Advanced technologies for biomass boilers in the MW-Range

Solving the problem of heavy metals

Due to environmental pollution heavy metals as Cd and Zn are deposited in trees and can pose an environmental risk when the wood is burned. Dedicated research on the behaviour of heavy metals during wood combustion has led to an understanding of the mechanisms relevant for their deposition in the ashes. Technical measures were developed and realised in a 5 MW district heating plant that lead to a fractionated deposition of heavy metals in the biomass ashes: 80% of Cd are deposited in fine fly ashes that amount to only 5% of the mass of total ashes. Thus the greatest part of ashes can be used for agricultural purposes safely while a small amount must be deposited as toxic waste. Present research is aiming at further increasing the amount of heavy metals in fine fly ashes to 90-95%. As fine fly ashes cannot be removed sufficiently in multicyclone precipitators a second stage of flue gas cleaning by electric precipitators, a flue gas condensation unit or the innovative rotating particle seperator are necessary.

Contact: University of Technology Graz

Rotating particle separator: a cost effective technology for the abatement of fine fly ashes

As of January 1998 a new limit value of 50mg/m³ for dust emissions applies in Austria for biomass combustion units with a nominal boiler capacity of more than 2 MWth. The rotating particle separator (RPS) is a technology newly adapted for the abatement of fine fly ashes from biomass boilers.

Fig. 6: Rotating particle separator: a cost-effective technology for the abatement of fine fly ashes Source: Energy from Biomass - R&D in Austria (Ministry of Science and Transport)

As the diagram shows it is almost as effective as an electric precipitator but significantly cheaper. The RPS consists of three main parts:

• a static body which is designed like a cyclone,
• a filter element rotating around a vertical axis and
• a cleaning system to remove the precipitated particles from the walls of the filter element.

Fig. 7: Separation efficiency and investment costs of different technologies Source: Energy from Biomass - R&D in Austria (Ministry of Science and Transport)

The RPS can be equipped with an impeller fixed at the top of the filter element that covers the pressure drop of the RPS and the boiler so no additional draught fan is necessary. The core of the RPS is the filter element which consists of many small parallel channels with a diameter of about 1.5 mm, rotating as one body around a common axis.

The flue gas is pre-cleaned by the cyclone, enters the filter element from the bottom and flows through the channels in axial direction to the top. As a result of centrifugal forces particles are moved in radial direction towards the walls of the filter channels and are precipitated there. The dust collected and agglomerated on the channel walls is removed periodically by injecting pressurised air at high velocity in reverse flow direction into the channels. This is done by a nozzle moving over the rotating filter element at periodic intervals without disturbing the operation of the RPS. Test runs with a pilot RPS have shown that, due to agglomeration effects of aerosols in the RPS, the precipitation efficiencies reached for particles smaller than 1.0 µm are even better than expected according to calculations. Numerous dust emission measurements were performed during test runs in 1997. The results show that the dust emissions of the plant could be reduced from 120-300mg/m³ (with multi-cyclone only) to 40-50 mg/m³ (dry flue gas, 13.0 percent by volume O₂).

Fig. 8: Scheme of a rotating particle separator (RPS) Source: Energy from Biomass - R&D in Austria (Ministry of Science and Transport)

As the pressure drop of a RPS is similar to that of a multi-cyclone the operating costs are comparable and about equal to those of electrostatic filters. A further advantage of the RPS is its low space demand which is equal to a multi-cyclone but significantly lower than the space needed for electrostatic or fibrous filters. Taking these cost factors and the dust pre-cipitation efficiency of the RPS into consideration, the economic as well as ecological advantages of RPS are considerable.

Contact: BIOS

Reduction of NO_x-emissions

Due to the nitrogen content of the fuel, biomass boilers have relatively high NOx emissions, particularly if wastes such as e.g. particle board residues are burned. Austria is involved in international research activities to bring down NOx emissions, with a focus on primary measures, i.e. by optimising combustion parameters. Especially for fuels with high N content NOx reductions by primary measures of up to 50% were possible.

Contacts: Joanneum Research; BIOS; Technical University of Vienna

Active flue gas condensation with a resorption heat pump

Due to the high water content of bark and woodchips from sawmill industry (typically between 30% and 50%) significant amounts of energy are used for the evaporation of water and lost through the chimney. Flue gas condensation units which are installed in a number of Austrian district heating plants, allow to recover this energy by cooling down flue gases to the point of condensation of the evaporated water. In practice however, the temperature of the water returning from the district heating system is not low enough, to take advantage of this considerable amount of energy.

A new concept for a resorption heat pump with a mechanical compressor has been developed which should allow to recover 6-10 times more energy, than it needs for operation. The power output of such a unit could reach 50-90% of the boiler power, as shown in the graph. This would reduce both fuel costs and boiler costs due to the lower boiler power requirement. The compressor of the heat pump could be powered by a Stirling engine, described in the section on electricity production from biomass. The realisation of a pilot plant is under preparation.

Contact: Joanneum Research

State of the art straw combustion

Straw is a large biomass resource which is yet hardly exploited. One of the reasons for the limited use of straw as an energy carrier, are its difficult properties for combustion, particularly the low ash melting temperature. Another problem is straw feeding. The milling of straw for combustion requires a lot of energy and rather robust milling equipment. In Austria a new type of straw boiler for the requirements of small district heating plants was developed.

Advanced control technology and special construction features allow highly reliable straw combustion at extremely low emissions (20mg CO/Nm³).

Contact: Kohlbach Inc.

Technologies for enhanced fuel management and boiler efficiency

A continuous process of technical improvement is accompanying the diffusion of biomass district heating. Among the critical elements of biomass use is the mechanical management of the fuel transport to the boiler. A decade´s experience was necessary to develop fully reliable feeding technology. The most recent innovation serves to remove the need to fill the temporary storage by a tractor or caterpillar. A "Walking Floor" allows to empty the whole contents of the fuel storage barn automatically.

Contact: Mawera

For the operator of a district heating plant fuel humidity is a critical parameter as it deter-mines the usable energy content of the fuel (unless a flue gas condensation unit is installed). Research has led to a fast, simple, reliable and precise device for fuel humidity.

Contact: Pandis

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