content). Tar is the main constraint in producing this gas as it can condense on valves and fittings, obstructing proper functioning. Tar, alkaline metals and dust also cause corrosion and erosion of cylinder walls and pistons. Hence the producer gas has to be cleaned before it can be used in a gas engine or turbine. Due to this tar problem, the availability of commercial wood gasifier systems is still limited, but more systems are slowly entering the phase of mass production.
Combustion Combustion means burning the crop with enough oxygen to convert nearly all the material to carbon dioxide (CO2), with water and ash as waste products. The heat that is produced can be used directly, or it can run steam engines, steam turbines or Stirling engines to produce electricity. Compared to gasification, combustion has the advantage that feedstock requirements are less exacting (up to 60 per cent moisture content, heterogeneous consistency and particle size) and the applied technology is more robust and simple. Turbines are designed for large-scale operations, while steam engines are available in the power range of 50kW to 1 MW. These smaller plants have a net efficiency of 10 – 15 per cent of electricity conversion. Stirling engines work by the repeated heating and cooling of a sealed amount of working gas, and can operate using any type of heat source. Electric efficiencies of about 20 per cent have been achieved with CHP (Combined Heat and Power) technology, and are expected to reach 30 per cent. Stirling engines have low maintenance costs and fewer combustion problems due to the use of solid biomass fuel. Cogeneration (CHP) produces heat and electricity simultaneously from the same plant, using a single primary energy source. More than 150 CHP plants in the range of 0.6-700MWe are running in Europe while others are installed in the USA. Cogeneration is useful for large processors who need large amounts of heat (sugar processing, organic oil pressing, cement production) and significant amounts of electricity. But investment costs are high.
Business models
Many parameters are involved in calculating Eucalyptus spp can be planted the best solution for very densely (up a given case. It is to 10,000 stems/ important to note that ha) to be cut just bio-energy systems two years later can either be standfor production of wood to be burned alone units (also called under controlled island solutions) or be supply of oxygen connected to the grid. for electricity But connecting small to generation. (Photo: medium systems to the BGF) grid has its problems, like irregular voltages of the grid that can damage the equipment, or synchronisation, which is expensive. Therefore, the study focuses on stand-alone units. The most important criteria to consider are the type of energy (heat and/or electricity) and the required amount (base load and peak load). The study focuses on two different scenarios. 1) Electricity production only with a capacity of 250kW The technology chosen here is a down-draught fixed bed gasifier, combined with an adequate combustion engine, sourced from Ankur Technologies in India. 2) Cogeneration through combustion with a capacity of 1MW This scenario, suitable for an industrial complex needing heat in its processing, employs a firetube steam boiler and an adequate steam piston engine. Heat exchangers are used for capturing the process heat. The latter is used on-site while electricity can also be exported to nearby communities with single earth-return-wire technology to battery charging stations. The acreage of required feedstock for both above scenarios is important. Scenario 1 would require a biomass of 1,400T annually, translated into 100ha. That means a total plantation area of 200 - 300ha to play it safe. Scenario 2 would require 7,700T annually or 500ha (total plantation area 1000-1500ha) which is five times as much, due to the lower electric efficiency of the employed installation. The wood has to be chipped, and the gasifier plant will only accept
Table 1: Some financial information on the two proposed scenarios Item
Scenario 1
Scenario 2
Investment costs
330,000 Euro
1 million Euro
Overall cost/kWhe
0.05 Euro
0.058 Euro
Investment pay-back period
4 - 5 years
3 1/2 – 4 1/2 years
Internal Rate of Return
18.5%
22%
chips of maximum diameter 5cm, length 10cm and a moisture content less than 20 per cent. The cogeneration plant can be less strict on chips dimensions, with a moisture content of 15 – 20 per cent. Table 1 provides some financial information for the two proposed systems, but the study stresses that this is a generic assessment, for which several assumptions that do not apply to every case, were made. More reliable numbers can only be obtained through actual running technology in the Ugandan context. The overall cost per kWhe for a diesel generator is at least five times higher, at 0.2-0.23 Euro. Actual electricity cost (per kWhe) as taken from the grid is 0.1 Euro (commercial), 0.083 Euro (medium industry) and 0.034 Euro (large industry). However, even smaller systems (e.g. 30kW) for community-based electricity production are possible, where the local community provides the wood and in turn is provided with electricity.
Conclusion There is significant potential for electricity production through bioenergy production from woody biomass. It is viable financially. Grants and development loans (green energy!) make this even more attractive. The potential is highest in remote rural areas not connected to the grid, but with real need for electricity. There is however little experience regarding feedstock production from energy forests, while the main barrier to implementing these systems is lack of local experience. The technology has, however, worked in Europe, India and Brazil. The writer is the Executive Director, Better Globe Forestry Ltd Email: jan@betterglobeforestry.com
kWhe: Kilo Watt per hour electricity IRR based on calculated electricity price of 0.114 Euro (75% grid, 25% diesel generator), no grants
Miti October-December 2011
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