This paper briefly discusses the relevance of small-scale biomass combustion systems for European renewable energy policy. Moreover, the state-of-the-art of modern small-scale biomass combustion systems is presented. The different technologies are reviewed regarding emissions and efficiency for different biomass. On-going developments and innovative approaches are presented and discussed.

Straw, Miscanthus, maize, and horse manure were reviewed in terms of fuel characteristics. They were tested in existing boilers, and the particulate and gaseous emissions were monitored. The ash was analyzed for the presence of sintered material. All the fuels showed problems with ash lumping and slag formation. Different boiler technologies showed different operational performances. Maize and horse manure are problematic fuels regarding NO_x and particulate emissions. Miscanthus was the best fuel tested. Due to the big variation of fuel properties and therefore combustion behavior of agricultural biomass, further R&D is required to adapt the existing boilers for these fuels.

Experimental data on the release of NO_x precursors from solid biomass fuels during thermal conversion are necessary to study N release in general and to supply reliable data for the purpose of packed bed and gas phase conversion model development and validation. In this work the release of NO_x precursors was studied at a lab-scale pot furnace (batch reactor) by taking measurements during the conversion process of solid biomass in a packed bed. The investigations were carried out with relevant woody biomass fuels, which cover a broad range of fuel N contents: sawdust, bark, waste wood and MDF board. The most important NO_x precursor detected above the fuel bed under fuel rich conditions was NH₃, while HCN was almost insignificant with the exception of sawdust. NO was detected mainly under air rich conditions. Furthermore, the experimental data were utilised to derive release functions for the relevant NO_x precursors NO, NH₃ and HCN. The release functions were implemented in an in-house empirical packed bed combustion model, which serves as a basis for a subsequent CFD N species gas phase calculation. © 2007 Elsevier Ltd. All rights reserved.

For an appropriate operation of a heat exchanger it is very helpful to know its dynamic behaviour. To this a simple sufficient accurate nonlinear model for the description of the dynamic behaviour is derived on the basis of a gas tube heat exchanger. Due to the general approach used for the derivation the model could be adaptet easily for other types of heat exchangers. The presented model can be used to estimate not measured physical values, to monitor the deposit formation in the heat exchanger and as a basis for the design of a model based control strategy. © Oldenbourg Wissenschaftsverlag.

The availability of energy is important to our every day lives. Biomass-fuelled heating systems are comfortable and reach an efficiency form over 90 %. With a thermoelectric generator (TEG) it’s possible to convert a part of the heat directly into electrical power and so become self sufficient from electicity. The purpose of this thesis was to optimise an existing prototype of a combined heat and power (CHP) plant based on a pellet heating system and a thermoelectric generator. Balancing the energy flows, especially the losses, was also part of the thesis.
Tests with the prototype were done. Some with the originial prototype, some with additional insulation and some with preheating the combustion air. To examine the part load behaviour, tests were done at 10, 7 and 4 kW fuelheat input.
By insulating the TEG the performance rose from 153 W to 174 W. The insulation and the preheating of the combustion air from room temperature to 350 degree lead to an power output from 194 watt. All at 10 kW fuellheat input. Finally the following conclusions can be drawn: For the series product it is recommended to optimize the insulation of the TEG. As the preheating of the combustion air didn’t lead to the expected effects it should be left out.

This paper presents one part of the results of a project dealing with straw pellets combustion in small
scale combustion systems. Whereas the other part of the work investigates gaseous and particulate emissions, this part focuses on the results of experiments to determine corrosion of refractory material. Three different types of straw
pellets are combusted in a prototype of a 15 kW residential heating boiler. The fuel samples are natural wheat straw,
wheat straw with alumina based additive and wheat straw with a mixture of calcium-/magnesium carbonate based
additive. Combustion experiments are performed under different operating conditions of the test boiler. Three
different types of refractory material are used as combustion chamber material. The refractory materials are different
mixtures of alumina, silica, zirconia and silicium-carbide. The degree of deterioration of these materials is
investigated for temperatures between 700 and 1300 deg C in the presence of slag formed during combustion of the
straw samples and the influence of the fuel additives on corrosion effects is analysed.

The primary energy consumption world-wide is rising constantly. Therefore it is necessary to open up renewable resources for energy production. Besides wood, the application of agricultural resources and residuals for energy production is possible, also within the range of small scale combustion units. These fuels still pose a challenge, concerning gaseous and particulate emissions. This work examines the application of straw pellets in a small scale combustion unit. Gaseous and particulate emissions, as well as the separation eciency of a secondary heat exchanger with scrubber were investigated. Compared with wood-like fuels a strong slagging of the combustion chamber could be determined. Gaseous emissions as NO_x, SO₂ and HCl, as well as the emission of particles were clearly higher than with combustions of wood. The gaseous emissions were below the considered limit value for other biogenous fuels after Art. 15 a B-VG 2007 [1]. The burnout of the gaseous phase, which can be evaluated by the emission of CO, was always good and comparable with the combustion of wood.
Using a secondary heat exchanger with scrubber (Hydrocube R of the company Schräder ) particulate emissions could be reduced by 20%. Element analysis of the particulate emissions as well as particle size measurements showed that primarily large particles were separated. A retrot of the Hydrocube R by an ionizing electrode increased the degree of separation on 60%. Besides the separation of particles, the Hydrocube R also reduced gaseous emissions like SO₂ and HCl. The absorption of these components in the condensate phase caused a decrease of the pH value. Low ph value increased the corrosion of the Hydrocube R , what could be detected by rising concentrations on Fe, Ni and Cr in the condensate.