Klaus R. G. Hein

Effect of co-combustion of biomass on emissions in pulverized fuel furnaces

Abstract Biomass not only has a considerable potential as an additional fuel source but it also shows a reasonable cost level in comparison to other renewable energies. The practicable fuel types are both residual material from forestry and agriculture, such as wood or straw, and especially cultivated reproducible feedstock such as Miscanthus Sinensis, whole cereal plants, poplars, or willows. Besides, as single fuel, it is also considered to be sensible to utilize biomass in co-combustion in existing firing systems, such as pc-fired power stations. In order to promote the employment of biogenetic fuels, the European Union, within the framework of the APAS Clean Coal Technology Programme, initiated a research project concerning the co-combustion of biomass in existing firing systems. The focus of the investigations in experimental and industrial-scale plants was to establish the impact of co-combusting biomass and sewage sludge in coal-fired systems with regard to combustion behaviour and pollutant formation. The investigations were carried out on laboratory scale and pilot scale, as well as in large-scale plants. The task of the Institut fur Verfahrenstechnik und Dampfkesselwesen (IVD), University of Stuttgart, within the EU clean coal technology programme, was to investigate the effects of co-combustion of solid biomass (Miscanthus, straw) and municipal sewage sludge together with the primary fuel hard coal. The experimental plant used was a semi-industrial pulverized fuel test rig (0.5 MW). Within the scope of this publication, a synopsis shall be given of the outcome of the project, with special regard to the effect of co-combustion on emissions. The investigations reveal that biomass addition has a positive effect on emissions. Since biomass in most cases contains considerably less sulphur than coal, an increasing biomass share in the thermal output makes the SO 2 emissions decrease proportionally. In addition, SO 2 can partly be captured in the ash by the alkaline–earth fractions of the biomass ash. As for sewage sludge, the emissions of SO 2 correlate with the sulphur content of the fuel and, hence, rise with an increasing share of this biomass. Due to the high volatile content of the biomass, low NO x emissions can be achieved both by air staging and by reburning.

Basic effects on NOx emissions in air staging and reburning at a bench-scale test facility

Investigations on air staging and fuel staging were carried out with an electrically heated tube reactor. The effects of stoichiometry and residence time in the fuel-rich zone and the effects of temperature were evaluated for air staging with different coals and, in the case of reburning, for different gaseous reburn fuels. The results show the strong influence of residence time and stoichiometry, which may not be considered independently. The effects of temperature depend on the stoichiometric parameters. In air-deficient conditions, given by air staging and fuel staging, the NOx emission decreased with increasing temperature, whereas in oxygen-rich conditions the opposite trend occurred. In pulverized bituminous coal combustion, reburning is superior to air staging. While minimum NOx emissions of textasciitilde250 mg ms-3 could be achieved with air staging, reburning brought about values textasciitilde200 mg ms-3. For brown coal combustion, the minimum emissions far below 200 mg ms-3 with air staging could not be achieved with reburning.

Investigation of slagging in pulverized fuel co-combustion of biomass and coal at a pilot-scale test facility

One option to reduce the CO2 emissions is biomass used for energy production. Co-combustion of biomass in existing coal-fired power stations offers a great potential. In pulverized coal combustion facilities, one implementation problem is the tendency of slagging and fouling of biomass. At the IVD, experiments were carried out to obtain information about biomass deposit characteristics in pulverized fuel (pf) co-combustion. This paper presents results from the IVD 0.5 MW pf combustion experimental facility obtained from different tests with coal-biomass mixtures ranging from small biomass shares up to a pure biomass firing. Samples of fuels, the fly ash path, deposition and slagging probes, and slags on furnace walls were taken. The amounts of deposits found on probes inserted into the combustion chamber, their macroscopic characteristics, elemental composition and fusion temperatures were analyzed and compared. Generally, the deposits from the co-combustion experiments softened in ash fusion tests like the main component (coal ash) rather than like a mixture. When co-firing biomass with 25

Legislative and environmental issues on the use of ash from coal and municipal sewage sludge co-firing as construction material

For the economy of any co-firing process, it is important that the common waste management options of ash remain practical. Ash from bituminous coal combustion is typically handed to the construction industry. This paper describes the current European legislation on use of ash for construction purposes. Also, it presents an experimental study on the suitability of fly ash from combustion of mixtures of bituminous coal and municipal sewage sludge as additive to cement and concrete, and for use in open-air construction works, based on the ash chemical composition and the characteristics of the extract of the ash. Presently, two European standards forbid the use of ash from co-firing as additive to cement or concrete. This study shows that ash derived from coal and sewage sludge co-firing contains generally less unburned carbon, alkali, magnesium oxide, chlorine, and sulfate than coal ash. Only the concentration of free lime in mixed ash is higher than in coal, even though, at least up to 25% of the thermal input, still below the requirements of the standards. This ash also meets the requirements for the use of fly ash in open-air construction works--concentration and mobility of few elements--although this management option is forbidden to ash from co-firing. The leaching of Cd, Cr, Cu, Ni, Pb and Zn was investigated with three leaching tests. The concentration of these metals in the extracts was below the detection limit in most cases. The concentration of Cu and Zn in the extract from fly ash was found to increase with increasing share of sewage sludge in the fuel mixture. However, the concentration of these two metals in the extract is not regulated. This study indicates that excluding a priori the use of ash from co-firing as a suitable additive for construction material could cause an unnecessary burden on the environment, since probably ash would have to be disposed of in landfill. However, allowing this requires the modification of current European standards to include limitations on all elements and compounds, absent in coal but which might be present in other fuels, that are deleterious for the quality of construction materials.

Effect of coal blending and particle size on NOx emission and burnout

Investigations were undertaken with a 0.5 MW(th) test combustor burning pulverized coal, with burner or in-furnace air-staging. Significant dependences of NOx emission and unburnt carbon in fly ash on the flow conditions in the furnace, the blending ratio of high- and low-volatile coals and the fineness of the coal were observed. The lowest NOx emission, 180 ppmv, was achieved with in-furnace air-staging, at an unburnt carbon content of ~10 wt%, acceptable for small test facilities. The results indicate limits to the use of NOx-reduction techniques.

Study on trace metal partitioning in pulverized combustion of bituminous coal and dry sewage sludge

A study of the influence of co-firing of dry municipal sewage sludge on the behavior of the metals Cr, Hg, Mn, Ni, Pb, Zn during pulverized coal combustion is presented. Sewage sludge contains higher concentrations of the metals listed above than the reference coal, but a lower concentration of Cl, that enhances the volatility of many metals. Experiments were performed in a semi-industrial scale pulverized fuel combustion chamber (500 kW). Ash was collected at four locations: bottom hopper (T=850 K), air preheater (T=750 K), cyclone (T=620 K), and bag filter (T=480 K). From the bottom hopper to the filter, the particle size decreased and ash particles were progressively enriched in volatile elements. Mass balances of the metals were performed and the enrichment trends on the ash collected at the different locations were calculated. Increasing the sewage sludge share in the blend caused a significant increase in the recovery rate in the solid phase. In spite of that, the calculated concentrations in the flue gas of Hg and Zn increased. Sewage sludge co-firing influences the combustion process and the post-combustion environment in many ways. This study focuses on the effect of the different flue gas composition on the condensation temperature of metal species. The system was modeled by assuming thermodynamic equilibrium. The results indicated that the increasing recovery of Zn might be caused by enhanced condensation and the increasing recovery of Hg by adsorption on ash particles. The increasing recovery of the other metals seemed referable to failure in vaporization and it cannot be studied with an equilibrium approach.

Tar quantification with a new online analyzing method

This paper presents a tar quantification method developed at the University of Stuttgart that allows quasi continuous on-line measurement of the content of condensable hydrocarbons (textquotelefttextquotelefttarstextquoterighttextquoteright) in the gas from biomass gasification. The method is based on the comparison of the total hydrocarbon content of the hot gas and that of the gas with all tars removed. Hot gas from the gasifier is led directly into the set up. Hydrocarbons are measured with a flame ionization detector. The method considerably simplifies tar measurements compared to other methods described in literature. A prototype based on the method has been developed and tested. Experiments with the gas from a fluidized bed gasifier and a fixed bed gasifier have been conducted yielding very good results. Tar contents between 200 and 20 000 mg/m3 have been measured reliably. The experimental procedure is simple and repeatability is good. Sampling and analysis time is two minutes. Continuous on-line monitoring of the tar content in the gas is possible.

A Kinetic Model for the Prediction of NO Emissions from staged Combustion of Pulverized Coal

Emissions of nitrogen oxides from coal combustion are a major environmental problem because they have been shown to contribute to the formation of acid rain and photochemical smog. Air staging and the application of low-NOx burners are effective in reducing the In-furnace-formed NOx. Fuel staging, or reburning, is another effective method to reduce NOx emissions in the combustion chamber. For a successful application of these processes on industrial scale, the governing parameters must be evaluated very carefully. Computer modeling is an efficient tool for acquiring a better knowledge of the optimum process parameters. In order to predict NO emissions from furnaces operated with advanced combustion technologies, a knowledge of the fare of coal nitrogen during the combustion process is paramount. An advanced NOx model for staged combustion of pulverized coal is presented. It is closely connected to a coal combustion model that includes primary pyrolysis and secondary reactions of tars formed during primary pyrolysis. The present NOx model takes into account the different pathways of coal nitrogen release during primary and secondary pyrolysis, char, and soot combustion. The subsequent conversions of nitrogen-containing species comprise formation of NO from fuel nitrogen and air nitrogen (thermal NO) as well as reduction of NO by hydrocarbon and NHi radicals, char, and soot. The interaction between turbulence and chemistry is modeled by an advanced eddy dissipation concept (EDC). The NOx model is used to predict NO profiles that are compared to measurements obtained from combustion tests carried out at a bench-scale entrained-flow reactor. Comparisons are made for air-staged and fuel-staged combustion of pulverized coal using methane and coal as reburn fuel.

Generation and conversion of carbonaceous fine particles during bubbling fluidised bed gasification of a biomass fuel

This article presents an experimental research concerning the biomass gasification in bubbling fluidised bed. The attention is focused on the part played by comminution phenomena during the conversion of fuel particles, and on the occurrence of post-conversion of elutriated chars in the freeboard. The aim is pursued by means of measurements of bed carbon load, elutriation rate as well as particulate concentration at various heights of the freeboard in a laboratory scale facility equipped with an isokinetic probe. The results of experiments show that fines generation is relevant during gasification, and post-conversion of particles takes place after their release from the bed. The carbon post-conversion in the freeboard achieves a maximum value of 70%, even when the operating conditions are unfavourable for gasification. The results are also confirmed by measurements of gas composition in the freeboard. The laboratory analyses of samples taken during experiments could confirm that a percolative fragmentation takes place in parallel with carbon post-conversion.

Detailed modeling of hybrid reburn/SNCR processes for NOx reduction in coal-fired furnaces

Abstract Mechanism reduction has made the detailed kinetic modeling of combustion problems much easier; it also offers potential improvement of modeling accuracy and flexibility in comparison to global mechanisms. The present work applies mechanism reduction in conjunction with the CHEMKIN library and develops an automatic reduction program code. Regarding the hybrid re-burn/selective non-catalytic reduction (SNCR) (“advanced re-burning”) conditions in coal-fired furnaces and based on a full mechanism “GADM98,” a skeletal mechanism with 39 species, 105 reactions, and further a 10-step/14-species reduced mechanism were established. The reduced mechanism was implemented into a 3D-combustion computational fluid dynamics (CFD) code. The eddy-dissipation-concept model was used to describe the influence of turbulence on the combustion chemistry. A large number of simulations for reburning and hybrid reburn/SNCR processes in a coal-fired reactor were executed; the predicted results were compared with experimental measurements. The reduced mechanism and the comprehensive modeling give quite satisfactory results over a wide range of mole ratios for β = [NH 3 ]/[NO] and air/fuel equivalence ratios λ 2 in the reburn zone. From the modeling results, it was found that adding ammonia premixed with reburn fuel (CH 4 ) effects no further reduction of NO x or even impairs the reduction efficiency compared to pure reburning, and in contrast, staged addition of ammonia downstream of the CH 4 injection in the reburn zone provokes a significant further reduction of NO x over a wide range of parameters. According to the predictions, NO x -reduction rates of 50–60% and of 70–80% can be achieved through pure reburning and hybrid reburn/SNCR approaches, respectively, at λ 2 = 0.95 and β = 1.5. Concerning the computational procedure, essential measures were taken to optimize convergence and computing time. The computing time with the present reduced mechanism is ∼2.5 times that with the traditional global mechanism for the same iteration number. Tabulation of the rate constants reduced the computing time of the reaction kinetics by ∼50%.