Efficient and clean use of biomass for the production of heat and electricity. Phase 1 of a long-term strategic research project

The project and this final report consists basically of three different parts: 1) a study of pre-treatment processes of straw for power production, 2) a study of bioful combustion in fluidized beds and 3) a study of formation of ash and depositions in biofuel-fired thermal conversion processes. The study of pre-treatment of biofuels for power production may be subdivided into the following activities: a) a laboratory investigation of the release of K and Cl from straw during pyrolysis, b) a laboratory investigation of the extraction of K and Cl from straw char, c) laboratory investigations of particle characterization, pyrolysis kinetics and char combustion, and, finally, d) a technical and economical evaluation in order to evaluate and industrial scale pre-treatment process capable of treating 20 tons of straw per hour. The study of biofuel thermal conversion in fluidized bed combustors (FBCs), consist of: 1) a detailed lab-scale investigation of agglomeration in FBCs, b) a study of reduction and decomposition of NO and N2O over char and bed material, c) an evaluation of NOx emissions from biofuel-fired FBCs, and finally, d) a study of the hydrodynamics of the 80 MWth Grenaa CFB boiler Full-scale measurements of ash and deposit formation in biofuel-fired boilers have been conducted at the Rudkøbing combined heat and power (CHP) production plant, at the Kyndby Power Station, Unit 11, at the Midtkraft Studstrup Power Station, Unit 1, and in the multi fuel combustor (MFC) at Sandia National Laboratories, Livermore, California. In each of these measuring compaigns, a number of bottom and fly ashes, and deposites have been collected and analyses by means of standard wet chemical analyses and advanced scanning electron microscopy analyses. Two ph.d.-theses have been finalized covering ash and deposit formation in straw-fired systems and in systems co-fired with coal and straw. A number of thermodynamic modelling activities have been conducted as part of this project. First, an outline of potassium chemistry in systems fired with straw or co-fired with straw and coal is provided. Secondly, biofuel ash chemistry is outlined considering thermal conversion of salix, straw and wood fuels in a number of combustion and gasification concepts. As part of the study, a number of models for non-ideal liquid mixtures of ash compounds have been investigated. A round-robin comparison of the performance of four well-documented algorithms and databases for minimization of the total Gibbs energy of a mass-balance constrained system is also described. Finally, three cases of application of the in-house Gibbs energy minimization algorithm, MINGTSYS, and its database GFEDBASE are outlined: a) thermal conversion of oil-shale in a power station in Estonia (a study conducted in cooperation with VTT Chemical Technology, Finland), b) combustion of blends of coal and sewage sludge (a study conducted in cooperation with the Technical University in Stuttgart, Germany), and c) trace element transformations in straw-fired utility boilers. Finally, an outline of a 1st generation algorithm for residual fly ash formation, developed in order to make it possible to model both ash and deposit formation in utility boilers without having to collect ash from a plant , is provided. The model have been tested with good results on a number of fly ashes generated in Danish power stations as well as on literature data from a US lab-scale study of ash formation