamic equilibrium calculations. The extended fuel analysis is a fractionation method that consists of sequential leaching of a solid fuel with water, ammonium acetate and hydrochloric acid. In order to cover a broad spectrum of fuels a coal, a peat, a forest residue and Salix (i.e. willow) were studied. The last was taken with and without soil contamination, i.e. with a high and low content of silica, respectively. Results from the fractionation showed clear differences in mineral distribution in the fuels. More ash-forming elements were present as included minerals in the older fuels. In relatively young fuels, almost half of the inorganic material was found in the soluble fractions after leaching with water and ammonium acetate. Fouling and slagging predictions based on the combined use of the extended fuel analysis and the advanced global equilibrium analysis indicated that no ash-related problems should be expected in FBC boilers firing the studied coal. The peat that was studied could cause minor ash depositions in the flue gas channel at temperatures above 700°C. The studied forest residue could form fly ash deposits in the flue gas channel at temperatures between 600 and 860°C. The Salix could cause fly ash depositions at temperatures between 840 and approximately 1000°C. If soil contamination was present as well, Salix could cause bed sintering at temperatures above 1030°C.

Cement manufacturing using alternative fuels and the advantages of process modelling

Energy costs and environmental standards encouraged cement manufacturers world-wide to evaluate to what extent conventional fuels can be replaced by alternative fuels, i.e., processed waste materials. Clinker burning is well suited for various alternative fuels. In order select a suitable alternative fuel, a commercial modelling tool (ASPEN PLUS®) is used to model the four-stage pre-heater kiln system of a full-scale cement plant (clinker production ∼2900 tons/day), using petcoke as fuel. The goal is to optimise process control and alternative fuel consumption, while maintaining clinker product quality. Calculations with varying amounts of different fuels are compared with a reference case. The dependence of process performance on the amount of combustion air is clearly demonstrated and the energy demand of the process could be predicted for varying fuel mixes.

Sintering mechanisms of FBC ashes

The agglomeration of solid particles is discussed on the basis of various sintering mechanisms. Three major mechanisms are identified and considered to be important in CFB combustion: sintering caused by 1. (1) partial melting, 2. (2) viscous flow and 3. (3) gas-solid chemical reactions. Recent results achieved with a laboratory sintering test are related to these sintering mechanisms. The test, based on compression strength measurements on heat-treated cylindrical ash pellets, showed clear differences in sintering tendency among five coal ashes. The temperature at which sintering began varied between 500 and 900 °C. For two brown coal ashes, partial melting was found to cause sintering, whereas viscous flow was probably the dominant sintering mechanism for ashes from a bituminous coal and an anthracite. SO2 in the atmosphere increased the sintering tendency for two of the brown coal ashes. One brown coal also showed a slight increase in sintering at 600 °C when CO2 was present in the atmosphere. This increase could not be detected at higher temperatures. For the bituminous coal ash no change in sintering tendency could be detected when SO2, CO or both were present in the gas phase.

Characterization of the sintering tendency of ten biomass ashes in FBC conditions by a laboratory test and by phase equilibrium calculations

Ash sintering may cause problems in fluidized bed combustors. It may contribute to both bed agglomeration in the furnace of the FBC, as well as to deposit formation in the cyclone and plugging of the cyclone return leg. The sintering process may be promoted by a molten phase present in the ash. In this paper, we compare the sintering tendency of an ash with the melting behavior of the same ash. The sintering tendency was measured by a laboratory test method based on compression strength measurements, and the melting behavior was evaluated with a multicomponent, multiphase equilibrium model. Ten biomass ashes were chosen for the study. The compression strength test showed significant differences in the measured sintering tendencies for the various ashes. The comparison between the measured sintering tendencies and the melting behavior calculations revealed that in 7 cases out of 10, an estimated amount of some 15% molten phase present in the ash may have been the reason to the increased degree of sintering in the tested ash pellets.

The ash chemistry in fluidised bed gasification of biomass fuels. Part II : Ash behaviour prediction versus bench scale agglomeration tests

This paper is part II in a series of two. Ash behaviour modelling of the gasification of four biomass fuels is compared with pilot-scale experiments carried out in a pressurised fluidised bed gasifier at the Royal Institute of Technology (KTH) and an atmospheric test rig of Termiska Processer AB (TPS). Experiments were provocative with respect to agglomeration of the bed material. Thus, in the experiments, the agglomeration was allowed to happen without any corrective changes in the operation. Small-scale experiments showed clear defluidisation in five cases. Some degree of bed disturbance or agglomeration occurred in seven out of 13 cases. In nine of these cases, agglomerates were also found in the samples analysed with SEM/EDX analyses. In six out of 13 cases, the thermodynamic multi-phase multi-component equilibrium calculations were in agreement with SEM/EDX analysis, i.e. predicted formation of agglomerates. In two cases, no or small amounts of agglomerates were predicted, nor were these found with SEM/EDX analysis. In two cases out of 13, the modelling predicted some degree of agglomeration while no agglomerates could be detected with SEM/EDX analysis. However, in these cases, agglomerates were found in the pilot-scale experiments. Thus it is shown that the thermodynamic multi-phase multi-component equilibrium calculations are a useful prediction tool for the formation of agglomerates in (pressurised) fluidised bed gasification of biomass fuels thereby enhancing the understanding of the chemistry involved.

Estimation of trace element release and accumulation in the sand bed during bubbling fluidised bed co-combustion of biomass, peat, and refuse-derived fuels☆ ☆

The experimental study and thermodynamic equilibrium calculations were performed to investigate the interactions of fuel-based lead, copper, zinc, and manganese with the bed material of the bubbling fluidised bed boiler (BFB) during the co-combustion of sawdust, peat, and refuse-derived fuel. Flue gas trace element measurements, a chemical analysis of process streams, and mass balance calculations over the boiler reveal that the bed material captures substantial amounts of Pb, Cu, Zn and Mn, but also that these metals are released from the bed when the fuel characteristics or the process parameters are changed. The study shows that the trace metal emissions of a BFB boiler are not necessarily directly related to metal concentrations of the fuel but are rather a result of a complex process combining the release of trace metals from the fuel, the interaction between the fuel and fuel ash particles within the bed material, and the release of trace metals from the bed. The EDS analysis of the bed material particles shows that the original sand particles are covered with a Ca- and K- rich layer. In combustion temperatures, the layer is assumably in a melt form and has an important role in the trace metal accumulation/release process. The thermodynamic multi-phase multi-component equilibrium calculation for zinc suggests that the release of zinc from the bed material is strongly increased with an increasing Cl-concentration of the fuel, due to the conversion of the bed-bound zinc to ZnCl2.

Ash Behavior in a CFB Boiler during Combustion of Salix

A study on the combustion characteristics of Salix Viminalis, a fast growing willow, was conducted at a 12 MW circulating fluidized bed boiler. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed agglomeration or slagging and fouling problems that may occur during the combustion of this type of a fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling and bed agglomeration. Samples from all in-going (bed material, fuel) and out-going solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg were collected and analyzed chemically. Selected bed samples and ash samples were also analyzed with a scanning electron microscope (SEM/EDAX). Deposit samples were collected at the cyclone inlet and from two different locations in the convective path using specially designed surface temperature controlled deposit probes. All collected probe deposits were photographed and characterized visually. Selected samples from both windward (front) side and leeward (back) side of the sampling probes were analyzed chemically as well as with SEM/EDAX. On top of these samples, the boiler operation was monitored carefully. This included collection of operational data (fuel feed, air distribution and total air), collection and monitoring of pressure drops in the furnace, fluegas temperature profiles and emissions. Multi-component multi-phase thermodynamic equilibrium calculations were then performed for predictions of the fly ash the thermal characteristics, using the fly ash chemical composition as input data. The thermal characteristics i.e. the melting behavior, was predicted for the different ash samples and compared with the results from the full scale fouling measurements. The paper discusses the impact of the ash chemistry on the bed agglomeration and fouling tendency, found during the combustion tests and draws conclusions about their relevance to the operation of the boiler.

Interaction of the Behavior of Lead and Zinc With Alkalis in Fluidized Bed Combustion or Gasification of Waste Derived Fuels

Combustion of waste derived fuels in fluidized bed boilers may result in fly ashes containing increased amounts of lead and zinc, besides the common alkali and alkaline earth metal compounds. Although the absolute concentrations of lead and zinc may be relatively low, around 1%, in the bulk ash, they may induce unwanted effects in combustors, partly due to their significant enrichment in the fly ash. First, lead and zinc in fly ashes may lead to unwanted heavy metal emissions. Further, they can also alter the behavior of the fly ash and cause it to become sticky and possibly corrosive. This paper discusses the mechanism of volatilization of lead and zinc and stickiness properties of their fly ash compounds under different conditions, based on data from a FBC gasifier using waste fuels with significant amounts of lead and zinc. Advanced thermochemical calculations using the data bases developed at Abo Akademi show that both lead and zinc can form volatile compounds and thus be strongly enriched in the fly ash. They can be volatilized as elemental gases, Pb(g) and Zn(g), or they can form gaseous chlorides, PbCl2 (g) and ZnCl2 (g). But they can also form non-volatile oxides. Thus their behavior is very dependent on the combustion conditions, particularly on the availability of chlorine. This way there is also a direct coupling of the volatilization behavior of lead and zinc with the chemistry of the alkali metals and calcium, all of which govern the availability of chlorine. Simplified thermochemical diagrams are shown to explain the complex interaction of the lead and zinc chemistry with the rest of the flue gas and fly ash chemistry. The thermochemical data can be used to explain the practical results from full scale boilers.Copyright

Co-Firing in FBC: A Challenge for Fuel Characterization and Modeling

Co-combustion of coal with biomass or firing biomass alone is used more and more in a first step in meeting the Finnish commitments under the Kyoto protocol. A frequently used technique for firing mixtures of fuels is fluidized bed combustion (FBC). Firing coal, co-combustion with biomass or firing biomass alone may, however, lead to unwanted ash-related problems. Prediction of ash formation behavior can help to avoid these problems before taking new fuels into use. Standard fuel analyses have shown to provide insufficient information for proper prediction especially when considering fuel mixtures. In an attempt to minimize the number of lab scale and pilot scale combustion experiments an extensive database is under development. This database contains data used as input for prediction models such as standard fuel analyses, results from stepwise leaching experiments, SEM/EDS analyses of original and partly burned-out fuels and thermodynamic estimations of the melting behavior of the fuels. Today the database contains 51 fuels, i.e. 8 bark fuels, 10 wood fuels, 3 annual biomasses, 8 peats, 6 coals and 16 miscellaneous fuels, such as RDF, sludge, hulls and husks, bagasse and other residues. Standardized fuel analysis is available for all fuels; melting calculations have been carried out for some 33 fuels. SEM/EDS analysis has been carried out for 20 fuels. The extended utilization of these data with computational fluid dynamic modeling (CFD) has proven to be a useful tool in prediction of deposits in FBC boilers. An example of the prediction tool shows the ability of deposit formation prediction.Copyright

THE ROLE OF ALKALI SULFATES AND CHLORIDES IN POST CYCLONE DEPOSITS FROM CIRCULATING FLUIDIZED BED BOILERS FIRING BIOMASS AND COAL

High amounts chlorine and sulphur in a fuel is generally connected with ash related operational problems in the boiler in which the fuel is fired. The problems occur as fireside deposits in different locations of the fluegas channel or as corrosion problems. Sulphur and chlorine together with alkali and earth alkali metals are known to strongly affect the thermal behaviour of the ash. First melting temperatures as low as 515 °C may be found if unsuitable amounts of alkali, sulphur and chlorine is present in the ash. A vast experience on the matter exists from coal firing [Sarofim and Helble 1994; Bryers 1992; Harb and Smith, 1990] as well as from firing different types of waste sludges [Backman et al., 1987] Backman et al., 1996; Salmenoja et al., 1996]. Forrest derived fuels such as wood, bark or forest residue (branches and tops) contain usually low amounts of sulphur and chlorine. The low potential for sulphur dioxide emissions from combustion of these kind of fuels as well as the indication of a fairly well behaving ash in most kind of combustion systems, are generally considered as two important advantages for the fuels. Sometimes these general indications are applied also on any type of biomass. This may, however, lead to serious errors since other biomasses such as straw or annually grown energy crops may contain significant amounts of both chlorine and sulphur [Nordin 1993]. In Fig. 1 the amount of chlorine and sulphur is shown for a number of different fuels, including coal, peat, wheat straw and forest derived fuel. Fluidized bed combustion is regarded as a very flexible combustion system with a capacity to burn a wide range of fuels. From the ash behaviour point of view the low combustion temperature of some 800–900 °C is favourable compared to conventional pulverised systems since operational problems due to a low melting ash are assumed to be avoided. The FBC technique has, however, limits. Common knowledge from conventional