Wood-fuel supply and preparation
The wood-fuel for the plant is supplied from a 274000 ha large forest area, with an estimated total quantity of 57 million tons of fuelwood and a natural growth rate of 2 — 3 m /ha/а. Within the zone under influence the use of fuelwood has grown from 3300 tons in 1981 to 10500 tons in 1984. With the third gasifier taken into use, the Loma Plata power plant is projected to use approximately 15000 tons of fuelwood per year. The other biggest fuelwood consumers in the area are the peanut oil and cotton industries, brick and charcoal producers, bakeries and furniture manufacturers, consuming annually between 600 to 1100 tons each. Thus, the supply of fuelwood to the power plant will by all probability not be endangered in the near future. The fuelwood resources of the Chaco region are estimated to cover the needs of the power plant and the industries for more than 3000 years. The fuelwood is brought into the plant from an area of 45 km radius.
The fuelwood delivered to Loma Plata is mainly of Quebracho bianco origin. The diameter of the stems is between 50 and 250 millimetres. It is necessary to provide the gasifier with clean and fairly dry wood-fuel without stones, clay or extra mineral matter; otherwise the ash
in the gasifier becomes slag and disturbs the gasification process. Therefore an area with hard floor, for the feedstock preparation, is arranged near to the gasifier plant. It is recommended that trunks of wood are first cut to lengths from 1.5 to 2.5 metres for easier preparing and handling in the wood-fuel comminution system. To accelerate the drying process it is advantageous to split the stems in the open air. A simple hydraulic splitter is used. Before chipping, the moisture content of the fuelwood is on average 40-42% and it dries in sixty days in open air to 28% and covered by a roof to approximately 25% (see table 6.1). For practical and safety reasons the plant stores woodfuel for roughly three months consumption within the site.
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Table 6.1 Drying properties of the fuelwood (9)
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The gasifier fuel should preferably be smaller than 70 to 90 millimetres in length of 300 cm in volume. A drum chipper, with a capacity of approximately 20 m /h, and driven by a wood-gas engine (Mercedes-Benz, 110 kW power output) is used for this purpose. The chipper has a diameter of 1000 mm and rotates at 1000 rpm. By running it for 4 — 5 hours/day the required woodfuel amount for 24 hours operation can be produced. Table 6.2 shows a normal particle size distribution of the produced wood chips.
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Table 6.2 Particle size distribution of the qasifier-fuel(9)
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The bulk density of the wood chips is approximately 400 kg/m3 at 25% moisture content.
From the chipper the wood chips are transported to a buffer storage with a conveyor. The capacity of this covered buffer is approximately 300 m and its surface area 200 m. The woodchips are loaded from the buffer storage into the feed hoppers of the gasifiers with the aid of conveyors. The feed hoppers are equipped to use the exhaust gases from the engines, to further reduce the moisture content of the wood-fuel. The moisture content of the chips decreases with 5-10% units after contact with the approximately 150°C hot exhaust gases during 12-15 minutes. An analysis of 15 samples of wood chips gave an average moisture content of 19.2%, with a variation between 10.2 and 28.7%. The size of one feed hopper is 8 m, which will suffice for three hours operation, at full gasifier load.