• Energy Research

Condensing biomass boilers gained increasing importance in the residential heating sector within recent years but the fuels for residential condensing biomass boilers are almost exclusively restricted to high quality wood fuels (namely pellets and wood chips). As it is a dedicated aim of the EC and the EU member states to promote the utilisation of biomass fuels for energy production as a measure for CO2 emission mitigation and since high quality wood fuel resources are not available in high volumes in many European regions, the fuel spectrum to be applied in residential biomass combustion systems must be broadened in order to cover the rising fuel demand while avoiding increasing fuel prices in future. Moreover, measures to further increase the thermal efficiency of residential biomass boilers are urgently required to use the available biomass potentials as efficiently as possible and to keep them competitive in comparison to fossil fuel based systems, also in terms of energy labelling. Within an EU funded project a condenser based on the diffusion bonding technology has been developed for a fuel flexible biomass boiler based on extreme air staging with ultra-low emissions and optimised in terms of compactness and high efficiency. The development work has been supported by CFD (computational fluid dynamics) simulations. Subsequently, a condenser prototype has been manufactured and tested during a comprehensive test run program with two different biomass fuels (forest residues, miscanthus pellets). The results, which are presented in this paper, show, that an economically attractive solution could be reached which is able to fulfil the requirements for fuel flexible operation and to ensure high efficiencies of the overall plant. Proceedings of the 27th European Biomass Conference and Exhibition, 27-30 May 2019, Lisbon, Portugal, pp. 454-461

The application of CFD simulations is gaining increasing importance for the development of furnaces and boilers. Despite the high complexity of the underlying processes, BIOS has developed a CFD model for biomass grate furnaces and successfully applied it for the development and optimisation of numerous plants of all scales. In this work, selected results from the simulation of continuously operated furnaces, wood log fired stoves and boilers, as well as results from CFD simulations of NOx and fine particle formation are shown in order to demonstrate the advantages and application possibilities of advanced CFD simulations for small­scale biomass combustion plants. The results show, that the CFD­aided technology development leads to reduced emissions, a better utilisation of the furnace volume, an increased fuel flexibility and an enhanced plant efficiency. Furthermore, CFD simulations result in reduced development times, test efforts and an increased security during plant development. Finally, a deepened understanding of the processes in biomass combustion plants is achieved. The applications of CFD models for small­scale plants show, that they are highly efficient tools for the development of new plant series. Proceedings of the 19th European Biomass Conference and Exhibition, 6-10 June 2011, Berlin, Germany, pp. 4-12

The work presented is part of a project that aims towards a deeper understanding of various pellet properties on the process of pellet combustion by means of test runs and simulations. A new transient CFD model for pellet combustion and packed bed compaction due to pellet shrinkage is described in detail and comparisons of the simulation results with the results from dedicated lab-scale reactor test runs are presented. The correspondence between simulated and measured mass loss, temperature and species concentration trends is well agreeable, however, space for improvement with regard to the velocity of charcoal burnout is still left. Proceedings of the 28th European Biomass Conference and Exhibition, 6-9 July 2020, Virtual, pp. 263-270

This paper presents experimental results derived from test runs performed with a laboratory-scale updraft fixed-bed gasifier coupled to a combustion chamber to produce data for the investigation of the release behaviour and the conversion of fuel-bound nitrogen during gasification and subsequent staged combustion of the producer gas using softwood pellets. The concentrations of relevant nitrogenous gas species including tars have been measured in the producer gas and at different positions in the combustion chamber. Based on the experimental measurements and results derived from the test runs, the pathway of the fuel-bound nitrogen could be described. Results show that during updraft fixed-bed gasification the fuel-bound nitrogen is mainly released as N bound in tars from the packed bed and is then subsequently released as HCN, NO, NH3 and N2 as a result of tar cracking during combustion. This strong N-fixation in the tars was not expected. It is of great relevance for the understanding of the behaviour of the fuel-bound nitrogen as a basis for a low-NOx combustion of the producer gas. Proceedings of the 19th European Biomass Conference and Exhibition, 6-10 June 2011, Berlin, Germany, pp. 996-1001

Wood log fired stoves increasingly constitute effective heating systems due to new innovative concepts including heat storage devices. These devices accumulate a certain fraction of the heat released in a special storage medium for a certain period of time and release it after opening of discharge channels. The combustion of wood logs in small­scale stoves itself is a highly transient and complex process and the transient character of the operation of wood log stoves becomes even more important, when a heat storage system is included. The operation of a heat storage device is divided into 3 phases: heat­up, heat storage and heat discharge. We developed an innovative methodology based on transient CFD simulations in order to analyse the transient heat­up/heat discharge processes in heat storage devices. Selected results are presented including the heating rate of the storage material during the heat­up, the energy release during the storage and the discharging rate during the discharge phase. The influence of the air­flow in the discharging channels and the flue gas flow in the charging channels as well as material properties on the charging/discharging processes is discussed. Proceedings of the 21st European Biomass Conference and Exhibition, 3-7 June 2013, Copenhagen, Denmark, pp. 578-584

A detailed analysis of the fate of aerosol and deposit forming elements in an industrial scale waste-wood-fired updraft gasifier (fuel heat input 36 MW) is presented. The work included a comprehensive analysis of all mass flows (fuel, bottom ash, fly ash, additives and flue gas) followed by mass and element balances for selected evaluation periods to determine the release rates of the relevant aerosol and deposit forming elements from the fuel bed to the gas phase. The results show that especially Cl, Zn and Pb are very unevenly distributed over the fuel and their real concentration in the fuel is obviously significantly higher than the analyses results of the fuels indicate. The results of high temperature thermodynamic equilibrium calculations showed that elevated Cl contents are the reason for the observed high release rates of K and Na (compared to updraft gasifiers operated with virgin wood). Finally, the impact of the ash forming elements released from the fuel bed with the gas on boiler deposits was investigated by 1D simulations of aerosol and ash deposit formation by a specially developed software. The results showed that elevated contents of Cl, Zn, Pb, K, Na and S lead to increased boiler deposits which was also confirmed by measurements. Zn and Pb compounds amount to almost 50 wt% of the boiler deposits. The main share of the deposits are originally formed as chlorides and ZnO, but get quickly sulphated once they are deposited on the boiler tubes. Proceedings of the 29th European Biomass Conference and Exhibition, 26-29 April 2021, Online, pp. 425-434

The growing demand for more economical and environmentally friendly power generation forces the industry to search for fuels that can replace conventional fossil fuels. Fast pyrolysis oil is an example of these alternative fuels. Pyrolysis of biomass is one of the most promising ways to directly generate liquid fuels from biomass. However, pyrolysis oil may have several major drawbacks which suppress its application. Within the ERA-NET EnCat project several aspects of pyrolysis oil comprising production processes, characterization as well as combustion in gas turbines and engines are approached. The present work focusses on the application of fast pyrolysis oil in a gas turbine. A combined numerical and experimental approach has been applied to a real scale gas turbine combustor for the OPRA OP16 gas turbine. This includes full scale combustor tests with various fuels and advanced CFD simulations. When operating on solely pyrolysis oil, it was found that stable operation could be achieved in the 30-100% load. Advanced char burnout simulations have been performed, which determined the optimal conditions for pyrolysis oil combustion. The newly developed NOx formation and pyrolysis oil combustion model show a good agreement with experimental data. Proceedings of the 28th European Biomass Conference and Exhibition, 6-9 July 2020, Virtual, pp. 957-965

A detailed CFD analysis of the fuel bed of a pilot-scale (nominal fuel power 450 kW) updraft gasifier powered by softwood chips (spruce) is presented, and the results are compared to measurement data. The CFD model allows an in-depth analysis of the conversion process in the gasifier bed, as well as a detailed description of tar formation which is of great relevance regarding the subsequent gas treatment. The simulation results show good agreement with the measured data for the composition of the producer gas and for the temperature profiles inside the fuel bed. Furthermore, the simulation allows the identification of the main conversion zones for drying, pyrolysis, charcoal gasification and combustion inside the fuel bed. Proceedings of the 29th European Biomass Conference and Exhibition, 26-29 April 2021, Online, pp. 451-457