Reality Check for Agricultural Biofuels

Transcription

1 Elisabeth Wopienka, Günther Friedl, Walter Haslinger Alle: Austrian Bioenergy Centre GmbH, Außenstelle Wieselburg, Rottenhauserstraße 1, 3250 Wieselburg Ansprechperson: DI Elisabeth Wopienka, Tel , Reality Check for Agricultural Biofuels 1 Introduction The use of renewable energy sources for heat production has increased significantly during the past years. Due to high costs and an unstable market for fossil fuels as well as political directives pushing the use of CO 2 -neutral energy sources a further increase in this field can be expected. The increasing popularity of biomass combustion leads to a higher demand of wood fuels, and consequently to a stronger competition for this raw material with other sectors of industry. Amongst other things, these developments may explain the growing interest for the utilisation of agricultural products and by-products as raw materials for the production of biofuels. Various types of biomass like e.g. Miscanthus, energy grain or straw are taken into account looking for potential raw materials for heat production, depending on local conditions and market prices for agricultural products. Several boiler manufacturers followed this trend coming up with new products developed for the combustion of nonwooden biomass. In Austria fuel standards are available e.g. for energy grain or Miscanthus up to now. In view of the fast market development and the big number of potential biofuels, legislative framework and standardisation lags behind, and more detailed knowledge is required as a basis for such decisions. 2 Description of the work Small scale combustion units in the range of 16 to 35 kw have been investigated in field tests over a period of 3 years, operated with Miscanthus, straw, energy grain, maize whole crop but also residues from corn production. Once a year, the each boiler has been evaluated with regard to emissions and boiler efficiency. Fuel and ash samples have been taken and their combustion relevant parameters as well as elemental composition have been analysed. In addition users documentations have been evaluated in order to obtain information about the amount of Austrian Bioenergy Centre GmbH Firmensitz Graz Außenstelle Wieselburg Inffeldgasse 21 b Rottenhauserstr. 1 T ++43(0) Firmenbuch: FN k 8010 Graz 3250 Wieselburg F ++43(0) Landesgericht für ZRS Graz Österreich centre@abc-energy.at UID-Nr.: ATU [Beitrag_ doc, Druckdatum ] Als Mitglied des Fachverbandes vertreten bei

2 fuel consumed and ash that has been removed, incidents and costs and time needed for the maintenance of daily operation. This information was also used for the evaluation of economic efficiency. Overall system efficiency was determined from fuel consumption and the metering of heat provided to the heating system. Moreover, indications of wear and corrosion have been investigated annually and documented in order to identify higher stress due to the fuels used. 3 Results 3.1 Fuel properties Compared to wood, which is a relatively homogenous raw material, most agricultural biofuels show a big variety with regard to fuel properties. In general, the ash content is significantly higher than in wood, and at the same time ash melting temperatures are found to be much lower than for wooden biomass. These properties can lead to slag formation in the combustion zone and consequently cause problems with ash removal resulting in a breakdown of the combustion process. Moreover, combustion conditions might change and cause higher emissions of unburnt gases like carbon monoxide (CO) or volatile organic compounds (VOCs). Besides the higher ash content, agricultural biofuels often contain increased elemental concentrations of nitrogen, sulphur, potassium or chlorine. These elements can form harmful emissions like NO x, SO 2, HCl and particulate emissions. On the other hand sulphur and chlorine play a major role in corrosion reactions. In the presented work Miscanthus, straw, maize whole crop, corn, residues from maize production and different types of energy grain have been investigated. Fuel analyses showed broad variations with regard to fuel quality even within the same type of raw material. The ash content varied from 2-3,5% for Miscanthus and different types of energy grain, a bigger amount of ash was found for straw with 6-8%. Consequently the heating value showed the highest values for Miscanthus with 18 MJ/kg TS followed by energy grain with 17-17,5 MJ/kg TS and straw with 17 MJ/kg TS in average. The content of sulphur found in the investigated fuels was 40 times as high as in wood and for chlorine 30 to 100 times as much respectively. For nitrogen clear differences could be observed between the raw materials. The nitrogen content in Miscanthus was found to be in the range of 0,2-0,3% followed by straw with 0,4-0,7%. With 1,9-2,5% for energy grain and 1,1-1,5% for maize products and residues the values were significantly higher. 3.2 Combustion performance and efficiency For all appliances investigated, the boiler efficiency was determined between 80% and 90%. The biggest losses are due to the amount of heat in the flue gas. High flue gas temperatures can be caused by insufficient operation of the heat exchanger due to depositions. Therefore, an efficient heat exchanger cleaning device is essential. On the other hand a higher volume flow due to an increased excess air ratio can reduce the 2/5

3 residence time of the flue gas in the heat exchanger and therefore also increase the temperature of the outgoing gas. The efficiency of the total heating system was determined between 58 75%, and no clear correlation to the efficiency of the combustion unit could be found. However, not the combustion unit itself but more the concept of the whole plant was identified to be relevant influencing factor. In general, all users were content with the performance of the combustion unit, though all of them reported that the time needed for the support of the combustion unit has increased compared to appliances operated with wood or fossil fuels 3.3 Emissions In general, CO emissions were below the threshold value of 500 mg/mj. In some cases the CO emissions were high, but could be reduced by optimising the combustion parameters. Especially during the combustion of straw and maize residues, high emission values were measured. NO x -emissions are increasing with increasing nitrogen content in the fuel. This correlation could also be verified from the experimental results. Furthermore, a remarkable influence of the combustion technology could be observed. The threshold value of 300 mg/mj was kept for most combustion experiments with straw and Miscanthus, emissions from the combustion of maize products and energy grain generally exceeded the threshold value. A high excess air ratio during combustion can cause increased CO-emissions by reducing the temperature in the combustion chamber, and also NO x -emissions might increase with more oxygen available in the combustion zone. Moreover, efficiency is reduced by discharging bigger amounts of heat with the flue gas. Dust emissions from complete combustion mainly consist of aerosol-forming elements like e.g. potassium, sulphur or chlorine. Emission values from all field tests except the combustion of Miscanthus exceeded the threshold value of 60 mg/mj 1. By trend, a correlation of dust emissions and the amount of aerosol forming elements in the fuel could be observed. The highest values were found from the combustion of straw, the lowest ones for the combustion of Miscanthus. 3.4 Corrosion Severe corrosive attack on refractory materials was particularly found in appliances operated with energy grain - especially in areas of the combustion zone where highest temperatures are reached. The material in heat exchangers was mainly affected in the rear part before entering the flue gas pipe. The maximum abrasion after three years of operation was about 10% of the total thickness of the material. It has to be mentioned, that only surface abrasion was considered by the measurements and e.g. pitting corrosion was not taken into account. 1 Entwurf einer Vereinbarung zwischen dem Bund und den Ländern gemäß Art. 15a B-VG Inverkehrbringen und Überprüfung von Feuerungsanlagen. Wien, /5

4 The combustion of straw mainly caused corrosive attack on metal parts in the combustion zone. The combustion units operated with Miscanthus suffered only from little corrosion attack. After operation with high chlorine pellets, some parts had to be changed after a period of 4 weeks. 3.5 Economic Efficiency In order to evaluate the economic efficiency, the investigated plants have been compared to systems operated with wood pellets, wood chips and oil. Moreover, three different capacities have been considered for the calculations (20-25 kw, kw and kw), and the influence of reduced durability due to a higher corrosion potential of agricultural biofuels has been taken into account. In general it can be concluded that with increasing capacity of the unit the influence of consumption-related costs like fuel costs become more important. Combustion units operated with maize whole crop where found to be the most efficient ones for all three capacities investigated, followed by appliances operated with wheat and wood chips. The use of barley or corn is even a little more expensive due to higher fuel costs. Combustion units operated with oil are mainly competitive for low capacities due to the high fuel costs. From the economic point of view the combustion of straw pellets is not attractive either, because of the costs of fuel production. In order to overcome that, an optimization of the whole logistics chain from the raw material on the field to the product is required. Assuming a reduced durability of the combustion units with 15 years when operated with agricultural biofuels instead of 20 years for wood combustion appliances, the use of energy grain is not efficient for all three capacities investigated when compared to wood chips. Compared to pellets boilers, average annual costs are comparable for a capacity of about 40 kw. For capacities of kw the combustion of energy grain is even more efficient, despite the reduced durability. 4 Conclusions The biggest challenge in the technological development of agricultural biofuel combustion systems is the handling of big amounts of ash with a strong tendency to form lumps, slags and depositions. Moreover, a reduction of NOx and dust emissions is required in order to fulfil future legal requirements. A strong influence of the operating conditions and control settings on the release of products of incomplete combustion as well as the efficiency of the combustion unit was found. Considerable improvements could be reached by adjusting each plant to the particular fuel. Corrosion mechanisms especially for refractory materials are not determined at the present time. However, the corrosion potential of agricultural biofuels is much higher than of wood and thus and thus usually more expensive materials are required in order to withstand the increased corrosive attacks. 4/5

5 From the economic point of view, the combustion of agricultural biofuels in small scale appliances is not competitive to wood combustion by now due to the high investment costs. Starting at a capacity of 40 to 70 kw combustion units for energy grain e.g. might be efficient when compared to wood pellets. 5 Future perspectives Aiming at a share of 34% of renewables for energy production by 2020, an efficient use of all available resources is required. Knowledge about the combustion properties of different types of biomass builds the basis for decisions regarding the use of these raw materials. A lot of work has been done focusing on ash removal technologies during the last years, and further developments are required especially for high-ash fuels like e.g. straw. Further development of the combustion technologies is also required in order to reduce emissions from biofuel combustion. In order to enable an adequate adaption of the combustion conditions to a wide range and strong variations of fuel properties, more flexible and more reliable control strategies will be necessary. Further research is required for an investigation of corrosion mechanisms especially for refractory material. Therefore, long term experiments under different conditions and with varying fuels are recommended. In general, further research is required in order to identify the interaction of fuel properties and combustion behaviour, and to be able to guarantee environmentally friendly and cost-effective biomass combustion. 6 Acknowledgement The presented work has been funded by the Regional Government of Upper Austria in the frame of the Upper Austrian Energy Technology Programme and by the Regional Government of Lower Austria, which shall both be highly acknowledged. Moreover, the authors would like to thank the cooperating partners: Upper Austrian Biomass Association, FJ-BLT Wieselburg, Fröling, Gerlinger, Hargassner, KWB, ÖkoFEN, Seifried GmbH and TGK GmbH. 5/5