Wood log stoves are a common residential heating technology that produce comparably high pollutant emissions. Within this work, a detailed CFD model for transient wood log combustion in stoves was developed, as a basis for its optimization. A single particle conversion model previously developed by the authors for the combustion of thermally thick biomass particles, i.e. wood logs, was linked with CFD models for flow and turbulence, heat transfer and gas combustion. The sub-models were selected based on a sensitivity analysis and combined into an overall stove model, which was then validated by simulations of experiments with a typical wood log stove, including emission measurements. The comparison with experimental results shows a good accuracy regarding flue gas temperature as well as CO2 and O2 flue gas concentrations. Moreover, the characteristic behavior of CO emissions could be described, with higher emissions during the ignition and burnout phases. A reasonable accuracy is obtained for CO emissions except for the ignition phase, which can be attributed to model simplifications and the stochastic nature of stove operation. Concluding, the CFD model allows a transient simulation of a stove batch for the first time and hence, is a valuable tool for process optimization.

Wood pellets have been used in domestic heating appliances for three decades. However, because the share of renewable energy for heating will likely rise over the next several years, alternative biomass fuels, such as short-rotation coppice or energy crops, will be utilized. We tested particulate emissions from the combustion of standard softwood pellets and three alternative pellets (poplar, Miscanthus sp., and wheat straw) for their ability to induce inflammatory, cytotoxic, and genotoxic responses in a mouse macrophage cell line. Our results showed clear differences in the chemical composition of the emissions, which was reflected in the toxicological effects. Standard softwood and straw pellet combustion resulted in the lowest PM₁ mass emissions. Miscanthus sp. and poplar combustion emissions were approximately three times higher. Emissions from the herbaceous biomass pellets contained higher amounts of chloride and organic carbon than the emissions from standard softwood pellet combustion. Additionally, the emissions of the poplar pellet combustion contained the highest concentration of metals. The emissions from the biomass alternatives caused significantly higher genotoxicity than the emissions from the standard softwood pellets. Moreover, straw pellet emissions caused higher inflammation than the other samples. Regarding cytotoxicity, the differences between the samples were smaller. Relative toxicity was generally highest for the poplar and Miscanthus sp. samples, as their emission factors were much higher. Thus, in addition to possible technical problems, alternative pellet materials may cause higher emissions and toxicity. The long-term use of alternative fuels in residential-scale appliances will require technological developments in both burners and filtration.

Describing the combustion of log wood and others solid fuels with complex geometry, considerable water content and often heterogenous struture is a nontrivial task. Stochastic Cellular Automata models offer a promising approach for modelling such processes. Combustion models of this type exhibit several similarities to the well-known forest fire models, but there are also significant differences between those two types of models. These differences call for a detailed analysis and the development of supplementary modeling approaches. In this
article we define a qualitative two-dimensional model of burning log wood, discuss the most important differences to classical forest fire models and present some preliminary results.

The necessity of recycling anthropogenically used phosphorus to prevent aquatic eutrophication and decrease the economic dependency on mined phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The aim of this work was to identify the P recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these fuel blends in comparison with pure sewage sludge. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could increase the degree of slag formation and enhance the volatilization of K in comparison with combustion conditions. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is emphasized that the results of this theoretical study represent an idealized system since in practice, non-equilibrium influences such as kinetic limitations and formation of amorphous structures may be significant. However, applicability of thermodynamic calculations in the prediction of molten and solid phases may still guide experimental research to investigate the actual phosphate formation in the future.

The use of biomass as feedstock for gasification is a promising way of producing not only electricity and heat but also fuels for transportation and synthetic chemicals. Dual fluid bed steam gasification has proven to be suitable for this purpose. Olivine is currently the most commonly used bed material in this process due to its good agglomeration performance and its catalytic effectiveness in the reduction of biomass tars. However as olivine contains heavy metals such as nickel and chromium no further usage of the nutrient-rich ash is possible and additional operational costs arise due to necessary disposal of the ash fractions. This paper investigates possible alternative bed materials and their suitability for dual fluid bed gasification systems focusing on the behavior of the naturally occurring minerals olivine, quartz and K-feldspar in terms of agglomeration and fractionation at typical temperatures. To this end samples of bed materials with layer formation on their particles were collected at the industrial biomass combined heat and power (CHP) plant in Senden, Germany, which uses olivine as the bed material and woody biomass as feedstock. The low cost logging residue feedstock contains mineral impurities such as quartz and K-feldspar which become mixed into the fluidized bed during operation. Using experimental analysis and thermochemical it was found that the layers on olivine and K-feldspar showed a significantly lower agglomeration tendency than quartz. Significant fractionation of particles or their layers could be detected for olivine and quartz, whereas K-feldspar layers were characterized by a higher stability. High catalytic activity is predicted for all three minerals once Ca-rich particle layers are fully developed. However quartz may be less active during the build-up of the layers due to lower amounts of Ca in the initial layer formation.
 

By using a microwave generator as energy source wood gets converted into three products: (1) condensate (“product oil”), (2) product gas and (3) charcoal (“material residue”). In this microwave-based specific kind of pyrolysis process wood is used as standard input material in order to have the possibility to compare the three generated products either with products of already established conventional pyrolysis processes [1] or other processes like gasification within thermo-chemical conversion [2]. Therefore, a discontinuous microwave apparatus of technical standard size (magnetron power: 6 kW, magnetron frequency: 2.45 GHz) is used.

This paper presents the virtual biomass grate furnace, which comprises of comprehensive CFD models of all relevant processes for the simulation of biomass grate furnaces. The models consist of a 3D packed bed model, a gas phase combustion model for laminar to highly turbulent flows and a model to account for the influence of the flue gas streaks arising from the fuel bed in the freeboard. The simulation results of a 20 kW underfeed stoker furnace show that the overall CFD model is able to provide valuable insight on the processes occurring in the packed bed and freeboard and their interactions.

In modern buildings with tight shells, often room-independent air supply is required for proper operation of biomass stoves. One possibility to arrange this supply is to use a double-wall chimney with flue gas leaving through the pipe and fresh air entering through the annular gap. A one-dimensional quasi-static model based on balance equations has been developed and compared with experimental data. Inclusion of leak air is crucial for reproduction of the experimental results. © 2014, Springer-Verlag Berlin Heidelberg.

The utilization of biomass for the substitution of fossil fuels to reduce greenhouse gas emissions in biomass steam gasification plants is a promising technology for the production of electricity, heat, and fuels for transportation. Experience from industrial scale dual fluidized bed steam gasification plants showed a modification of the bed material due to the interaction of the bed material (olivine) with biomass ash components and additives. In this paper the influence of bed material modification on the gasification properties of used olivine from an industrial scale plant in Güssing is compared with the case of fresh olivine. The trials were carried out under similar conditions in a pilot plant at the Vienna University of Technology. The pilot plant trials showed an increase in hydrogen and carbon dioxide in the product gas with the used bed material while the content of carbon monoxide in the product gas decreased. The exothermal water–gas shift reaction is enhanced by the used bed material, resulting in a lower energy demand for the gasification. Tar content was decreased by around 80% for tars detected by gas chromatography–mass spectrometry (GCMS) and the composition of the tar showed less components during the trial with used bed material.

The results obtained with the used bed material at the 100 kW pilot plant are in good agreement with those for the 8 MW industrial plant in Güssing, confirming good scale-up properties from the 100 kW plant to industrial scale plants.

The bed material and especially its catalytic activity plays an important role in biomass steam gasification in dual fluidized bed gasifiers. The bed material is modified by interaction with biomass ash during operation of the gasification plant forming layers at the particles which are induced by the biomass ash. Optimization of dual fluidized biomass steam gasification will have significant influence on the process variables such as temperatures, inorganic composition and product gas composition. The influence of these changes on layer formation is still unknown. This paper summarizes results of investigations about bed material characteristics taken from the industrial-scale biomass steam gasification plant in Güssing where woody biomass is used as fuel. Analyses of the surface and the crystal structures of the bed material particles treated in gasification and combustion atmospheres were carried out. The thermal behavior of used olivine and fresh olivine in different atmospheres was analyzed. A suggestion for the mechanism of formation of the layers is presented and the influence of possible optimization measures is discussed. A change in the elemental composition of the surface was not detectable but a slight change in the crystal structure. Thermal investigations show a weak endothermic weight loss with used olivine in a CO₂-rich atmosphere which could not be determined with fresh olivine. The formation of layers at the olivine particles is considered to be caused by the intensive contact with burning char particles in the combustion reactor. © 2013 Elsevier B.V.

The phenomenon of off-gassing from wood pellets during storage has been the cause of several, in some cases fatal, accidents due to toxic atmospheres in storages. To optimize safety measures the nature of the responsible processes needs to be clarified. In this study the impact of O₂ availability, which is a decisive factor for the presumed oxidation of fatty acids, is pointed out. Off-gassing rates of CO, CO₂, VOC, and CH₄ of pellets at relatively constant O₂ levels of approximately 35%, 20%, and <1% over a period of 20 d at approximately 295 K were investigated. For this purpose 7 kg of spruce pellets was stored under simulated ventilation of the atmosphere in a 31 L tank. Gas concentrations were determined every 24 h by GC-FID/TCD. Compared to the mean emission rates at 35% O₂ of CO (0.22 mg kg^–1pellets_d.b. in 24 h) and CO₂ (0.76 mg kg^–1pellets_d.b. in 24 h) the lowest O₂ concentration of <1% resulted in a significant reduction of off-gassing rates of 40% for both gases. In contrast the release rates of VOCs and also CH₄ decreased with the higher O₂ concentration (0.035 to 0.025 mg kg^–1pellets_d.b. in 24 h; 0.0085 to 0.0061 mg kg^–1pellets_d.b. in 24 h), presumably, because of increased onward reactions to CO and CO₂. Since off-gassing was not prevented by the lack of O₂ (<1% O₂-trial) it is assumed that the O₂ required for the reactions originated from the biomass itself. During the storage of pellets at 20% O_2, emission rates of CO (0.18 mg kg^–1pellets_d.b. in 24 h) and CO₂ (0.79 mg kg^–1pellets_d.b. in 24 h) at the start decreased by more than 20% and those for VOCs (0.032 mg kg^–1pellets_d.b. in 24 h) by almost 30% after 3 weeks. It can be assumed that in ventilated storages the reactivity and thus a potential risk from off-gases from wood pellets decreases considerably in only a few weeks. The effects of aging, in terms of declining reactivity at relatively constant tank conditions, on off-gassing rates could be clarified for the first time. A realistic development of the decline of reactivity of the material itself could be determined.