ENK5-2000-00324 Advanced Biomass Reburning in Coal Combustion Systems - ABRICOS

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EESD (Energy, Environment and Sustainable Development) : Solid Biofuels : Thermochemical Conversion

Proposal No:	ENK5-2000-00324
Date Prepared:	February 2004
Source:	European Bio-Energy Projects (EUR 20808)

Introduction

The NO_x targets to be met by power plants in the coming years, set both by European and national directives, are still to be specified. These will require plants to continue operating at relatively high load factors to achieve NO_x reduction, aiming to meet emissions of around 200 mg/ Nm³. A technically feasible option for this reduction, and with a high potential for the retrofitting of existing industrial boilers, is offered by fuel staging or reburning. The relatively high volatile content of biomass makes this type of fuel a perfect candidate for this technique.

The use of biomass as a reburn fuel allows for a secondary improvement at the same time by contributing to the mean CO₂ reduction policy of the European Community. However several scientific and technologic aspects should be worked out before this potentially very interesting technique can spread in the industrial world.

Activities

The scientific approach used in the project consisted of three steps. Firstly, the physical and chemical mechanisms were investigated in laborator y scale pilots and described with numerical models. Secondly, tests were carried out in semi-industrial scale facilities to assess the technique and validate the models developed. Finally the results were implemented in a simplified model and extrapolated for a full scale industrial plant.

Results

The ABRICOS project is now in its final year and the major part of the experimental and model development work has been carried out. Concerning the use of biomass as a reburn fuel, three biomass fuels (poplar, straw, eucalyptus) have been extensively characterised. Their specificity as a reburn fuel is firstly their high volatile matter content (typically up to 80% dry basis, compared with typically 30% for bituminous coals).

The analysis of these volatile matters measured at 1350°C showed also a greater relative content in species such as C₂H₂, C₂H₄ and C₂H₆ which are believed to play an important role in the reburning chemistry. The biomass chars were all found to be very reactive compared with coal char. This is important since the ash quality (i.e. the unburned carbon content of the fly ash) is one of the issues of the reburning technique. It also means that the biomass char combustion is controlled by the oxygen diffusion to the biomass particles; the size distribution of the biomass is probably the most important fuel characteristic regarding the ash quality.

The detailed chemistry of the biomass reburning process was analysed and an augmented reduced mechanism has been implemented in the computational fluids dynamic software AIOLOS#. Experimental investigations in a 0.5 MWth combustion rig showed that biomass reburning can reduce the NO_x concentration by up to 60% of the value at the end of the primary zone within a split time of half a second.

The optimum NO reduction is obtained with an air ratio in the reburn zone of between 0.8 and 0.85. This optimum corresponds to a share of the secondary fuel typically between 15 and 20% of the thermal input (depending on the primary section conditions and biomass composition). The residence time in the reburn zone is not as important for the NO reduction as for the ash quality.

The efficiency of the biomass reburning technique has been proved in a semi-industrial facility of 5MWth located in Santa Gilla (Italy). NO_x emissions as low as 190 mg/Nm³ at 6% 0₂ were obtained. This result corresponds to a NO_x reduction of 55% compared with the reference case (without reburning and without OFA) and still a reduction of 25% compared with the case with OFA only. The measurements carried out during these tests were also used to validate the previously developed NO_x models.

Concerning the impact of the biomass reburning on the fly ash characteristics, the measurements made during the combustion tests showed the following:

• the resistivity of the fly ash is almost unchanged,
• it results in the formation of the finest particles and a so-called bi-modal size distribution at the outlet of the boiler. This modification of the fly ash size distribution is important since it will impact negatively on the efficiency of the electrostatic precipitators,
• the shape and density of the biomass fly ash may be very different from those of the coal fly ash. One of the reasons is that biomass fuels usually present a needle structure, in contrast with coal particles commonly considered as spherical. This characteristic will impact in particular on the transportation and deposition rate of the particles in the ducts going from the boiler to the electrostatic precipitator.

To take into account these modified fly ash characteristics, a previously existing ESP numerical model was enhanced. Based on laboratory tests, a new model describing in detail the ash layer behaviour and the reentrainment processes was developed.

Comprehensive measurements of particles and flue gas characteristics were performed at the duct inlet and downstream in an ESP pilot of 10 000 Nm³/h in Porto Maghera (Italy). A detailed mass balance including each plate and hopper allowed the influence of the electrical parameters on each zone to be quantified in well-defined conditions. Finally a complete ESP model was validated with good agreement.

Concerning the extrapolation of the results of the project to an industrial plant, the power station of Porto Maghera, Italy (70 MW_e) was chosen. A full, simplified model of the plant in a retrofitted configuration is currently under development and based on this model and economical analysis, a first assessment of the industrial relevance of the biomass reburning technique will be proposed.

Impact and exploitation

The expected impacts of the ABRICOS project are scientific, technical and economic. Several articles based on the results obtained during the project have already been accepted in scientific reviews and a better knowledge of the biomass reburning technique is thought to have been gained. The exploitation and diffusion of the results will directly concern the members of the consortium. The end-users intend to work together with the technology suppliers to find the best and quickest way to exploit the results as soon as possible after the project.