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Co-firing of biomass, coal waste and coal in mining sites for electricity generation

Exploitable results

The present project has covered the different aspects required to implement the best technology for co-firing of biomass (mostly forestry and sawdust residues) with coal and coal waste in Narcea I, power plant and to determine the overall feasibility. Initially an estimation of biomass waste was carried out in the Tineo area (Spain), where the plant is located, not only the amount produced in sawmills, but also all wastes left in the woodland as a result of forestry activities. According to the assessment carried out, the use of coal residues is not possible from an economic point of view, and is also environmentally unacceptable. Due to this circumstance, other coal-based products were studied such as high-ash coals or coal from the mine without any kind of pretreatment, and consequently with less additional costs. Information about gross coal production and production foresees were obtained from the mining companies in the area. Afterwards representative biomass and coal samples for carrying out the analysis were collected. These analyses have consisted of proximate analysis, ultimate analysis, particle size distributions and determination of the heating value that were carried out according with standard accepted procedures (ISO standards when applicable). Simultaneously a review of the state of art was performed which allowed to identify the advances and operational problems in relation to pretreatment, energy generation and waste production. Once the most promising technological solutions for Narcea I were identified, the overall process with Aspen Plus was modelled in order to predict the behaviour of the plant with the changes suggested. Considering the results obtained in the modelling stage the testing was designed, which can be divided in: - Lab scale tests: led to the determination of the "universal" kinetic parameters for different types of coal and biomass and the determination of the "high-temperature volatile yield". - The main conclusion of this stage is that the high-ash coal and standard coal have similar kinetic parameters. Therefore the high-ash coal does not offer any possible advantage to the overall process because it does not improve the kinetic aspect and it worsens the thermodynamic and environmental aspects (lower LHV and higher ash production than standard coal). - Semi-industrial tests: two different strategies have been considered, combustion of coal/biomass blends and separation injection of biomass. It has not been considered to introduce high-ash coal or coal from mine without any kind of pretreatment. According to the results obtained, the oak wood has been selected as the biomass fuel, in the proportion of 20% of total thermal input and the reburning technique has been selected as the best one considering the geometry of the Narcea I furnace (arch-fired). Based on the results of the activities previously mentioned and in the engineering experience of different companies, the technological modifications required to shift coal combustion to co-firing were described in detail and the technoeconomic feasibility of these modifications. Afterwards the economical feasibility of the project was studied. To determine this feasibility not only the modifications were considered (because the avoided costs will not possibly compensate the investment required) but the overall economical situation of the plant. After performing the economic analysis of the feasibility of the modifications, it was concluded that the modifications are not economically feasible because the incomes associated to the electricity selling cannot compensate the investment required and the operational and maintenance costs. Additional benefits in relation to NOx reduction are not valuable, due to the fact that no regulation applies to set incomes/cost reduction about this environmental effect. If the regulation applicable, the remaining life of the plant and the market conditions are modified, the economic feasibility of the proposed modifications will be affected. The modifications could become economically feasible.