(517e) Biomass Co-Firing for CO2 Management: Full-Scale Field Test and Modeling

An increasing emphasis on mitigating global climate change (global warming) over the last few decades has created interest in a broad range of sustainable or alternative energy systems to replace fossil fuel combustion. Co-firing biomass with coal in traditional large-scale coal power plants represents one of the lowest risk, least costly, near-term methods of CO2 mitigation. Simultaneously, it is one of the most efficient and inexpensive uses of biomass. Cofiring biomass reduces greenhouse gases and additionally reduces SO2 and in many cases NOx emissions. This is primarily due to lower biomass sulfur and nitrogen contents than in the replaced coal. There are some challenges to cofiring biomass with coal. The majority of co-firing demonstrations have focused on the challenges of preparation, storage, and handling. However biomass also poses additional fireside issues. This investigation has focused on one of these issues: the effects of biomass addition on flame structure and composition in the near burner region. Greater understanding in this region will be helpful in other fireside issues: such as emissions, corrosion, fouling, and slagging. Several investigations have been performed with measurements in the near-burner region of pf flames. These investigations include some or all of the following information: velocity fields, gas temperatures, particle temperatures, gas composition data, and carbon burnout. Most of these investigations have been on coal flames and most of these have been in pilot-scale systems. There are relatively few biomass or biomass-coal blend flame investigations and these are almost entirely in pilot-scale systems. Some important flame characteristics, such as the stoichiometry and intermittency of oxygen near the flame center, quite likely change with increasing scale. This research is a continuation of the work performed by Brad Damstedt, Chunyang Wu, Hong Lu, and Warren Roberts in this research group. Their work included detailed flame composition maps of pure coal, pure biomass, and co-fired flames in a 150 kWth swirl-stabilized burner and detailed particle combustion data and model development. These investigations developed the first detailed gas composition maps of a non-preblended dual fed burner and significantly changed perceptions about the structure of co-fired, pure biomass, and many coal-based flames. Although these analyses involved among the largest scale flames that can be generated at laboratory scale, some of the features noted, specifically the presence of oxygen at least momentarily essentially everywhere in the flame region, may not be as significant at the much larger flames of commercial scale. In addition, although the measured and predicted velocity profiles measured in the small-scale system reached a new level of agreement in this research group, no reacting flow velocity results were available for comparison. This research focuses on the flame composition and structure from a single burner in a utility-scale 350 MWe power plant. Both traditional extraction methods and in situ spectroscopic methods were utilized to determine gas composition profiles in the near-burner region, some of which are the first of their kind by these measurement techniques and all of which are the first of their kind at this scale for cofired burners. Gas composition measurements include major components: H2O, CO2, CO, and O2; minor components: CH4, C2H2, other UHC, and PAH; and nitrogen species: NO, HCN, and NH3. Gas and particle temperatures can also be obtained simultaneous with the composition measurement by in situ IR spectroscopy. Gas velocities in the axial and vertical direction were obtained through laser Doppler anemometry (LDA). Particles will be sampled throughout the flame and analyzed on a basis of composition and size distribution. These field tests and accompanying model predictions are being developed and carried out in coordination with partners at Risø DTU, Dong Energy, and Aalborg University. Experimental measurements will be complemented with full boiler simulations using Computational Fluid Dynamics CFD modeling. Greater node density will be placed in the near-burner region of the burner from which data will be obtained to give higher spatial resolution in this region. Inlet conditions for this model will come from a separate CFD simulation of the detailed wind box. The boiler mesh, with specific emphasis on burner dimensions will be assisted by Søren Hvid of Dong Energy's CFD modeling group. A newly-developed particle conversion model that better accounts for intra-particle gradients and simultaneous combustion steps due to larger particle sizes and different shapes will be compared against traditional particle models such as by the Discreet Phase Model (DPM) laws. Both methods will be compared against trends and order of magnitude estimates from experimental data. Preliminary data analysis have agreed with pilot scale data in that the flame length and diameter are increased with biomass addition. Flame intermittency is seen throughout the whole flame region. Some differences include separation of coal and biomass fuels and non-symmetric flames. These results were expected due to much larger biomass particles used than in pilot scale projects and horizontally fired burners in the full-scale power plants. Analysis is ongoing. Specifically data from traditional gas extraction methods will be compared with in situ measurements. Also work has begun on CFD modeling of the boiler with grid emphasis in the near-burner region.