The use of low calorific value or high sulphur content coal mixed with biomass is interesting due to the complementary properties of the two fuels since, as opposed to the typical characteristics of low quality coals, biomass has low ash and sulphur contents and a high volatile concentration that is favourable to a clean combustion of the coals under consideration. However, the performance of this technique in fluidised bed facilities leads sometimes to considerable operating problems.

Within the scope of this concept, NTUA's Steam Boilers and Thermal Plants Laboratory (NTUA-LSB) performed a series of experiments in a lab-scale Atmospheric Bubbling Fluidised Bed Combustor (ABFBC), using Megalopolis pre-dried lignite with biomass material, i.e. Wood matter from pressed oil-stone (WPOS) and straw. During the trials, the applicability limits of the lignite partial substitution by biomass as a function of the excess air ratio were determined, taking also into account the special features of the combustion technology used. In this way, the effects on combustion behaviour, flue gas emissions, volatiles release, residual matter and the ash tendency for agglomerates and deposits formation were thoroughly examined. Aiming to confront the ash agglomerates formation in addition to the deposition problems presented during the experiments, pre-treatment techniques were applied on the biomass fuels and further combustion trials were carried out. The results were very positive and showed that if the pre-treatment methods are industrially attainable then the fossil fuels combined combustion with biomass is an attractive option for large-scale firing installations.

Keywords : co-combustion, lignite, Wood matter from pressed oil-stone (WPOS), straw, fluidised bed, emissions, ash agglomerates, pre-treatment techniques