Bengt-Johan Skrifvars

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.

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.

Ash behaviour in a CFB boiler during combustion of coal, peat or wood

This paper presents selected results from an extensive on-site measurement campaign where the ash behaviour in a 12 MW CFB boiler was studied during firing of coal, peat and wood. Samples were taken 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. Deposit samples were further collected from the cyclone inlet and from two different locations in the convective path. In addition, the boiler operation was monitored, including collection of operational data, flue gas temperature profiles and emissions. The paper discusses the differences in the ash chemistry that were detected between the three different combustion cases and draws conclusions on the impact of the chemistry on the bed agglomeration and fouling tendency for each fuel.

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.

Fouling Tendency of Ash Resulting From Burning Mixtures of Biofuels

Specified mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor that simulates conditions in the superheater region of a biomass-fired boiler. Deposits were collected on an air-cooled probe that was inserted into the reactor at the outlet. For both mixtures, the deposition behaviour followed a non-linear pattern, which suggests that physico-chemical interaction between the ashes of the different fuels has taken place. The results indicate that it is possible to burn up to 30 wt-% bark (renewable biofuel and pulp mill waste) and up to 70 wt-% straw (renewable biofuel and agricultural waste) in mixtures with peat without encountering increased deposition rates in the reactor. The deposit composition was compared to the fuel ash composition using chemical fractionation analysis and SEM/EDX. While the composition of deposits obtained from pure fuels resembles the feed composition, a considerable change is observed in deposits obtained from mixtures. K and S compounds are attached to Si spheres and the substrate surface. The deposition rate is significantly lowered when removing K, S, Cl and Na in bark prior to burning by washing and mechanical/thermal dewatering.Copyright

Agglomeration and Defluidization in FBC of Biomass Fuels — Mechanisms and Measures for Prevention

The use of biomass fuels in fluidized bed combustion (FBC) and gasification (FBG) is becoming more important because of the environmental benefits associated with these fuels and processes. However, severe bed agglomeration and defluidization have been reported due to the special ash forming constituents of some biomass fuels. Previous results have indicated that this could possibly be prevented by intelligent fuel mixing. In the present work the mechanisms of bed agglomeration using two different biomass fuels as well as the mechanism of the prevention of agglomeration by co-combustion with coal (50/50%w) were studied. Several repeated combustion tests with the two biomass fuels, alone (Lucerne and olive flesh), all resulted in agglomeration and defluidization of the bed within less than 30 minutes. By controlled defluidization experiments the initial cohesion temperatures for the two fuels were determined to be as low as 670°C and 940°C, respectively. However, by fuel mixing the initial agglomeration temperature increased to 950°C and more than 1050°C, respectively. When co-combusted with coal during ten hour extended runs, no agglomeration was observed for either of the two fuel mixtures. The agglomeration temperatures were compared with results from a laboratory method, based on compression strength measurements of ash pellets, and results from chemical equilibrium calculations. Samples of bed materials, collected throughout the experimental runs, as well as the produced agglomerated beds, were analysed using SEM EDS and X-ray diffraction. The results showed that loss of fluidization resulted from formation of molten phases coating the bed materials; a salt melt in the case of Lucerne and a silicate melt in the case of the olive fuel. By fuel mixing, the in-bed ash composition is altered, conferring higher melting temperatures, and thereby agglomeration and defluidization can be prevented.

Ash deposition prediction in biomass fired fluidised bed boilers ? combination of CFD and advanced fuel analysis

Operational problems caused by ash such as slagging, fouling and corrosion of boiler surfaces continue to be the most usual single reason for unscheduled shut downs of these boilers. The amount and distribution of various ash forming elements in the fuel and the local operational conditions in a boiler strongly affect the type of fly ash formed during the firing process, which again will affect the deposition behaviour. To avoid these operational uncertainties, it will be very beneficial to be able to predict ash deposition behaviour based on advanced fuel analysis combined with computational fluid dynamic (CFD) calculations. This paper presents for the first time a concept combining CFD calculations and advanced fuel analysis to predict the ash deposition behaviour on heat exchanger surfaces and boiler walls in the freeboard of a 105 MW biomass fired bubbling fluidised bed boiler. Extensive experimental investigations of the untreated fuel deliver the composition of the ash forming elements in the fuel. These fuel specific data are used as input for advanced thermodynamic equilibrium analysis leading to a detailed description of the temperature dependent melting behaviour of the ash. Based on this melting behaviour, a fuel-specific stickiness criterion is determined. This stickiness criterion, experiences from field studies and the operational set up of the boiler serve as boundary conditions for the CFD calculations. In these calculations physical and chemical processes occurring in the freeboard region of a bubbling fluidised bed combustor - starting from the bed up to the second super heater - are predicted in the form of gas phase and ash particle trajectory simulations. The exact positions of ash particle impacts on the boiler surfaces are recorded and the particle temperatures at these locations are the linking parameter to the fuel-specific stickiness criterion. The predicted locations of high ash deposition probability on boiler walls and super heater surfaces are compared with observations made in the boiler and very good qualitative agreement is found.

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.

Characterization of Biomass Ashes

Energy production from combustion or gasification of biomass has recently attracted increased interest. These fuels form a valuable indigenous energy resource for some countries. They represent also an attractive way to decrease CO2 emissions from the energy production. The development of new combustion and gasification techniques, such as atmospheric or pressurized fluidized bed combustion and gasification has also made it possible to utilize biomass in a more feasible way than before. The availability of these new energy conversion systems is, however, still unknown. Among other questions the behavior of ash can be critical.