Wood Residue Combustion for Energy Conservation. and Skodras, G. Aristotle University of Thessaloniki, Thessaloniki, Greece

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1 Wood Residue Combustion for Energy Conservation *Stavropoulos, G.G 1 1, 2, 3 and Skodras, G. 1 Lab of Chemical Process Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece 2 Laboratory of Environmental and Energy Processes, Chemical Process Engineering Research Institute, Thessaloniki, Greece 3 Institute for Solid Fuels Technology and Applications, Ptolemais, Greece *P. O. Box 1520, Thessaloniki, Greece Tel : , Fax gstaurop@cheng.auth.gr Abstract In a chipboard production plant, wood residues (barks and chips) were used to replace conventional fuel i. e. LPG. The project consist in the installation of oil-heating boiler and a chip drier both fueled with residues. In this paper energetic and environmental benefits are enumerated together with the economic incentive for the implementation of the project. Keywords: wood residue, combustion, energy conservation 1. Introduction The increased use of renewable energy sources is a key issue for many countries. For example, the European Union has set a target to double the share of renewable energy in the European primary Energy supply from a level of 6 % in 1997 to 12 % by Biomass is one of the most important sources of renewable energy and its contribution to this end may be significant. The use of biomass reduces the dependency on fossil fuels and contributes to the security of the fuel supply by using indigenous fuel. It also provides savings from decreasing or eliminating landfill costs and possible tax credits. 1

2 One of the most important sources of biomass except of forestry and energy crops is the waste wood from wood manufacturing industries. Unlike most other industries, the wood industries are able to use their waste to help meet their energy needs. In wood processing the greater part-if not all- of the thermal energy (and electricity) can be met from the available residues. For example the saw milling industry has the potential to produce a surplus of heat and electricity to support other deficient processes in an integrated complex producing, for example, plywood and particleboard or in the rural areas, to supply energy for the needs of surrounding community. In the past years wood waste was considered as a troublesome by-product to be disposed. Latter, residues demand was increased to furnish paper-pulp and panel board manufacture. In coincidence, contentious environmental issues and rising costs of energy, forced many wood industry owners to consider the merits of using residues. Actually, most wood processing plants incorporate wood combustion burner in order to safeguard against certain and costly fossil fuel supply. However, the residue energy potential is not always used due to the investment capital involved in the installation of the heat generating equipment. Although the heat produced from wood residues is less than that from oil and gas, its cost compared to fossil fuels makes it an attractive source of ready available heat and power. In the last few decades many efforts have been made to minimize pollutants emission by combustion, i.e. oxides of sulphur and nitrogen, soot, and other particulates. Nevertheless, expected, new emission regulations will place even stronger regulations on combustion not only to conventional pollutants (SO 2, NOx,) but also CO 2 associated with possibility of greenhouse effect. Biomass combustion has a low environmental impact [1] due to its no net CO 2 emissions and very low contents of sulphur compounds. Theoretically, produces zero CO 2, essentially eliminates SO 2 emissions and reduces air toxic emissions. In Table 1, a comparison is made of the emissions from an LPG and a waste wood boiler, [1]. The residues of the wood industry such as saw milling, plywood 2

3 production, particleboard production, are bark, dusts, and coarse residues. All those are well suited for combustion in boilers or for heating kilns and dryers. This paper presents a techno economic evaluation of a particleboard production plant in which wood waste is used to satisfy the plant heat energy requirements. 2. Plant Description The plant-a typical particleboard production plant-can be divided in four parts: a) particle preparation and drying b) blending and mat forming, c) pressing, and d) board finishing, fig. 1. In the first part tree trunks are fed to a chipping machine where bark is separated and the particle sizes as required for board forming are produced. Particles need to be dried so that the overall moisture level is in the order of 3-8 % for the purpose of bonding with liquid resins. Particle drying is a continuous process with particles moving along the length of the rotary drier whilst exposed to hot flue gases produced by the combustion of process residues. After screening the particles are blended under controlled conditions with adhesives. The mix is then fed to the former-a wholly mechanical process that results in mat formation. Mats are transported to the press for consolidation. Press is heated by passing thermal oil trough the platens to attain a temperature of 220 o C. The boards are cooled, conditioned and cut to size by trimming saws. Finally, in order to meet thickness and surface standards, boars are sanded and send to storage. Plant capacity is tn/yr of chipboard; it operates 24 h/d in three shifts for 11 months per year. 3. Residue Use There are two kinds of residues in the plant: 1. Bark produced in the chipping machine 2. Wood dust from the sanding machine The heat requirements of the plant are: 3

4 1. Heat for the thermal oil used in board press 2. Heat for the chips drier A total heat amount of Gkal/h is required for plant operation and can be satisfied by using the waste produced in the plant without the need for supplementary fuel. Wood waste will replace LPG used conventionally as fuel. For the implementation of this project two process additions should be made: 1. A biomass boiler fed with bark as fuel should be installed including fuel transport and feeding system, particle collection device (cyclone), and ash withdrawal mechanism. A stand-by LPG boiler should also provide for start-up and emergency purposes. 2. For using wood dust as fuel in the drier, a dust recovery and pneumatic transportation system will be installed including silos, screens etc. The waste quantities, heating values and heat supply to users are shown in Table Economic Evaluation The project cost can be seen in Table 3. It includes equipment, installation and other costs. The evaluation of economic potential of the project has been accomplished by determining the total cost and calculating the revenues and expenses. The usual economic indices were used i. e. the internal rate of return (IRR), the payback period (PBP) and the net present value, (NPV), table 2. Reported costs are real costs from equipment manufacturers. Total equipment cost is high because includes major parts. The most expensive equipment parts are those related to drier as can be seen in table 3. The revenues of the project are due to the cost of LPG that was used prior the project implementation. The savings are calculated based on total plant heat requirement and LPG price (0.35 Euro/kg). The expenses are due to the production cost of waste wood dust. Although residues can be recycled in the production or sold to other wood 4

5 industries, their real cost to the plant is their production cost, which includes raw material and operating costs. So the expenses (losses) of the plant from the use of wood dust as fuel are calculated from the wood cost (38.6 Euro/tn) and labour cost per ton of product. Economic indices are encouraging considering that regard an energy conservation project and must be evaluated not only with monetary benefits but also in a context of other hidden costs. Those costs include environmental and health effects and dependency on fossil fuels which are not included in the fuel prices. 5. Conclusion In the present case study the use of waste wood as fuel, replacing LPG, was demonstrated to be technically feasible and economically attractive. Obvious benefits are gained from the reduction of plant fuel cost although the high capital costs involved. Of course, the economics of wood waste energy generation should become more attractive as traditional fuel prices should increase. Other, non obvious, benefits are the pollutant emission reduction and the fuel self-sufficiency in self-generating fuel. The results can motivate other wood industries to consider the prospects of opting for wood energy. References 1. Wood residue combustion in boilers, EPA Document AP-42, Section 1.6, 5th Edition, July

6 Table 1. Pollutant emissions Component LPG, (kg/t fuel) Waste wood, (kg/t fuel) CO CO NO x SO HC Particulates Table 2. Energy balance Waste Quantity, Kg/h Heating value, kcal/kg Total heat, Gcal/h Heat use Bark Oil boiler Dust Drier 6

7 Table 3. Economic evaluation Installation Equipment Item Cost/benefit, K Euro Drier 1,272 Pneumatic conveying 2,080 Other related costs 1,242 Bark boiler 502 LPG stand-by boiler 69 Press oil supply system 72 Other 477 Mechanical erection 727 Civil works 289 Project management and start-up 294 Personnel 59 Revenues Displaced LPG (keuro/y) 6,493 Expenses Waste production cost (keuro/y) 5,572 Economic indices NPV (keuro) 300 IRR (%) 9.1 PBP (years) 9.7 7

8 Raw material-tree trunks Chipper Drier Screening and classification Blending Forming Hot press Coolingconditioning Cutting Sanding Product Figure 1. Particleboard production process 8