Mikko Hupa

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.

Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses

The effect of the composition and bonding configuration of the bioactive silica-based glasses on the initial stage in vitro bioactivity is presented. Information of the IR active Si–O groups of glass in the system SiO2–P2O5–CaO–Na2O–K2O–MgO–B2O3 was obtained by fourier transform Infrared (FTIR) spectroscopy. Two different bands associated to non-bridging oxygen stretching vibrations (Si–O–1NBO and Si–O–2NBO) and a gradual shifting of the bridging oxygen stretching vibration (Si–O) have been observed and evaluated. Both effects are attributed to a decrease of the local symmetry originating from the incorporation of alkali ions into the vitreous silica network. The Si–O–NBO(s)/Si–O(s) absorbance intensity ratio increases with a gradual incorporation of the alkali ions (diminution of SiO2 content) following a linear dependence up to values close to 50 wt % of SiO2. In vitro test analysis by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) showed a correlation between the amount and type of the non-bridging oxygen functional groups and the growth of the silica-rich and CaP layers. It was found that a minimum concentration of Si–O–NBO bonds in the glass network is required in order to have an efficient ion exchange and dissolution of the silica network. Finally, the bioactivity of the glass is favored by the presence of the Si–O–2NBO groups in the glassy network. The role of these functional groups in the dissolution of the silica network through the formation of silanol groups and the adsorption of water is discussed.

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.

Homogeneous N2O chemistry at fluidized bed combustion conditions : a kinetic modeling study

The importance of homogeneous gas-phase reactions of fuel-nitrogen species to the formation of N2O in fluidized bed combustion was studied based on detailed chemical kinetic modeling. A kinetic scheme consisting of over 250 elementary reactions was applied for an ideal plug flow reactor, and the most important reaction paths leading to N2O were identified and quantified for a number of conditions relevant for fluidized bed combustion. In addition, the effect of different operating parameters on N2O formation was investigated, and the results were compared to the existing data from laboratory and full-scale experiments. Calculations showed that if fuel-nitrogen species in form of simple cyano species (HCN) enters a fuel-lean gas phase between 1000 and 1200 K, a significant conversion to N2O is found. The N2O formation occurs principally through the reaction NCO+NO→N2O+CO where NCO originates from HCN mostly via the reaction HCN+O→NCO+H For ammonia-based fuel-nitrogen derivates (NHi) hardly any N2O was formed. Calculations indicated that N2O formation increases steeply as the temperature decreases. At high temperatures less N2O is produced because the key intermediate NCO is rapidly removed by the radicals, mostly via the reactions NCO+H→NH+CO,NCO+O→NO+CO Generally, the trends obtained in this study are in good agreement with existing data from laboratory and full scale measurements. Thus, the work indicates that the homogeneous gas-phase reactions are an important contributor to the N2O formation in fluidized bed combustion.

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.

Antibacterial effects and dissolution behavior of six bioactive glasses

Dissolution behavior of six bioactive glasses was correlated with the antibacterial effects of the same glasses against sixteen clinically important bacterial species. Powdered glasses (<45 microm) were immersed in simulated body fluid (SBF) for 48 h. The pH in the solution inside the glass powder was measured in situ with a microelectrode. After 2, 4, 27, and 48 h, the pH and concentration of ions after removing the particles and mixing the SBF were measured with a normal glass pH electrode and ICP-OES. The bacteria were cultured in broth with the glass powder for up to 4 days, after which the viability of the bacteria was determined. The antibacterial effect of the glasses increased with increasing pH and concentration of alkali ions and thus with increased dissolution tendency of the glasses, but it also depended on the bacterium type. The changes in the concentrations of Si, Ca, Mg, P, and B ions in SBF did not show statistically significant influence on the antibacterial property. Bioactive glasses showed strong antibacterial effects for a wide selection of aerobic bacteria at a high sample concentration (100 mg/mL). The antibacterial effects increased with glass concentration and a concentration of 50 mg/mL (SA/V 185 cm(-1)) was required to generate the bactericidal effects. Understanding the dissolution mechanisms of bioactive glasses is essential when assessing their antibacterial effects.

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.

A reduced mechanism for nitrogen chemistry in methane combustion

A four step mechanism for combustion of methane in perfectly stirred reactors with special emphasis on formation and destruction of hydrocarbon radicals has been developed using steady state and partial equilibrium assumptions for minor species. The reduced mechanism has been extended to include the nitrogen chemistry with NO and HCN as independent reactive species. The reduced nitrogen scheme includes thermal NO and prompt NO formation, as well as the NO to HCN recycle reactions, and conversion of HCN to NO and N2. We have tested the reduced mechanism by comparing perfectly stirred reactor calculations performed with full and reduced chemistry over a wide range of stoichiometries, temperatures and residence times. The reduced model generally provides a good description of the methane oxidation process as well as formation and destruction of nitrogen oxides. However, at low temperatures or very fuel-rich conditions reduced model predictions deteriorate, partly due to neglection of the C2-chemistry and partly because the OH partial equilibrium assumption becomes less accurate.