Christian Mueller

PCDD/F reduction in incinerator flue gas by adding urea to RDF feedstock

The effect of urea on PCDD/F formation in a pilot incinerator was studied by incinerating urea with refuse-derived fuel (RDF) at three concentrations (0.1%, 0.5% and 1.0%, of the fuel feed). A distinct reduction in both PCDD/F and chlorophenol concentrations could be noticed when urea was introduced into the system. Partial-least-square (PLS) analysis of the data showed the importance of certain chlorophenol isomers as PCDD/F precursors, pointing to the possibility that the impact point of the urea inhibitor could be before the precursor molecules, i.e. chlorophenols, have been formed.

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.

Urea as a PCDD/F Inhibitor in Municipal Waste Incineration

ABSTRACT Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from municipal waste incineration have been widely studied because of their extensive toxicity, and many efforts have been made to restrict their emissions. Although a number of chemical compounds have been shown in laboratory-scale tests to inhibit the formation of PCDD/Fs, few have been tested in pilot- or full-scale plants. This work evaluates the effect of urea as a PCDD/F inhibitor in a pilot-scale incinerator that uses refuse-derived fuel (RDF). The decomposition of urea under the test conditions was also studied using detailed kinetic modeling. An aqueous solution of urea was injected into the flue gas stream after the furnace at ~730 °C, with varied urea concentrations and flue gas residence times used between the furnace and the sampling point. The results demonstrate that urea can successfully inhibit PCDD/F formation in waste incineration if concentrations and injection points are properly adjusted. The kinetic model showed that urea can be rapidly decomposed under appropriate flue gas conditions, indicating that in addition to the urea molecule itself, decomposition products of urea can also be responsible for the reduction of PCDD/F production during incineration.

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

CFD Based Ash Deposition Prediction in a BFBC Firing Mixtures of Peat and Forest Residue

Fuels currently used for energy production in thermal power plants are characterized by their huge variety ranging from fossil fuels to biomass and waste. This multitude of fuels offers opportunities to the energy industry and nowadays many power plants do not fire either of these fuels but mixtures of them are burnt. While this procedure may lead to overall economic and environmental advantages it is very demanding for the boiler operators to still meet expectations concerning boiler performance, boiler availability and emission regulations. In the course of this latest trend in boiler operation, ash related operational problems such as slagging, fouling and corrosion are ranking very high on the list of reasons leading to significant reduction of boiler availability. Ash related problems strongly dependent on fuel specific aspects, such as the mineral matter distribution in the fuel, aspects specific to the used combustion technique as well as design aspects unique for the combustion chamber of any operating power plant. The overall goal in combustion related research is therefore the prediction of potential operational problems originating from fuel streams entering the combustion chamber as well as those originating from the design of individual furnaces. In our earlier work we have strongly focused on developing an advanced ash behavior prediction tool for biomass combustion combining computational fluid dynamic calculations (CFD) and advanced fuel analysis. In this paper the tool is applied to analyze the slagging and fouling tendency in a 295 MW bubbling fluidized bed boiler fired with mixtures of peat and forest residue. In addition to the overall deposition prediction this work focuses on details of the models used in the computational fluid dynamic calculations. These include a study on the importance of the accurate description of the fuel feeding system and related to this aspect the advanced description of the bubbling bed with regard to release of primary gas and ash particles from its surface to the freeboard. Evaluation of the predictions comparing simulation results with deposits on the furnace walls show good agreement.Copyright

A CFD-applicable discrete combustion model for thermally large particles

The objective of this paper is to present a comprehensive new single-particle combustion model that can be used effectively in Computational Fluid Dynamics (CFD) applications. As a major difference from standard particle sub-models used in CFD software packages, the new model is suitable for simulating thermally large particles, which means that it considers the intra-particle temperature gradients and overlapping stages of drying, pyrolysis and char conversion. The model was validated by comparison with experiments and also with a more detailed model presented in our earlier work. Although the model was applied here for black liquor combustion, it is also suitable and can be used for other different applications, such as the combustion of biomass, coal or heavy fuel oil and metallurgical processes.

CFD Based Ash Deposition Prediction in a BFB Firing Mixtures of Peat and Forest Residue

Fuels currently used for energy production in thermal power plants are characterized by their huge variety ranging from fossil fuels to biomass and waste. This multitude of fuels offers opportunities to the energy industry and nowadays many power plants do not fire either of these fuels but mixtures of them are burnt. While this procedure may lead to overall economic and environmental advantages it is very demanding for the boiler operators to still meet expectations concerning boiler performance, boiler availability and emission regulations. In the course of this latest trend in boiler operation, ash related operational problems such as slagging, fouling and corrosion are ranking very high on the list of reasons leading to significant reduction of boiler availability. Ash related problems strongly dependent on fuel specific aspects, such as the mineral matter distribution in the fuel, aspects specific to the used combustion technique as well as design aspects unique for the combustion chamber of any operating power plant. The overall goal in combustion related research is therefore the prediction of potential operational problems originating from fuel streams entering the combustion chamber as well as those originating from the design of individual furnaces. In our earlier work we have strongly focused on developing an advanced ash behavior prediction tool for biomass combustion combining computational fluid dynamic calculations (CFD) and advanced fuel analysis. In this paper the tool is applied to analyze the slagging and fouling tendency in a 295 MW bubbling fluidized bed boiler fired with mixtures of peat and forest residue. In addition to the overall deposition prediction this work focuses on details of the models used in the computational fluid dynamic calculations. These include a study on the importance of the accurate description of the fuel feeding system and related to this aspect the advanced description of the bubbling bed with regard to release of primary gas and ash particles from its surface to the freeboard. Evaluation of the predictions comparing simulation results with deposits on the furnace walls show good agreement.Copyright

Analysis of combustion processes using computational fluid dynamics—A tool and its application

Abstract Numerical simulation by means of Computational Fluid Dynamics (CFD) has developed over recent years to a valuable design tool in engineering science. In the beginning mainly applied to address fluid dynamic questions it is nowadays capable to predict in detail conditions in various complex technical processes. State of the art commercial CFD codes are almost always set up as multi-purpose tools suitable for a wide variety of applications from automotive industry to chemical processes and power generation. However, since not highly specialized in all possible fields of application, CFD codes should be rather seen as a collection of basic models that can be compiled and extended to individual tools for special investigations than as readily applicable tools. In power generation CFD is extensively used for simulation of combustion processes in systems like utility boilers, industrial furnaces and gas turbines. The purpose of these simulations is to analyze the processes, to optimize them with regard to efficiency and safety and to develop novel techniques. Since combustion processes have been the target for CFD software for long, also standard models available in the codes are of high quality as long as modelling of conventional combustion systems is concerned. However, as soon as characteristics of novel combustion systems or fuels or detailed effects within a certain process are of interest, the limits of these standard models are reached easily. At this point extension of standard models by process specific knowledge is required. This paper presents some of the opportunities CFD offers when applied to analyse different combustion systems. Practical examples presented are ash deposition predictions on heat exchanger surfaces and walls in a bubbling fluidised bed furnace and detailed nitrogen oxide emission predictions for the same furnace type. Furthermore, the extension of a standard model using process specific data is presented for the fuel conversion process in a black liquor recovery furnace.