Objective
The aim is to demonstrate that the efficiency of biomass boilers can be significantly improved by decreasing the temperature of the flue gases to levels at which condensation of combustion products occurs. The addition of a condensation exchanger with high heat transfer coefficients increases the amount of sensible heat which results from the latent heat of the condensed products.
The plant furthermore will demonstrate automatic regulation of excess air and an improved dust separation technique.
Corrosion problems in the tubes of the condensation heat exchanger have resulted in temporary shutdown and bypassing of the heat exchanger. The cause of the corrosion was investigated. It was found that corrosion was caused by surface deposition of the ashes on the horizontal tubes of the condensation heat exchanger. The heat exchanger was replaced with another one made of iron (same material) which was also corroded within one heating season. The boiler operates successfully while the condensation heat exchanger is bypassed. It can be concluded that for boilers in this small scale, this kind of technology is not adequately developed because of the need to remove completely the ashes and organic gases upstream the heat exchanger when operating with biomass of high moisture content.
A plant for producing heat from wood blocks (L 300 mm, D 100 mm) and wood chips as its source of energy will be established at the heating plant of SCI des Mauges, 49300 Cholet France. The plant will provide heating to 672 houses in this area. The boiler has an output of 3000 kW and the condenser produces an additional 600 kW. The overall efficiency is 95 %. The fuel is wood of high moisture content (50 %).
The wood is fed into the boiler by a hydraulic piston, into a flat water-cooled or refractory steel grate. Combustion takes place in a large flame furnace divided into two sections : at the base the fuel is gasified by an adiabatic process and the gas burns at the top by means of secondary air injected through a narrow section providing a proper lean-gas burner. The ash is transported by means of a second piston which advances at regular intervals.
A special process enables the high moisture content fuel to be burnt. It consists of recirculating hot gas taken from the top of the furnace and reinjecting it through the bed of fuel in the furnace inlet zone with a recycling rate of 10-15 %.
A scrubber-heat exchanger is used to condense the flue gases and to remove any remaining dust particles (after the flue gases have been cleaned in a multicyclone). Condensation increases the efficiency of moist biomass boilers.
The control of excess air is based on the measurement of the oxygen concentration (by zirconium dioxide detector) at the flue gases. Depending on this value the boiler programmer adjusts the feed rate of wood fed into the furnace in order to maintain a constant oxygen concentration.
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Coordinator
44042 Nantes Cedex
France
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