Prediction of slagging with enhanced biomass co-firing in Dutch utility boilers using an engineering method

摘要

Co-firing of biomass is practised in the Netherlands for over fifteen years. For 2020 the EU-govemmeni has set a target of 20% of greenhouse gas emission reduction in comparison with 1990 in EU energy systems which could possibly mean 30 - 35% reduction in Dutch power generation. Carbon Capture and Storage (CCS) is a possible way to reduce CO_2 emissions Nevertheless, compared to co-firing of biomass these technologies are less developed and demonstrated and, dependent on the technology, it causes an expected efficiency loss of 4 (chilled ammonia process) to 9-12 (oxyfuei, amtne washing) percent points whereas efficiency losses with biomass co-firing compared to pure coal tiring are only minor Therefore, increasing the percentage of co-tiring of biomass is a realistic scenario. However, (operational) problems are expected with increased co-firing percentages in a number of fields, amongst others slagging and fouling. This paper focuses on aspects of slagging with biomass co-combustion.An engineering approach is chosen to obtain results in a relatively short period of time, "this approach consists of a combination of calculating boiler temperatures using a one dimensional boiler model and thermodynamic equilibrium calculations (FaetSage) to calculate slag composition and mass flow at a given temperature. Using the viscosity model of Urbain a prediction of the viscosity can be made and ultimately a ranking of slagging propensity tor different fuels or fuel blends. In this paper this has been done for coal, and co-firing straw and clean wood with coal. Straw is not being co-fired in the Netherlands, neither it is the expectation that it will be, but acts as a reference for international literature. Wood ts foreseen as major biomass for co-firing in the future of the NetherlandsVarious aspects of modelling are discussed which often lack in papers on thermodynamic equilibrium A short literature overview of thermodynamic equilibrium is given, especially in relation to deposition related aspects (molten slags and salts). Also the restraints of equilibrium calculations are mentioned Availability of elements is higher in many biomasses than in coal. However, the translation of solvability (chemical fracuonatiort) to availability is unclear and its sensitivity is evaluated. Important is also the choice of products, E g,, the equilibrium of SO_2 and SO_3 is far from what is measured in practice. Furthermore, formation of more complex minerals is less likely, hence an evaluation of minerals found in fly ash is necessary.The results are not conflicting with sooner obtained results by others, however, validation and comparison with more accurate modelling (i e. CFO) is required. The following results can be drawn from the presented calculations: the choice of the critical viscosity has a great impact Oil the results. As measurement of viscosity at high temperatures (> 1000 °C) is difficult, validation by measurement of the deposition rate is the most logic route; the availability of elements (in biomass) has an impact but minor compared to the choice of the critical viscosity. It is expected that for higher percentages of biomass co-firing availability of elements in biomass will have more impact, also on fouling behaviour, temperature will be lower near the wails as the calculated temperature with the ID model are average temperatures in a horizontal plane. Also here the sensitivity is minor compared to the critical viscosity Nevertheless, more changes in temperature can be expected with higher biomass percentages and hence it should be accounted for more accuracy in the ID boiler model. Further work is aimed at the improvement of the accuracy of modelling, modelling validation and putting results into practise