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Combustion of horse manure for heat production
Bioresource Technology 100 (2009) 3121–3126
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Combustion of horse manure for heat production J. Lundgren a,*, E. Pettersson b a b
Division of Energy Engineering, Luleå University of Technology, S-971 87 Luleå, Sweden Energy Technology Centre, Box 726, S-941 28 Piteå, Sweden
a r t i c l e
i n f o
Article history: Received 20 January 2009 Accepted 23 January 2009 Available online 3 March 2009 Keywords: Biomass combustion Horse manure Heat production
a b s t r a c t The main objectives of this paper have been to evaluate the use of horse manure and wood-shavings as a fuel for heat production and to provide sets of data on the chemical composition, ash characteristics and ash forming elements of the fuel. Another objective has been to investigate the possibility to use the ash as fertiliser by analysing the heavy metal and nutrient contents. The results showed that the fuel is well suited for combustion for heat production causing low emissions of products of incomplete combustion. The emissions of NOx were however high due to the high content of fuel bound nitrogen. Emissions of CO and NOx were typically in the range of 30–150 mg/Nm3 and 280–350 mg/Nm3 at 10 vol% O2, respectively. The analysis of the ash showed on sufficiently low concentration of heavy metals to allow recycling. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction There are roughly five million horses living in stables in Europe today. The residues from the horses contain solid as well as liquid portions of waste, typically about 60% solids and 40% liquids (Wheeler and Smith Zajaczkowski, 2002). Bedding materials are used on the floor of the stall boxes to absorb the liquid part and are exchanged regularly in order to keep a hygienic environment for both, people working in the stables and for the horses. The residues from the stables consist therefore of a mixture of manure, urine and bedding material. There are several different types of bedding materials that are used. The most common are woodshavings, sawdust, straw (silage or pellets), peat or paper pieces. The choice is strongly depending on which material that is available at low cost. In northern Sweden, wood-shavings are generally used due to the great accessibility. From an inquiry made, it was found that the used annual volume varied from 9 to 29 m3 per horse. The large variations are due to that the stables and riding schools often pay for the bedding material, and the share of the bedding material in the mixture depends strongly on how careful the keepers are when they clean the stall boxes (Pettersson and Lundgren, 2002). The options for handling the residues are recycling to agricultural land either through composting or via biogas production, deposition at landfills, combustion or other usage. In the study by Pettersson and Lundgren (2002), it was found that currently the largest part of the horse manure is recycled to agricultural land followed by deposition at landfills. Combustion and other usage such as soil production for lawns were of small importance. Wheeler and Smith * Corresponding author. Tel.: +46 920 491307; fax: +46 920 491047. E-mail address: [email protected] (J. Lundgren). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.01.050
Zajaczkowski (2002) have found that approximately 20 m3 of bedding material is used per horse each year, which is close to the average value of the inquiry. According to a law passed in the year 2001, landfill of organic material will be prohibited from 1st of January 2005 in Sweden (Swedish Codes of Statute, 2001). This law is based on a European Commission Directive (European Commission, 1999) which forces the member states to lower the landfill of biodegradable municipal waste to less than 35% of the amount produced in 1995. This means that the Swedish law is more stringent than the directive. Currently, composting of the horse manure to be used as fertiliser on arable land is a viable alternative for many horse stable owners if the arable land is located in the vicinity of the stable. Otherwise costly transports of large waste volumes would be required, which many horse owners can not afford. Moreover, cereal farmers often hesitate to accept composted horse manure as fertiliser since it may contain oat weeds. This would force the farmer to weed the fields manually as no other weed control is available. Many do not recommend spreading of composted manure mixed with wood-shavings because of a rumour saying that lignin and terpene contents tend to restrain the growth. According to a study by Steineck and Svensson (2000), there are no valid theories confirming this rumour. Due to the above mentioned reasons, many horse owners have large problems to get rid of the residues. For many stables, the energy content of the annual volume of bedding material used would easily cover the space heating demand and the demand for hot tap water over a year in a normal stable facility. There is a great interest in burning the manure for heat production amongst horse stable- and trotting course owners. By doing so, the stable and trotting course owners will on the one hand decrease the cost for heating of their facilities and on the other hand, decrease the waste volume. One important condition
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for motivating combustion of horse manure is that the emissions of harmful substances are kept at an environmentally benign level. The ash could potentially be used as fertiliser in forests, if the amount of heavy metals, like cadmium, is less than the prevailing regulations. Very few studies have been found on thermochemical conversion of horse manure for energy generation. Schuster and Strömberg (1997) presented results from combustion experiments using horse manure mixed with straw as a fuel at the trotting course of Färjestad in Sweden. The results indicate a poor combustion process with very high emissions of CO. However, there are several studies made regarding thermochemical conversion of other types of animal manures. Zhu and Lee (2005) have presented experimental results from co-combustion of poultry wastes with natural gas in an advanced Swirling Fluidized Bed Combustor (SFBC). Sweeten et al. (2003) have thoroughly investigated the fuel properties of cattle manure, while Annamalai et al. (2003a,b) presents emission data and operational problems during co-firing of coal and cattle manure in two different burners. Priyadarsan et al. (2005) have experimentally investigated gasification of coal and animal waste-based fuels like feedlot biomass (cattle manure) and chicken litter biomass under batch mode operation. In a paper by Sánchez et al. (2007), experimental studies regarding pyrolysis of mixtures of sewage sludge and manure are presented. Möller et al. (2007) investigated co-combustion of treated pig manure. In this paper, results from combustion experiments performed in a 250 kWth furnace especially designed for wet and inhomogeneous biomass fuels is presented. The purpose has been to evaluate horse manure and wood-shavings as a fuel for co-firing for heat production in horse stable facilities. The results are compared with emission data from combustion of wood-chips with high moisture content. Furthermore, this paper provides sets of data on the chemical composition, ash characteristics and ash forming elements of the fuel. Another objective has been to investigate the possibility to use the ash as fertiliser by analysing the heavy metal and nutrient contents.
2. Test facility and experimental setup The combustion experiments have been carried out in a 250 kWth biomass-based district-heating plant located in the northern part of Sweden, around 70 km south of the Arctic Circle. The plant is connected to the local district-heating network of the town of Boden. The experimental facility includes, besides a furnace and a heat transfer unit, a water heat store to handle heat load variations as well as two cyclones that are used for flue gas cleaning.
2.1. The combustion chamber The furnace is divided into a primary and a secondary zone. The primary combustion chamber is of a counter-current grate type, meaning that the flame direction is the opposite of the fuel flow. This type is appropriate for wet biomass fuels, due to the increased convective heat transfer contributing to an improved drying process of the fuel. After the final gas combustion in the secondary zone, the gases enter a convection boiler where the heat is transferred to the water. The cooled gases continue through a cyclone system. Fig. 2 shows a sketch of the primary combustion chamber. Two fuel feeding screws (1) are used to transport the fuel from an intermediate fuel store into the furnace. The fuel enters the combustion chamber at a first horizontal plane (2) and moves slowly towards a
slope. The purpose of the two planes is to dry the fuel before the combustion process starts, using heat transfer by radiation and convection from the combustible gases. Pyrolysis starts in the end of the slope and at the beginning of the fuel grate (4). The burning fuel bed is moved slowly forward towards the steps (5) and the ash bin (6) by short strokes from a rectangular piston (3). Final charcoal combustion takes place at the end of the second horizontal plane (4) and on the steps (5). The largest difference compared to a conventional counter-current grate furnace is that the primary air is supplied from above the fuel bed and not through the grate. In this furnace, the combustion air is introduced partly through slotted steel pipes integrated in the sidewalls and partly through a pipe in the front of the furnace as illustrated in Fig. 2. The secondary combustion chamber is cylindrically shaped in order to create a re-circulating flow and thereby enhance the large scale mixing and the combustion intensity. It is assumed that the most important factors for a good burnout rate are to create a good mixing between gases leaving the primary zone and secondary air and to maintain a high gas temperature. A proper method of achieving this is to supply pre-heated combustion air as high velocity air jets. To make the air jet penetration easier it is advantageous to have a low momentum of the primary gases and short distances for the secondary air jets to travel. Unfortunately, these two statements contradict each other since a low primary flow momentum requires a large cross sectional area, leading to larger penetration distances being required. The secondary air is pre-heated outside the cylinder to a few 100 °C depending on the thermal output. Also the gas residence time in the secondary combustion zone varies mainly with the thermal output, but is typically in the range of 0.6–1.6 s. The design was developed on the basis of CFD simulations and previous experiments. A thoroughly description of the design of the primary and secondary combustion chamber can be found in Lundgren et al. (2003, 2004a,b, 2005). 2.2. Measuring equipment The analysis of the flue gas composition is carried out immediately after the heat transfer unit shown in Fig. 1. A multi-component gas analyser for online measurements of NO, CO, CO2 and O2 (Maihak) and a heated THC analyser (JUM) were used. A NO/ NO2 converter (JNOx) was used to measure total NOx emissions. The gases were extracted using a heated probe and gas sample line maintained at a constant temperature of 120 °C. Table 1 shows the measuring methods and ranges for the different gas components. Gas temperatures have been measured before the secondary combustion chamber and after the heat transfer unit by using radiation-shielded thermocouples of type N. The location of the temperature gauges are shown in Fig. 1. Fuel samples have been dried in an electrical oven for at least 24 h at approximately 105 °C to determine the moisture content of the fuel. Samples have been taken regularly during the experiments.
3. Experimental results 3.1. Fuel characteristics and analysis The fuel consists of wood-shavings and horse manure (hereafter referred to as the fuel mixture), where the share of wood-shavings typically constitute 40–80 wt% depending on how careful the horse boxes are cleaned. The moisture content of the fuel mixture as received varies in the range of 45–65% (wet basis) mostly depending on if it is stored outdoors with cover or without.
J. Lundgren, E. Pettersson / Bioresource Technology 100 (2009) 3121–3126
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Fig. 1. Explanatory sketch of the test plant. Gas temperatures are measured between the primary and secondary zones as well as immediately after the heat transfer unit.
Fig. 2. 3-D sketch of the primary combustion chamber to the left and illustrations of the primary air supply to the right.
Several analysis of the ash content of the fuel mixture show that it varied typically in the range of 5–7% on dry basis. The volatile matter normally varied in the range of 72–74% on dry basis. An accredited laboratory SLU (The Swedish University of Agricultural Sciences, Umeå, Sweden) has performed analyses of the chemical composition of the horse manure and wood-shavings
Table 1 Measuring ranges and methods for different gas components.
O2 CO CO2 NO THC a b
Range
Method
0–25 vol% 0–1000/10,000 ppm 0–20 vol% 0–500 ppm 0–10/102/103/104/105 ppm
Paramagnetic O2 cell NDIRa NDIRa NDIRa FIDb
Non-dispersive infra red. Flame ionisation detector.
mixture. The results are shown in Table 2. For comparison, typical analyses of regular wood-chips and other types of manure fuels are also shown in the table. The nitrogen content of the horse manure fuel is higher than for typical wood-chips, but considerably lower than in poultry litter and feedlot manure. The sulphur and chlorine concentrations are also higher than wood-chips. The higher concentration of chlorine in the fuel will volatilise more potassium, while the sulphur will form potassium sulphate. Comparing the inorganic constituents of the fuel mixture with ordinary wood-chips show that the composition is fairly similar but manure has higher concentrations of silica, potassium and magnesium. The higher concentrations of potassium may decrease the sintering temperature of the ash. The horse manure does have lower concentrations of calcium which will influence the self-hardening ability of the ash. Analysis of the inorganic trace components of the horse manure fuel mixture is shown in Table 5.
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Table 2 Heating value, ultimate analysis, analysis of ash forming elements in different biomass fuels. Analysis
Wood-chips
Horse manure + wood-shavings
Poultry litterb
Feedlot manured
Heating value, calorimetric [MJ/kg dry basis] Lower heating value [MJ/kg dry basis]
20.56 19.21
19.37 18.14
– 6.03c
– 20.9
Ultimate analysis [wt% dry basis and ash-free] Carbon Hydrogen Nitrogen Sulphur Oxygena Chlorine Total
49.5–49.8 6.1–6.2 <0.1 <0.01 43.5–44 0.004 100
48.60 5.80 0.90 0.14 44.30 0.26 100
46.12 6.54 6.63 0.16 40.55 – 100
49.66 5.62 4.28 1.36 39.09 – 100
Ash forming elements [% of ash] Si K Na Ca Mg
7.0 4.3 0.3 24.3 3.0
15.5 7.3 0.8 11.4 5.8
– – – – –
– 11.7 2.93 13.6 5.08
a b c d
Calculated by difference. Data from Zhu and Lee (2005) re-calculated to ash-free. As received. Data from Sweeten et al. (2003) re-calculated to ash-free.
Table 3 Operating conditions during the experiments. Experiment
1
2
3
4
Average thermal output (kW) Fuel Fuel moisture contentb (wt%) Total air supply (m3/h) Primary/secondary air ratio Avg. combustion temperature (°C)
150 Wood-chips 50 356 2.06 985
150 Horse manurea 57 344 2.56 775
150 Horse manurea 49 490 1.38 921
150 Horse manurea 49 376 2.00 978
a b
Mixed with wood-shavings. On wet basis (w.b.).
3.2. Emissions Several combustion experiments have been carried out using horse manure mixed with wood-shavings as a fuel. During all experiments presented in this paper, the furnace was operating in between 4 and 15 h at the constant thermal output of 150 kW. As an attempt to improve the combustion process and reduce the emissions, the settings of the air supply (total volume flow, ration between primary and secondary air) were different in each of the experiments. Table 3 shows the most important operating param-
Fig. 3. Average emissions and O2 content during combustion of wood-chips and horse manure mixed with wood-shavings. All emissions are normalised to 10 vol% O2.
eters during the experiments. In experiment No. 1, wood-chips were used as a fuel and serves as a reference case. The combustion temperature is measured in between the primary and secondary zone. Fig. 3 shows the average emissions during the experiments. In experiment 2, the combustion temperature was too low causing an incomplete combustion process and thereby relatively high CO emissions. The O2 content was also high, approximately 12 vol%, which indicates that the secondary combustion air was diluting. In experiment 3, the water content of the fuel was lower, which led to higher combustion temperature and consequently lower emissions of CO. The O2 content was still unsatisfactory high. This was most probably caused by that too much secondary air was supplied. In experiment 4 using the same fuel as in experiment 3, the total air supply as well as the share of secondary air were reduced. These led to increased combustion intensity in the primary zone and consequently lower emissions of CO as well as lower air excess. It may be concluded that it is possible to burn horse manure mixed with wood-shavings with satisfactory CO emission levels with proper air supply and a water content of the fuel below 50 wt%. The most important difference between burning clean wood fuels like wood-chips or pellets and combustion of horse manure mixed with wood-shavings is the higher NOx emissions. The NOx emission levels are in some cases more than twice as high as during combustion wood-chips. As the fuel analysis shown in Table 2 shows, the horse manure mixture contains considerably more fuel bound nitrogen than wood-chips, which increases the formation of fuel-NOx.
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3.3. Analysis of the bottom ash Table 4 shows the major inorganic components of the bottom ash after combustion of the horse manure mixture. The analysis of inorganic constituents of the bottom ash compared with the fuel shows that potassium, phosphorus and sulphur have left the fuel. The chlorine content was not analysed due to the experience that chlorine is very seldom found in biomass ash since potassium chloride is volatile. Table 5 shows the inorganic trace elements in the ash as well as in the horse manure fuel mixture. The concentrations of chromium and nickel increased several times in the ash compared with the manure fuel. Table 4 shows that the iron content has increased substantially as well. This indicates that some stainless steel from the furnace has contaminated the ash. The fly ash was not collected during the experiments. Table 6 shows measured and recommended minimum and maximum concentrations of nutrients and trace elements in ash products to allow recycling to forests. The ash contained lower calcium concentration than the recommended limit. Wood ash contains high concentrations of calcium and does upon storage in humid climate self-harden. This is due to that the calcium in the ash form calcium hydroxide, which reacts with carbon dioxide forming limestone. A high calcium concentration means larger coverage of the particles with limestone Table 4 Major inorganic components of the bottom ash as well as in the horse manure mixture (% of ash). Element
Bottom ash
Fuel mixture
SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO2 Na2O P2O5 TiO2 S Cl
42.6 7.75 15.4 4.24 11.5 8.85 0.368 2.59 4.27 0.436 0.4 n.a.a
33.15 3.36 15.9 2.30 17.67 9.64 0.39 2.16 7.27 0.29 1.9 3.56
a
Not analysed.
Table 5 Inorganic trace components in the bottom ash and in the fuel (mg/kg dry basis). Element
Bottom ash
Fuel mixture
As Ba Be Cd Co Cr Cu Hg La Mo Nb Ni Pb S Sc Sn Sr V W Y Zn Zr
<3 656 0.892 <0.1 13.8 1000 105 <0.1 9.23 10.3 <6 378 5.4 4020 7.32 <20 464 70.8 <60 10.4 344 73.6
2.1 500 <0.55 1.55 6.85 72.5 104 0.2 <5.5 13.6 <5.5 24.8 5.2 19,178 4.6 <27 401 42.5 <55 6.5 683 46.7
Table 6 Recommended minimum and maximum concentrations in ash products to be recycled to forests (National Board of Forestry, 2008). Elements
Standard values
Lowest
Measured values
Highest
Macro nutrients g/kg DS Ca 125 Mg 15 K 30 P 7
110 53 95 18
Trace elements mg/kg DS B Cu Zn 500 As Pb Cd Cr Hg Ni V
800 400 7000 30 300 30 100 3 70 70
n.a.a 105 344 <3 5 <0.1 1000 <0.1 378 70.8
Organic pesticides mg/kg DS Total PAH (tentative)
2
n.a.a
a
Limiting values forest in Austria (Van Loo and Koppejan, 2007)
250 1500 100 8 250 100
Not analysed.
while a low concentration means a lower coverage corresponding to a more soluble ash product with less strength. The measured concentrations of nickel and chromium were also higher than recommended, but as may be seen from the iron analysis this is probably due to contamination of the ash by some stainless steel. Otherwise the ash fulfils the requirements except for the zinc content. The requirement on the lowest level for zinc is a strange requirement, since zinc is volatile both as a metal and as chloride. 4. Discussion The experimental results are so far promising. Further studies should be carried out to investigate the possibility to reduce the emissions of NOx. In Sweden, the emissions of NOx are in practice limited by a NOx-charge system, which includes plants with an energy production exceeding 25 GWh annually (Van Loo and Koppejan, 2007). Therefore, the current legislation in Sweden does not cover plants of the intended thermal output range. However, also handling, storing and dissemination of farmyard manure cause ammonia (NH3) emissions. According to the Swedish Board of Agriculture (2003) a large part of the total nitrogen content of the manure will be emitted as ammonium and the largest emissions occur during storage and dissemination. If the farmyard manure is used as fertiliser on arable land, it is recommended that the manure is composted. During this process, the nitrogen will be washed out partly as ammonia and partly as nitrate. The amount varies depending on what kind of bedding material is used. During combustion the fuel bound nitrogen content is emitted as nitrogen (N2) and nitrogen oxides (NOx). Additionally, results from earlier experiments with wood-chips have shown that it was possible to slightly reduce the NOx emissions by decreasing the air/fuel ratio (Lundgren et al., 2004a). This indicates that it should be possible to reduce the emissions of NOx for this application as well. This means that all types of manure handling practices contribute, more or less, to an enhanced acidification of soil and surface water. Further investigations are required to compare the NOx emissions from combustion with the escapes of NH3 regarding the acidification contribution.
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The concentrations of heavy metals are sufficiently low in order to allow recycling to forests, because contamination of the ash will not occur in continuous combustion practise. The lower calcium content resulting in a lower self-hardening capability should however be considered if recycling to forest is aimed at. The major drawback is that the nitrogen in the manure cannot be used as fertiliser on agricultural land, since all of the nitrogen leaves the chimney. In principle, the ash could be used as a fertiliser on agricultural land as well as in forests. The ash contains substantial amounts of phosphorus, which ought to be recycled to farmland in many cases in order not to lower the depot of phosphorus in the ground. It has however been shown that the phosphorus in the ash after combustion exists in chemical structures less accessible to plants than in the manure. According to Möller et al. (2007), 80% of the phosphorus is being transformed into apatite, which is unavailable to plants. This has been claimed as a drawback, but Linderholm (1997) stated that only 0.01% of the phosphorus in the ground is accessible to the plants, and that it is of little importance in which chemical form the phosphorus is added to the depot. More ash analysis is however needed before it can be stated whether the quota between phosphorus and heavy metals is higher or lower than in the phosphorus products from the fertiliser industry. If the quota were found to be acceptable, the ash could be considered for agricultural land as well forests. 5. Conclusions The fuel quality is of great importance when firing horse manure mixed with wood-shavings especially the water content. It is possible to obtain a good combustion process with low emissions of unburnt gases if the water content does not exceed 50 wt%. The horse manure should therefore be stored weather protected. The experimental results show that it is possible to obtain as low emissions of CO as during combustion of regular wood-chips. The emissions of NOx are significantly higher during combustion of horse manure due to the higher nitrogen content in the fuel. It should however be taken into account that a large amount of NH3 is emitted during handling, storing and dissemination of farmyard manure as well. The concentrations of heavy metals are sufficiently low to allow recycling to forests, if no stainless steel is added to the ash. The lower calcium content yielding lower self-hardening capacity should be considered. The main conclusion is that it is possible to use horse manure as fuel for heat production from the point of view of emissions. Longterm test runs and further studies are required to be able to draw conclusions regarding ash sintering and other ash related problems. Acknowledgements The authors would like to thank Mr. Mikael Jansson, Swebo Bioenergy AB, for fruitful co-operation and discussions during the project. The Network Institute for Future Energy systems (NIFES) and Swedish National Energy Administration are acknowledged for the
funding of this work. The authors would also like to thank our colleagues at the Division of Energy Engineering, Luleå University of Technology and at the Energy Technology Centre in Piteå. References Annamalai, K., Thien, B., Sweeten, J., 2003a. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part II. Performance results from 30 kWt (100,000) BTU/h laboratory scale boiler burner. Fuel 82, 1183–1193. Annamalai, K., Sweeten, J., Freeman, M., Mathur, M., O’Dowd, W., Walbert, G., Jones, S., 2003b. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part III: Fouling results from a 500,000 BTU/h pilot plant scale boiler burner. Fuel 82, 1195– 1200. European Commission, 1999. Council Directive 1999/31/EC on Landfill of Waste. Luxembourg. Linderholm, K., 1997. Plant availability of Phosphorus in Different Kinds of Sewage Sludge, Commercial Fertilizer and Ash. Serie: VA-Forsks Rapportserie, 11025638, vol. 6. ISBN: 91-88392-23-6 (in Swedish). Lundgren, J., Hermansson, R., Lundqvist, M., 2003. Design of a secondary combustion chamber for a 350 kW wood-chips fired furnace. In: Proceedings of the International Conference on Fluid and Thermal Energy Conversion, 7–11 December, Bali, Indonesia. Lundgren, J., Hermansson, R., Dahl, J., 2004a. Experimental studies of a biomass boiler suitable for small district heating systems. Biomass and Bioenergy 26 (5), 443–453. Lundgren, J., Hermansson, R., Dahl, J., 2004b. Experimental studies during heat load fluctuations in a 500 kW wood-chips fired boiler. Biomass and Bioenergy 26 (3), 255–267. Lundgren, J., Hermansson, R., Dahl, J., 2005. New furnace designed for small-scale combustion of wet and inhomogeneous biomass fuels. In: Proceedings of the 14th European Biomass Conference and Exhibition, 17–21 October, Paris, France. Möller, H.B., Jensen, H.S., Tobiasen, L., Hansen, M.N., 2007. Heavy metal and phosphorus content of fractions from manure treatment and incineration. Environmental Technology 28, 1403–1418. National Board of Forestry, 2008. Recommendations for the Extraction of Forest Residues and Compensation Fertilising. Meddelande 2-2008. Jönköping, Sweden. Pettersson, E., Lundgren, J., 2002. Kretsloppsanpassad förbränning av strömedel/gödsel från häststallar. Technical Report. NIFES 2002-2. Piteå, Sweden (in Swedish). Priyadarsan, S., Annamalai, K., Sweeten, J.M., Holtzapple, M.T., Mukhtar, S., 2005. Co-gasification of blended coal with feedlot and chicken litter biomass. Proceedings of the Combustion Institute 30, 2973–2980. Sánchez, M.E., Martínez, O., Gómez, X., Morán, A., 2007. Pyrolysis of mixtures of sewage sludge and manure: a comparison of the results obtained in the laboratory (semi-pilot) and in a pilot plant. Waste Management 27 (10), 1328– 1334. Schuster, R., Strömberg, B., 1997. Förbränning av gödsel – en orienterande litteraturstudie med kommentarer. Teknisk rapport O3-513. Stiftelsen för värmeteknisk forskning, Stockholm, Sverige. Steineck, S., Svensson, L., 2000. Hästar-gödselhantering. Teknik för lantbruket. JTIReport 82. Swedish Institute of Agricultural and Environmental Engineering, Uppsala, Sweden (in Swedish). Swedish Board of Agriculture, 2003. Yearbook of Agricultural Statistics 2003 including Food Statistics. Jönköping, Sweden, p. 179 (Chapter 12, in Swedish). Swedish Codes of Statute (SFS), 2001. Law (2001:512) on Landfill of Waste. Swedish Parliament, Stockholm, Sweden (in Swedish). Sweeten, J.M., Annamalai, K., Thien, B., McDonald, L.A., 2003. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part I. Feedlot biomass (cattle manure) fuel quality and characteristics. Fuel 82, 1167–1182. Van Loo, S., Koppejan, J. (Eds.), 2007. Handbook of Biomass Combustion and Co Firing. Prepared by Task 32 of the Implementing Agreement on Bioenergy under the Auspices of the International Energy Agency (IEA). Twente University Press, Enschede, the Netherlands. Wheeler, E., Smith Zajaczkowski, J., 2002. Horse Stable Manure Management. College of Agricultural Sciences. G-97. Penn State Cooperative Extension, USA. Zhu, S., Lee, S.W., 2005. Co-combustion performance of poultry wastes and natural gas in the advanced swirling fluidized bed combustor (SFBC). Waste Management 25, 511–518.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Combustion of horse manure for heat production J. Lundgren a,*, E. Pettersson b a b
Division of Energy Engineering, Luleå University of Technology, S-971 87 Luleå, Sweden Energy Technology Centre, Box 726, S-941 28 Piteå, Sweden
a r t i c l e
i n f o
Article history: Received 20 January 2009 Accepted 23 January 2009 Available online 3 March 2009 Keywords: Biomass combustion Horse manure Heat production
a b s t r a c t The main objectives of this paper have been to evaluate the use of horse manure and wood-shavings as a fuel for heat production and to provide sets of data on the chemical composition, ash characteristics and ash forming elements of the fuel. Another objective has been to investigate the possibility to use the ash as fertiliser by analysing the heavy metal and nutrient contents. The results showed that the fuel is well suited for combustion for heat production causing low emissions of products of incomplete combustion. The emissions of NOx were however high due to the high content of fuel bound nitrogen. Emissions of CO and NOx were typically in the range of 30–150 mg/Nm3 and 280–350 mg/Nm3 at 10 vol% O2, respectively. The analysis of the ash showed on sufficiently low concentration of heavy metals to allow recycling. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction There are roughly five million horses living in stables in Europe today. The residues from the horses contain solid as well as liquid portions of waste, typically about 60% solids and 40% liquids (Wheeler and Smith Zajaczkowski, 2002). Bedding materials are used on the floor of the stall boxes to absorb the liquid part and are exchanged regularly in order to keep a hygienic environment for both, people working in the stables and for the horses. The residues from the stables consist therefore of a mixture of manure, urine and bedding material. There are several different types of bedding materials that are used. The most common are woodshavings, sawdust, straw (silage or pellets), peat or paper pieces. The choice is strongly depending on which material that is available at low cost. In northern Sweden, wood-shavings are generally used due to the great accessibility. From an inquiry made, it was found that the used annual volume varied from 9 to 29 m3 per horse. The large variations are due to that the stables and riding schools often pay for the bedding material, and the share of the bedding material in the mixture depends strongly on how careful the keepers are when they clean the stall boxes (Pettersson and Lundgren, 2002). The options for handling the residues are recycling to agricultural land either through composting or via biogas production, deposition at landfills, combustion or other usage. In the study by Pettersson and Lundgren (2002), it was found that currently the largest part of the horse manure is recycled to agricultural land followed by deposition at landfills. Combustion and other usage such as soil production for lawns were of small importance. Wheeler and Smith * Corresponding author. Tel.: +46 920 491307; fax: +46 920 491047. E-mail address: [email protected] (J. Lundgren). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.01.050
Zajaczkowski (2002) have found that approximately 20 m3 of bedding material is used per horse each year, which is close to the average value of the inquiry. According to a law passed in the year 2001, landfill of organic material will be prohibited from 1st of January 2005 in Sweden (Swedish Codes of Statute, 2001). This law is based on a European Commission Directive (European Commission, 1999) which forces the member states to lower the landfill of biodegradable municipal waste to less than 35% of the amount produced in 1995. This means that the Swedish law is more stringent than the directive. Currently, composting of the horse manure to be used as fertiliser on arable land is a viable alternative for many horse stable owners if the arable land is located in the vicinity of the stable. Otherwise costly transports of large waste volumes would be required, which many horse owners can not afford. Moreover, cereal farmers often hesitate to accept composted horse manure as fertiliser since it may contain oat weeds. This would force the farmer to weed the fields manually as no other weed control is available. Many do not recommend spreading of composted manure mixed with wood-shavings because of a rumour saying that lignin and terpene contents tend to restrain the growth. According to a study by Steineck and Svensson (2000), there are no valid theories confirming this rumour. Due to the above mentioned reasons, many horse owners have large problems to get rid of the residues. For many stables, the energy content of the annual volume of bedding material used would easily cover the space heating demand and the demand for hot tap water over a year in a normal stable facility. There is a great interest in burning the manure for heat production amongst horse stable- and trotting course owners. By doing so, the stable and trotting course owners will on the one hand decrease the cost for heating of their facilities and on the other hand, decrease the waste volume. One important condition
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for motivating combustion of horse manure is that the emissions of harmful substances are kept at an environmentally benign level. The ash could potentially be used as fertiliser in forests, if the amount of heavy metals, like cadmium, is less than the prevailing regulations. Very few studies have been found on thermochemical conversion of horse manure for energy generation. Schuster and Strömberg (1997) presented results from combustion experiments using horse manure mixed with straw as a fuel at the trotting course of Färjestad in Sweden. The results indicate a poor combustion process with very high emissions of CO. However, there are several studies made regarding thermochemical conversion of other types of animal manures. Zhu and Lee (2005) have presented experimental results from co-combustion of poultry wastes with natural gas in an advanced Swirling Fluidized Bed Combustor (SFBC). Sweeten et al. (2003) have thoroughly investigated the fuel properties of cattle manure, while Annamalai et al. (2003a,b) presents emission data and operational problems during co-firing of coal and cattle manure in two different burners. Priyadarsan et al. (2005) have experimentally investigated gasification of coal and animal waste-based fuels like feedlot biomass (cattle manure) and chicken litter biomass under batch mode operation. In a paper by Sánchez et al. (2007), experimental studies regarding pyrolysis of mixtures of sewage sludge and manure are presented. Möller et al. (2007) investigated co-combustion of treated pig manure. In this paper, results from combustion experiments performed in a 250 kWth furnace especially designed for wet and inhomogeneous biomass fuels is presented. The purpose has been to evaluate horse manure and wood-shavings as a fuel for co-firing for heat production in horse stable facilities. The results are compared with emission data from combustion of wood-chips with high moisture content. Furthermore, this paper provides sets of data on the chemical composition, ash characteristics and ash forming elements of the fuel. Another objective has been to investigate the possibility to use the ash as fertiliser by analysing the heavy metal and nutrient contents.
2. Test facility and experimental setup The combustion experiments have been carried out in a 250 kWth biomass-based district-heating plant located in the northern part of Sweden, around 70 km south of the Arctic Circle. The plant is connected to the local district-heating network of the town of Boden. The experimental facility includes, besides a furnace and a heat transfer unit, a water heat store to handle heat load variations as well as two cyclones that are used for flue gas cleaning.
2.1. The combustion chamber The furnace is divided into a primary and a secondary zone. The primary combustion chamber is of a counter-current grate type, meaning that the flame direction is the opposite of the fuel flow. This type is appropriate for wet biomass fuels, due to the increased convective heat transfer contributing to an improved drying process of the fuel. After the final gas combustion in the secondary zone, the gases enter a convection boiler where the heat is transferred to the water. The cooled gases continue through a cyclone system. Fig. 2 shows a sketch of the primary combustion chamber. Two fuel feeding screws (1) are used to transport the fuel from an intermediate fuel store into the furnace. The fuel enters the combustion chamber at a first horizontal plane (2) and moves slowly towards a
slope. The purpose of the two planes is to dry the fuel before the combustion process starts, using heat transfer by radiation and convection from the combustible gases. Pyrolysis starts in the end of the slope and at the beginning of the fuel grate (4). The burning fuel bed is moved slowly forward towards the steps (5) and the ash bin (6) by short strokes from a rectangular piston (3). Final charcoal combustion takes place at the end of the second horizontal plane (4) and on the steps (5). The largest difference compared to a conventional counter-current grate furnace is that the primary air is supplied from above the fuel bed and not through the grate. In this furnace, the combustion air is introduced partly through slotted steel pipes integrated in the sidewalls and partly through a pipe in the front of the furnace as illustrated in Fig. 2. The secondary combustion chamber is cylindrically shaped in order to create a re-circulating flow and thereby enhance the large scale mixing and the combustion intensity. It is assumed that the most important factors for a good burnout rate are to create a good mixing between gases leaving the primary zone and secondary air and to maintain a high gas temperature. A proper method of achieving this is to supply pre-heated combustion air as high velocity air jets. To make the air jet penetration easier it is advantageous to have a low momentum of the primary gases and short distances for the secondary air jets to travel. Unfortunately, these two statements contradict each other since a low primary flow momentum requires a large cross sectional area, leading to larger penetration distances being required. The secondary air is pre-heated outside the cylinder to a few 100 °C depending on the thermal output. Also the gas residence time in the secondary combustion zone varies mainly with the thermal output, but is typically in the range of 0.6–1.6 s. The design was developed on the basis of CFD simulations and previous experiments. A thoroughly description of the design of the primary and secondary combustion chamber can be found in Lundgren et al. (2003, 2004a,b, 2005). 2.2. Measuring equipment The analysis of the flue gas composition is carried out immediately after the heat transfer unit shown in Fig. 1. A multi-component gas analyser for online measurements of NO, CO, CO2 and O2 (Maihak) and a heated THC analyser (JUM) were used. A NO/ NO2 converter (JNOx) was used to measure total NOx emissions. The gases were extracted using a heated probe and gas sample line maintained at a constant temperature of 120 °C. Table 1 shows the measuring methods and ranges for the different gas components. Gas temperatures have been measured before the secondary combustion chamber and after the heat transfer unit by using radiation-shielded thermocouples of type N. The location of the temperature gauges are shown in Fig. 1. Fuel samples have been dried in an electrical oven for at least 24 h at approximately 105 °C to determine the moisture content of the fuel. Samples have been taken regularly during the experiments.
3. Experimental results 3.1. Fuel characteristics and analysis The fuel consists of wood-shavings and horse manure (hereafter referred to as the fuel mixture), where the share of wood-shavings typically constitute 40–80 wt% depending on how careful the horse boxes are cleaned. The moisture content of the fuel mixture as received varies in the range of 45–65% (wet basis) mostly depending on if it is stored outdoors with cover or without.
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Fig. 1. Explanatory sketch of the test plant. Gas temperatures are measured between the primary and secondary zones as well as immediately after the heat transfer unit.
Fig. 2. 3-D sketch of the primary combustion chamber to the left and illustrations of the primary air supply to the right.
Several analysis of the ash content of the fuel mixture show that it varied typically in the range of 5–7% on dry basis. The volatile matter normally varied in the range of 72–74% on dry basis. An accredited laboratory SLU (The Swedish University of Agricultural Sciences, Umeå, Sweden) has performed analyses of the chemical composition of the horse manure and wood-shavings
Table 1 Measuring ranges and methods for different gas components.
O2 CO CO2 NO THC a b
Range
Method
0–25 vol% 0–1000/10,000 ppm 0–20 vol% 0–500 ppm 0–10/102/103/104/105 ppm
Paramagnetic O2 cell NDIRa NDIRa NDIRa FIDb
Non-dispersive infra red. Flame ionisation detector.
mixture. The results are shown in Table 2. For comparison, typical analyses of regular wood-chips and other types of manure fuels are also shown in the table. The nitrogen content of the horse manure fuel is higher than for typical wood-chips, but considerably lower than in poultry litter and feedlot manure. The sulphur and chlorine concentrations are also higher than wood-chips. The higher concentration of chlorine in the fuel will volatilise more potassium, while the sulphur will form potassium sulphate. Comparing the inorganic constituents of the fuel mixture with ordinary wood-chips show that the composition is fairly similar but manure has higher concentrations of silica, potassium and magnesium. The higher concentrations of potassium may decrease the sintering temperature of the ash. The horse manure does have lower concentrations of calcium which will influence the self-hardening ability of the ash. Analysis of the inorganic trace components of the horse manure fuel mixture is shown in Table 5.
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Table 2 Heating value, ultimate analysis, analysis of ash forming elements in different biomass fuels. Analysis
Wood-chips
Horse manure + wood-shavings
Poultry litterb
Feedlot manured
Heating value, calorimetric [MJ/kg dry basis] Lower heating value [MJ/kg dry basis]
20.56 19.21
19.37 18.14
– 6.03c
– 20.9
Ultimate analysis [wt% dry basis and ash-free] Carbon Hydrogen Nitrogen Sulphur Oxygena Chlorine Total
49.5–49.8 6.1–6.2 <0.1 <0.01 43.5–44 0.004 100
48.60 5.80 0.90 0.14 44.30 0.26 100
46.12 6.54 6.63 0.16 40.55 – 100
49.66 5.62 4.28 1.36 39.09 – 100
Ash forming elements [% of ash] Si K Na Ca Mg
7.0 4.3 0.3 24.3 3.0
15.5 7.3 0.8 11.4 5.8
– – – – –
– 11.7 2.93 13.6 5.08
a b c d
Calculated by difference. Data from Zhu and Lee (2005) re-calculated to ash-free. As received. Data from Sweeten et al. (2003) re-calculated to ash-free.
Table 3 Operating conditions during the experiments. Experiment
1
2
3
4
Average thermal output (kW) Fuel Fuel moisture contentb (wt%) Total air supply (m3/h) Primary/secondary air ratio Avg. combustion temperature (°C)
150 Wood-chips 50 356 2.06 985
150 Horse manurea 57 344 2.56 775
150 Horse manurea 49 490 1.38 921
150 Horse manurea 49 376 2.00 978
a b
Mixed with wood-shavings. On wet basis (w.b.).
3.2. Emissions Several combustion experiments have been carried out using horse manure mixed with wood-shavings as a fuel. During all experiments presented in this paper, the furnace was operating in between 4 and 15 h at the constant thermal output of 150 kW. As an attempt to improve the combustion process and reduce the emissions, the settings of the air supply (total volume flow, ration between primary and secondary air) were different in each of the experiments. Table 3 shows the most important operating param-
Fig. 3. Average emissions and O2 content during combustion of wood-chips and horse manure mixed with wood-shavings. All emissions are normalised to 10 vol% O2.
eters during the experiments. In experiment No. 1, wood-chips were used as a fuel and serves as a reference case. The combustion temperature is measured in between the primary and secondary zone. Fig. 3 shows the average emissions during the experiments. In experiment 2, the combustion temperature was too low causing an incomplete combustion process and thereby relatively high CO emissions. The O2 content was also high, approximately 12 vol%, which indicates that the secondary combustion air was diluting. In experiment 3, the water content of the fuel was lower, which led to higher combustion temperature and consequently lower emissions of CO. The O2 content was still unsatisfactory high. This was most probably caused by that too much secondary air was supplied. In experiment 4 using the same fuel as in experiment 3, the total air supply as well as the share of secondary air were reduced. These led to increased combustion intensity in the primary zone and consequently lower emissions of CO as well as lower air excess. It may be concluded that it is possible to burn horse manure mixed with wood-shavings with satisfactory CO emission levels with proper air supply and a water content of the fuel below 50 wt%. The most important difference between burning clean wood fuels like wood-chips or pellets and combustion of horse manure mixed with wood-shavings is the higher NOx emissions. The NOx emission levels are in some cases more than twice as high as during combustion wood-chips. As the fuel analysis shown in Table 2 shows, the horse manure mixture contains considerably more fuel bound nitrogen than wood-chips, which increases the formation of fuel-NOx.
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3.3. Analysis of the bottom ash Table 4 shows the major inorganic components of the bottom ash after combustion of the horse manure mixture. The analysis of inorganic constituents of the bottom ash compared with the fuel shows that potassium, phosphorus and sulphur have left the fuel. The chlorine content was not analysed due to the experience that chlorine is very seldom found in biomass ash since potassium chloride is volatile. Table 5 shows the inorganic trace elements in the ash as well as in the horse manure fuel mixture. The concentrations of chromium and nickel increased several times in the ash compared with the manure fuel. Table 4 shows that the iron content has increased substantially as well. This indicates that some stainless steel from the furnace has contaminated the ash. The fly ash was not collected during the experiments. Table 6 shows measured and recommended minimum and maximum concentrations of nutrients and trace elements in ash products to allow recycling to forests. The ash contained lower calcium concentration than the recommended limit. Wood ash contains high concentrations of calcium and does upon storage in humid climate self-harden. This is due to that the calcium in the ash form calcium hydroxide, which reacts with carbon dioxide forming limestone. A high calcium concentration means larger coverage of the particles with limestone Table 4 Major inorganic components of the bottom ash as well as in the horse manure mixture (% of ash). Element
Bottom ash
Fuel mixture
SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO2 Na2O P2O5 TiO2 S Cl
42.6 7.75 15.4 4.24 11.5 8.85 0.368 2.59 4.27 0.436 0.4 n.a.a
33.15 3.36 15.9 2.30 17.67 9.64 0.39 2.16 7.27 0.29 1.9 3.56
a
Not analysed.
Table 5 Inorganic trace components in the bottom ash and in the fuel (mg/kg dry basis). Element
Bottom ash
Fuel mixture
As Ba Be Cd Co Cr Cu Hg La Mo Nb Ni Pb S Sc Sn Sr V W Y Zn Zr
<3 656 0.892 <0.1 13.8 1000 105 <0.1 9.23 10.3 <6 378 5.4 4020 7.32 <20 464 70.8 <60 10.4 344 73.6
2.1 500 <0.55 1.55 6.85 72.5 104 0.2 <5.5 13.6 <5.5 24.8 5.2 19,178 4.6 <27 401 42.5 <55 6.5 683 46.7
Table 6 Recommended minimum and maximum concentrations in ash products to be recycled to forests (National Board of Forestry, 2008). Elements
Standard values
Lowest
Measured values
Highest
Macro nutrients g/kg DS Ca 125 Mg 15 K 30 P 7
110 53 95 18
Trace elements mg/kg DS B Cu Zn 500 As Pb Cd Cr Hg Ni V
800 400 7000 30 300 30 100 3 70 70
n.a.a 105 344 <3 5 <0.1 1000 <0.1 378 70.8
Organic pesticides mg/kg DS Total PAH (tentative)
2
n.a.a
a
Limiting values forest in Austria (Van Loo and Koppejan, 2007)
250 1500 100 8 250 100
Not analysed.
while a low concentration means a lower coverage corresponding to a more soluble ash product with less strength. The measured concentrations of nickel and chromium were also higher than recommended, but as may be seen from the iron analysis this is probably due to contamination of the ash by some stainless steel. Otherwise the ash fulfils the requirements except for the zinc content. The requirement on the lowest level for zinc is a strange requirement, since zinc is volatile both as a metal and as chloride. 4. Discussion The experimental results are so far promising. Further studies should be carried out to investigate the possibility to reduce the emissions of NOx. In Sweden, the emissions of NOx are in practice limited by a NOx-charge system, which includes plants with an energy production exceeding 25 GWh annually (Van Loo and Koppejan, 2007). Therefore, the current legislation in Sweden does not cover plants of the intended thermal output range. However, also handling, storing and dissemination of farmyard manure cause ammonia (NH3) emissions. According to the Swedish Board of Agriculture (2003) a large part of the total nitrogen content of the manure will be emitted as ammonium and the largest emissions occur during storage and dissemination. If the farmyard manure is used as fertiliser on arable land, it is recommended that the manure is composted. During this process, the nitrogen will be washed out partly as ammonia and partly as nitrate. The amount varies depending on what kind of bedding material is used. During combustion the fuel bound nitrogen content is emitted as nitrogen (N2) and nitrogen oxides (NOx). Additionally, results from earlier experiments with wood-chips have shown that it was possible to slightly reduce the NOx emissions by decreasing the air/fuel ratio (Lundgren et al., 2004a). This indicates that it should be possible to reduce the emissions of NOx for this application as well. This means that all types of manure handling practices contribute, more or less, to an enhanced acidification of soil and surface water. Further investigations are required to compare the NOx emissions from combustion with the escapes of NH3 regarding the acidification contribution.
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The concentrations of heavy metals are sufficiently low in order to allow recycling to forests, because contamination of the ash will not occur in continuous combustion practise. The lower calcium content resulting in a lower self-hardening capability should however be considered if recycling to forest is aimed at. The major drawback is that the nitrogen in the manure cannot be used as fertiliser on agricultural land, since all of the nitrogen leaves the chimney. In principle, the ash could be used as a fertiliser on agricultural land as well as in forests. The ash contains substantial amounts of phosphorus, which ought to be recycled to farmland in many cases in order not to lower the depot of phosphorus in the ground. It has however been shown that the phosphorus in the ash after combustion exists in chemical structures less accessible to plants than in the manure. According to Möller et al. (2007), 80% of the phosphorus is being transformed into apatite, which is unavailable to plants. This has been claimed as a drawback, but Linderholm (1997) stated that only 0.01% of the phosphorus in the ground is accessible to the plants, and that it is of little importance in which chemical form the phosphorus is added to the depot. More ash analysis is however needed before it can be stated whether the quota between phosphorus and heavy metals is higher or lower than in the phosphorus products from the fertiliser industry. If the quota were found to be acceptable, the ash could be considered for agricultural land as well forests. 5. Conclusions The fuel quality is of great importance when firing horse manure mixed with wood-shavings especially the water content. It is possible to obtain a good combustion process with low emissions of unburnt gases if the water content does not exceed 50 wt%. The horse manure should therefore be stored weather protected. The experimental results show that it is possible to obtain as low emissions of CO as during combustion of regular wood-chips. The emissions of NOx are significantly higher during combustion of horse manure due to the higher nitrogen content in the fuel. It should however be taken into account that a large amount of NH3 is emitted during handling, storing and dissemination of farmyard manure as well. The concentrations of heavy metals are sufficiently low to allow recycling to forests, if no stainless steel is added to the ash. The lower calcium content yielding lower self-hardening capacity should be considered. The main conclusion is that it is possible to use horse manure as fuel for heat production from the point of view of emissions. Longterm test runs and further studies are required to be able to draw conclusions regarding ash sintering and other ash related problems. Acknowledgements The authors would like to thank Mr. Mikael Jansson, Swebo Bioenergy AB, for fruitful co-operation and discussions during the project. The Network Institute for Future Energy systems (NIFES) and Swedish National Energy Administration are acknowledged for the
funding of this work. The authors would also like to thank our colleagues at the Division of Energy Engineering, Luleå University of Technology and at the Energy Technology Centre in Piteå. References Annamalai, K., Thien, B., Sweeten, J., 2003a. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part II. Performance results from 30 kWt (100,000) BTU/h laboratory scale boiler burner. Fuel 82, 1183–1193. Annamalai, K., Sweeten, J., Freeman, M., Mathur, M., O’Dowd, W., Walbert, G., Jones, S., 2003b. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part III: Fouling results from a 500,000 BTU/h pilot plant scale boiler burner. Fuel 82, 1195– 1200. European Commission, 1999. Council Directive 1999/31/EC on Landfill of Waste. Luxembourg. Linderholm, K., 1997. Plant availability of Phosphorus in Different Kinds of Sewage Sludge, Commercial Fertilizer and Ash. Serie: VA-Forsks Rapportserie, 11025638, vol. 6. ISBN: 91-88392-23-6 (in Swedish). Lundgren, J., Hermansson, R., Lundqvist, M., 2003. Design of a secondary combustion chamber for a 350 kW wood-chips fired furnace. In: Proceedings of the International Conference on Fluid and Thermal Energy Conversion, 7–11 December, Bali, Indonesia. Lundgren, J., Hermansson, R., Dahl, J., 2004a. Experimental studies of a biomass boiler suitable for small district heating systems. Biomass and Bioenergy 26 (5), 443–453. Lundgren, J., Hermansson, R., Dahl, J., 2004b. Experimental studies during heat load fluctuations in a 500 kW wood-chips fired boiler. Biomass and Bioenergy 26 (3), 255–267. Lundgren, J., Hermansson, R., Dahl, J., 2005. New furnace designed for small-scale combustion of wet and inhomogeneous biomass fuels. In: Proceedings of the 14th European Biomass Conference and Exhibition, 17–21 October, Paris, France. Möller, H.B., Jensen, H.S., Tobiasen, L., Hansen, M.N., 2007. Heavy metal and phosphorus content of fractions from manure treatment and incineration. Environmental Technology 28, 1403–1418. National Board of Forestry, 2008. Recommendations for the Extraction of Forest Residues and Compensation Fertilising. Meddelande 2-2008. Jönköping, Sweden. Pettersson, E., Lundgren, J., 2002. Kretsloppsanpassad förbränning av strömedel/gödsel från häststallar. Technical Report. NIFES 2002-2. Piteå, Sweden (in Swedish). Priyadarsan, S., Annamalai, K., Sweeten, J.M., Holtzapple, M.T., Mukhtar, S., 2005. Co-gasification of blended coal with feedlot and chicken litter biomass. Proceedings of the Combustion Institute 30, 2973–2980. Sánchez, M.E., Martínez, O., Gómez, X., Morán, A., 2007. Pyrolysis of mixtures of sewage sludge and manure: a comparison of the results obtained in the laboratory (semi-pilot) and in a pilot plant. Waste Management 27 (10), 1328– 1334. Schuster, R., Strömberg, B., 1997. Förbränning av gödsel – en orienterande litteraturstudie med kommentarer. Teknisk rapport O3-513. Stiftelsen för värmeteknisk forskning, Stockholm, Sverige. Steineck, S., Svensson, L., 2000. Hästar-gödselhantering. Teknik för lantbruket. JTIReport 82. Swedish Institute of Agricultural and Environmental Engineering, Uppsala, Sweden (in Swedish). Swedish Board of Agriculture, 2003. Yearbook of Agricultural Statistics 2003 including Food Statistics. Jönköping, Sweden, p. 179 (Chapter 12, in Swedish). Swedish Codes of Statute (SFS), 2001. Law (2001:512) on Landfill of Waste. Swedish Parliament, Stockholm, Sweden (in Swedish). Sweeten, J.M., Annamalai, K., Thien, B., McDonald, L.A., 2003. Co-firing of coal and cattle feedlot biomass (FB) fuels. Part I. Feedlot biomass (cattle manure) fuel quality and characteristics. Fuel 82, 1167–1182. Van Loo, S., Koppejan, J. (Eds.), 2007. Handbook of Biomass Combustion and Co Firing. Prepared by Task 32 of the Implementing Agreement on Bioenergy under the Auspices of the International Energy Agency (IEA). Twente University Press, Enschede, the Netherlands. Wheeler, E., Smith Zajaczkowski, J., 2002. Horse Stable Manure Management. College of Agricultural Sciences. G-97. Penn State Cooperative Extension, USA. Zhu, S., Lee, S.W., 2005. Co-combustion performance of poultry wastes and natural gas in the advanced swirling fluidized bed combustor (SFBC). Waste Management 25, 511–518.