Wood pellet production costs under Austrian and in comparison to Swedish framework conditions

Owing to the rapidly increasing importance of pellets as high-quality biomass fuel in Austria and Europe within the last years, many companies, mainly from the wood industry, are thinking of entering this market. The calculation of the production costs before starting a pellet plant is essential for an economic operation. Based on comprehensive investigations within the EU-ALTENER project “An Integrated European Market for Densified Biomass Fuels” calculations of the pellet production costs loco factory for different framework conditions with basic data based on already realised plants as well as a questionnaire survey of pellet producers in Austria, South Tyrol and Sweden have been performed.

The production costs for wood pellets are mainly influenced by the raw material costs and, in the case of using wet raw materials, by the drying costs. Depending on the framework conditions these two parameters can contribute up to one-third of the total pellet production costs. Other important parameters influencing the pellet production costs are the plant utilisation (number of shifts per week) as well as the availability of the plant. For an economic production of wood pellets at least three shifts per day at 5 days per week are necessary. An optimum would be an operation at 7 days per week. A low plant availability also leads to greatly increased pellet production costs. A plant availability of 85–90% should therefore be achieved.

The calculations show that a wood pellet production is possible both in small-scale (production rates of some hundred tonnes per year) as well as in large-scale plants (some ten thousand tonnes per year). However, especially for small-scale units it is very important to take care of the specific framework conditions of the producer, because the risk of a non-economic pellet production is considerably higher than for large-scale systems.

The direct comparison of typical pellet production costs in Austria and Sweden showed the Swedish pellet production costs to be considerably lower due to larger plant capacities, the combination of pellet production and biomass CHP or biomass district heating plants and the implementation of technologies which allow an efficient heat recovery from the dryers. Moreover, another difference between the Austrian and the Swedish framework conditions is the price of electricity, which is much lower in Sweden.     

Automated analysis of submicron particles by computer-controlled scanning electron microscopy

Automated analysis of submicron particles by computer-controlled scanning electron microscopy is generally possible. The minimum diameter of the detectable particles is dependent on the mean atomic number of the particles and the operating parameters of the scanning microscope. The main limitation with regard to particle size is set by the quality of the particle detection system, which generally is the backscatter electron detector. The accuracy of the results of the x-ray analyses is very often strongly affected by specimen damage, omnipresent especially for environmental particles even at low electron energies and probe currents. With the exception for light elements, the detection limit is approximately 1 wt%. Device-related limitations to automated analysis may be specimen drift and an unreliable autofocus function.     

Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behaviour

With respect to the use of densified biomass fuels in fully automatic heating systems for the residential sector a high quality of these fuels is required. Several European countries already have implemented standards for such fuels. In other countries such standards are in preparation or planned. Furthermore, in some countries also standards from associations are existing (e.g. from the Austrian Pellets Association). In addition to these national standards, European standards for solid biomass fuels are under development. For producers of densified biomass fuels, especially for pellet producers, it is therefore very important to produce high-quality fuels keeping the limiting values of the standards addressed. However, in this context it has to be considered that as a high fuel quality as is necessary for the combustion of densified biomass fuels in automatic small-scale furnaces is not necessary if these fuels are used in larger industrial furnaces as they are equipped with more sophisticated flue gas cleaning, combustion and process control systems. Two pellet qualities, one for industrial and one for small-scale consumers seem to be more meaningful.

Within the framework of the EU-ALTENER-project “An Integrated European Market for Densified Biomass Fuels (INDEBIF)” a questionnaire survey of European producers of densified biomass fuels was performed. In this connection the possibility was offered to the producers to participate in an analysis programme with their fuels. An overview was obtained of the qualities of densified biomass fuels offered in the European market, covering pellets and briquettes from Austria, Italy, Sweden, Spain, Norway and the Czech Republic.

The parameters analysed were the dimensions of the fuels, the bulk and the particle density, the water and the ash content, the gross and the net calorific value, the abrasion, the content of starch (as an indication for the use of biological binding agents), the concentrations of C, H, N, S, Cl, K as well as of the heavy metals Cd, Pb, Zn, Cr, Cu, As and Hg. These parameters have been chosen following the Austrian, German, Swiss and Swedish standards for densified biomass fuels.

The results showed that a majority of the participating producers produce fuels of high quality. However, wood pellets of some producers show a high abrasion, one of the most important quality parameters for pellets. An increased amount of fines often causes failures in the feeding systems used in the residential heating sector. In order to decrease abrasion, the addition of small amounts of biological binding agents (e.g. maize or rye) is possible. This kind of additive is most common in Austria.

Moreover, some producers obviously use not only chemically untreated raw materials or additives, which increase the content of pollutants. Such fuels cause problems regarding emissions, deposit formation and corrosion. Emission problems are expected due to increased contents of N, Cl, S as well as heavy metals. Increased concentrations of heavy metals additionally contaminate the ash, increased Cl concentrations raise the risk of corrosion. Moreover, an increased content of K has a negative effect on the ash melting behaviour and causes higher aerosol formation, which enhances deposit formation and particulate emissions.     

Characterisation of fuel bound nitrogen in the gasification process and the staged combustion of producer gas from the updraft gasification of softwood pellets

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 characterisation of fixed-bed gasifier operation and to investigate the release behaviour and the conversion of fuel-bound nitrogen during gasification and subsequent staged combustion of the producer gas using softwood pellets. Spatial temperature profiles and the composition of the producer gas of the gasifier have been measured for different air flow rates. In addition, the concentrations of relevant nitrogenous gas species including tars have been measured in the producer gas and at different positions in the combustion chamber. The air flow rate has a significant influence on the composition of the producer gas and the temperature profile of the packed bed of the gasifier. Results show that during updraft fixed bed gasification almost the entire fuel-bound nitrogen is 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 and is of great relevance concerning NO_x formation during combustion of the producer gas.     

From waste to raw material—the route from biomass to wood ash for cadmium and other heavy metals

Energetic utilization of biomass is considered an environmentally safe way of providing energy, especially for process heat and district- heating purposes. The main advantage of energy from biomass is the CO₂-neutrality of this energy-production process. However, this process produces a solid by-products, namely ash, that has to be considered. This ash contains nutrients like calcium, potassium and phosphorus that should be recycled to forest or agricultural soils, thus closing not only the carbon cycle but also the fluxes of mineral materials caused by these technologies. The problem is, however, that besides nutrients, the ash also contains heavy metals. Cadmium poses a special risk to the use of wood ash in agriculture. It pollutes a large fraction of the ash generated in a biomass combustion plant, namely the cyclone fly-ash and, to an even higher degree, the filter fly-ash or (where flue gas condensation is installed) the condensation sludge. A medium-term solution to the recycling of solid residues from biomass combustion is blending cyclone fly-ash and bottom ash and using the mixture in agriculture. Although a large part of nutrients might be recycled in this manner, care has to be taken of the relatively high amount of cadmium in this material. A new technology currently under development takes advantage of the different temperatures in a biomass combustion plant. This technology enables concentration of cadmium (and other volatile heavy metals) in a very small portion of the whole ash flux from a plant and the concentrations of environmentally relevant substances in the remainder of the ash is kept low. In this manner, wood ash from the process industry or district heating systems might be transformed from waste to raw material for agricultural use.     

Modelling of grate combustion in a medium scale biomass furnace for control purposes

A new mathematical model for the grate combustion of biomass has been derived from physical considerations. Various models for grate combustion can already be found in the literature. Usually their intention is to simulate the real situation in a furnace as precisely as possible. Hence they are very detailed, typically consisting of many partial differential equations. However, because of their complexity they are useless for control purposes. The new model is very simple, consisting of only two ordinary differential equations, which makes it particularly suitable as a basis for model based control strategies. To verify the model, experiments were performed at a pilot scale furnace with horizontally moving grate. The pilot plant is a downscaled version (180 kW_th) of a typical medium scale furnace in terms of geometry and instrumentation. Comparison of the measured and calculated values shows good agreement.     

Experimental investigation of nitrogen species release from different solid biomass fuels as a basis for release models

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.     

Modelling of an updraft fixed-bed gasifier operated with softwood pellets

This paper presents a one-dimensional steady state mathematical model for the simulation of a small scale fixed-bed gasifier. The model is based on a set of differential equations describing the entire gasification process of softwood pellets and is solved by a two step iterative method. The main features of the model are: homogeneous and heterogeneous combustion and gasification reactions, one-step global pyrolysis kinetics and drying, heat and mass transfer in the solid and gas phases as well as between phases, heat loss, particle movement and shrinkage within the bed. The pyrolysis model has been improved by partially cracking primary tar into lighter gases according to experimental data. The model is used to simulate a laboratory scale fixed-bed updraft gasifier. Good agreement is achieved between prediction and measurements for the axial temperature profiles and the composition of the producer gas. Moreover, results are presented for different air to fuel ratios and varying power inputs. The gasification process is improved by increasing the power input of the gasifier as a result of higher temperatures. Furthermore, a higher air to fuel ratio lowers the efficiency of the gasification process.