Biomass from Bushes has High Energy Production Potential

VTT Technical Research Centre of Finland has studied the profitability of using bush chips in electricity production in Namibia, where biomass from bushes has great energy production potential. Namibia suffers from the overgrowth of bush, which is disruptive to cattle raising, the country's primary source of livelihood. VTT also developed the production technology for bush chips.

According to the study, the production of chips for power plants is technically possible. However, electricity production in the power plants with a capacity of 5-20 MW which were studied is not economically profitable without investment aid and benefits from emissions trading. On the other hand, the conversion of one boiler in the coal power plant in the country’s capital, Windhoek, to use biomass is profitable. The largest costs in electricity production are incurred by fuel and investments in power plants.

Bush chips are an important source of raw material in electricity production in Namibia, which is located in southern Africa. It has been estimated that the overgrowth of bush greatly affects an area of about 10 million hectares in northern parts of central and eastern Namibia. There are 1,000-10,000 bushes per hectare in this area. The amount of biomass per hectare is 5-25 tonnes.

The overgrowth of bush can be managed by thinning and leaving 200-300 of the largest bushes to grow on the savannah. An average quantity of biomass obtained by thinning is about 10 tonnes per hectare. The thinned bushes re-grow from their roots, and thinning can be repeated in 10-15 years. In other words, the area affected by the overgrowth of bush produces in total 125 million tonnes of biomass, i.e. 500 TWh. The total consumption of energy in Namibia was 12.6 TWh in 1999.

The overgrowth of bush is harmful to, for example, cattle raising, as the bush reduces the growth of grass and it is almost impossible to move in thick bush. The production of bioelectricity would also contribute to Namibia’s energy self-sufficiency, which is currently low. Namibia imports most of its electricity from South Africa. There is a coal-fired power plant in the country’s capital, Windhoek, but it is used sporadically to supplement imported South African electricity. At the moment, the bush is used for firewood (about 1 million tonnes a year) and for charcoal production (0.3 million tonnes a year). Namibia also has one briquette factory, which produces about 6,000 tonnes of briquettes from bush chips.

The bush chip production trials for the VTT study were carried out on a farm owned by the CCF (Cheetah Conservation Fund). The farm is 36,000 hectares in size and is located in the area where bush growth is a problem, i.e. about 200 km north of Windhoek. Chips for the briquette factory owned by the CCF are produced on the farm.

In the CCF chip production process, the bushes are cut by axe. After cutting, the bushes are carried to the side of the skid road. The skid roads are 50 metres apart. The bushes are placed in piles by the skid road, where they are dried to 15-20% moisture content. In Namibian weather conditions, it takes two to three weeks of drying for the bush to achieve this moisture content. When the bush is sufficiently dry, it is chipped in a drum chipper. The chipper blows the chips straight into a tractor trailer, which takes them to the briquette factory 40 km away. Two trailers equipped with tipping gear are attached to the tractor.

The cutting trials were carried out on one-hectare test plots. Thirteen plots were used for the study. After thinning, 200-250 bushes remained in the cutting area. The average yield in the test plots was 7 tonnes per hectare. In the cutting tests, the productivity of the various work stages was measured in tonnes per time unit. The productivity of cutting was 0.25 ha (1.7 tons) and of transport 0.30 ha (2.0 tons) per worker per day. The productivity of chipping was 20.1 tonnes per day and of road transport 10.5 tonnes per day, when the transportation distance was 50 km.

The bioplant of 5 MW under review uses 32,000 tonnes of chips per year at a moisture content of 20%. To produce this amount using the existing production chain requires 198 men, in addition to the machines. The production cost of the chips, calculated for a transport distance of 30 km, is €5.1/MWh. Chipping costs are the largest individual cost item.

Cutting costs reduced by 15%

VTT also improved the efficiency of the chip production chain. Based on machinery tests, it was proposed that the chain be fully mechanised. In the mechanised chain, bushes are cut with a small tractor equipped with a rotating cutting head. The bushes are moved by a small tractor equipped with a grapple. The chipping is done by a drum chipper equipped with a feeder, and a tractor-trailer combination is used for road transport due to the short distance. According to VTT, the new mechanised chain could reduce the cutting costs by about 15%, in comparison with the chain used at the moment. If a 50 MW electricity plant uses 32,000 tonnes of chips (at a 20% moisture content) per year, 32 workers are needed for the production.

Electricity production in separate 10 MW and 20 MW plants is profitable if the investor investing in the plant receives 35% investment aid and €15 per tonne of carbon dioxide in emissions trading. According to the study, a 20 MW plant would repay itself in seven years and a 10 MW plant in 10 years. In the study, the price of electricity was €40/MWh. A 5 MW plant is too small, which is why the repayment period for it using the values provided above, is 16 years. The plants could be located in northern Namibia where the largest biomass resources are located.

There is a coal-fired power plant with four boilers in Windhoek. In the project, the conversion of one of these to biomass was considered. The boiler’s capacity using biomass would be 20 MW, which is somewhat less than when coal is used. The conversion of the boiler to biomass would be profitable even without aid. Electricity production costs would be €35.6/ MWh. Most of the costs would be incurred by fuel (€27/MWh). The capital cost would be €5.5/MWh. The investment would repay itself in seven years.

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