Experience on Combustion and Co-Combustion of Greek Brown Coal in Fluidized Bed Facilities

The paper aims to present the experience gained from the combustion trials of Greek brown coal in different installations, both in semi-industrial and laboratory scale. Specifically, these research activities are separated in two parts, i.e. combustion tests using only brown coal and co-combustion tests with brown coal and biomass. Combustion tests with Greek lignite were realised in three different Circulating Fluidized Bed Combustion (CFBC) facilities. Low rank lignite was burned in a pilot scale facility of approx. 100kW thermal capacity, located in Athens (NTUA) and a semi-industrial scale of 1.2 MW thermal capacity, located at RWE’s power station Niederaussem in Germany. The results include the determination of operating conditions to achieve proper fuel burnout, the examination of the influence of air staging on the temperature distribution inside the reactor and the investigation of the combustion behaviour of the particular fuel type and emitted pollutants. Several conclusions are drawn concerning the necessary modifications and requirements of the plant layout when a large scale CFBC installation is designed to utilize low grade brown coal. Co-combustion tests with Greek xylitic lignite and waste wood were carried out in the 1 MWth CFBC installation of AE&E, in Austria. During the tests, oxygen concentration and CO, SO₂, N₂O and NO_X emissions were continuously monitored. Ash samples were collected and analysed for heavy metals content in ICP-AES spectrophotometer. The improved combustion behaviour of this lignite type was more than evident, since it has lower moisture content and increased calorific value. In all co-combustion tests, low emissions of gaseous pollutants were obtained and metal element emissions were lower than the corresponding values anticipated by the guidelines. In addition, lab-scale co-combustion tests of Greek pre-dried lignite with biomass were accomplished in a bubbling fluidised bed. The main purpose of these experiments was to examine ash melting problems and differentiation to the emitted pollutants due to biomass addition. The obtained results of all aforementioned activities showed that fluidised bed is the appropriate combustion technology to efficiently exploit the low quality Greek brown coal either alone or in conjunction with other biomass materials.