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FP5

BIO-ENERGY CHAINS Informe resumido

Project ID: ENK6-CT-2001-00524
Financiado con arreglo a: FP5-EESD
País: Austria

Information on the performance of combustion tests

Combustion test runs with the four perennial crops were performed at a laboratory scale fixed-bed reactor as well as in a pilot-scale fixed-bed combustion plant.

The investigations at the pilot-scale plant focused on the identification of possible operational problems such as fuel feeding, ash melting on the grate and in the furnace, ash transport systems, emissions (NOx, SOx, HCl and particulates), formation and growth of deposits (fouling tendencies), fractionation of elements in the different ash fractions.

The results from the evaluation of the test runs performed indicates, that a temperature below about 1,100°C is necessary in order to prevent ash melting on the grate. It is therefore recommended that technologies used for wheat straw combustion such as water cooled furnace walls and a water cooled grate should be used for the combustion of these crops.

Another possibility to avoid slagging problems would be to blend these fuels with a less problematic fuel such as wood pellets.

However, the test runs at the pilot-scale combustion unit have shown, that Switchgrass, Miscanthus and Giant Reed could be utilised in such a combustion unit but not Cardoon samples, which was characterised by extremely high K, Si and Clcontents.

From the results of the test runs it can be concluded, that the combustion characteristics as well as the problems occurring during combustion are similar for all four perennial crops investigated. The design of a combustion plant using all these crops would be similar as well except for the Cardoon tested, which is not suitable for combustion.

The main problem would be to design a suitable fuel feeding system. Bales are probably the easiest and most efficient way for transporting for large-scale combustion plants. In the case of small-scale plants pelletised fuels are the best suitable solution in order to ensure high enough bulk and energy densities of the fuels, which makes them suitable for the fuel feeding systems usually applied. Although pelletised perennial crops have a higher energy density than chopped fuels, still problems with reaching the full load of the plants designed for wood pellets are observed.

Consequently, new designs with a larger combustion chamber in relation to the boiler capacity would be necessary for an optimised combustion of these crops.

Moreover, also the increased amount of ash produced from these crops has to be considered in the design of the ash removal systems of the plant.

According to the test run at the pilot-scale combustion plant about 87 to 95% of the ashes produced remained as bottom ash. Due to the generally low concentrations of environmentally relevant heavy metals in these ashes, the ashes could be recycled and used as fertilisers on the soils where the crops were grown.

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Ingwald OBERBERGER
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