Optimisation of combustion efficiency and emissions in coal-fired boilers using Computational Fluid Dynamics

J. Vanormelingen, Laborelec – Combustion for Power Generation

As more power generation facilities fall under increasingly strict environmental regulations, many difficult emissions control questions must be answered. In addition, the changes on the electricity market make efficient generation vital if a plant is to remain viable. Until recently an effective method for identifying optimal emissions control technologies has been elusive. Fortunately, emissions reduction schemes can now be evaluated in a scientific and unbiased manner using Computational Fluid Dynamics or CFD to model all the relevant physics and chemistry of the plant. Numerical simulation of boilers reveals a lot of information regarding combustion efficiency, emissions, corrosion and erosion damage, heat transfer, slagging, fouling and flow distribution

Fig. 1: Numerical grids of several boilers (from left to right: Langerlo 1 (B), Ruien 5 (B) and Gelderland (NL)

Laborelec has adopted CFD to improve plant profitability in several Belgian and Dutch power plants owned by the utility operator Electrabel (fig. 1). In order to achieve the project targets, the execution of a typical CFD-project is organised into four different phases: ?? Phase I: Characterisation of the boiler: this phase includes the determination of

geometrical parameters as well as the boundary conditions ?? Phase II: Baseline simulation ?? Phase III: Validation using measured emission values, temperatures, etc., but also e.g. in-

furnace measurements ?? Phase IV: Simulation of different scenarios: in this last phase the fuel type can be altered,

burner settings, etc. A simulation was carried out for boiler #1 (235 MWe) of the Genk -Langerlo power station (B). This boiler was built as a fuel oil-fired boiler, but has been retrofitted to a pulverised coal, front-wall fired boiler. A reacting-CFD study has been performed to assess the overall combustion performance. The CFD modelling allowed to identify problems related to the coal burner air distribution. Examining the NOx formation in the furnace revealed that some burners showed a jet burner pattern which was mainly attributed to the high core air momentum of these burners. This effect destroyed the internal recirculation

1E-05 2.5E-05 5E-05 0.0001 0.00025 0.0005 0.001 0.0025 0.005

Fig. 2: Total deposition rates on furnace walls in kg/m2.s

zone and led to high NOx production rates. An optimisation of the core air momentum led to a significant reduction of the NOx concentration at the furnace exit. This reduced operating costs by minimising the ammonia consumption of the SCR DeNOx installation and enhanced the lifetime of the catalyst. Furthermore, the 3D-furnace model allowed to identify the potential contribution of individual particle sizes of the different fuels to the total unburned carbon loss at the furnace exit. The computational results of the wall oxygen level indicated a potential risk for wall corrosion on the rear wall of the furnace displaying a large area of the furnace wall with wall oxygen values below 0.5 volume percent. A further question was whether the fine grind of the fuel was really necessary to avoid either slagging or burnout problems. An optimisation of the grind back to a slightly coarser level reduced the energy required for the milling devices and thus enhanced the efficiency of the power station. Moreover, deposition rates due to slagging were predicted (fig. 2). Another simulation was performed for the tangentially-fired boiler #5 (200 MWe) of Ruien power plant (B). In this power plant, co-gasification of biomass will be adopted as an attractive and efficient way to use biomass sources in renewable energy production. This is performed through partial gasification in a circulating fluidised bed reactor and injection of the syngas into the existing boiler. When using co-gasification, it is crucial to determine the optimal injection point for the low calorific gas in the existing coal-fired boiler, in order to avoid disturbances in the boiler that could severely affect coal combustion. This would result in damages, slagging, fouling or increased emissions.

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Fig. 3: Averaged values of unburned carbon in ash and NOx as a function of furnace height for baseline case and 2 syngas injection options.

In the first stage, a baseline simulation without circulating fluidised bed operation was carried out. Subsequently, the location of the syngas injection was varied in order to obtain minimal NOx levels. At the same time, the fouling on the superheater tube banks was restricted to a certain level. These simulations led to an optimal injection point for the syngases. The NOx concentration was reduced with approximately 70 mg/Nm3 compared to the baseline case, while the values for unburned carbon remained the same (fig. 3). Furthermore, fouling of the superheater tube banks was minimised. A pure experimental determination of different syngas injection positions would have been a time-consuming task that would have caused a high financial effort. A similar simulation was done for the boiler of Gelderland power station in Nijmegen (NL). This boiler is boxer-fired and represents 600 MWe. A feasibility study similar to Ruien 5 was carried out in order to investigate the impact of the injection of syngases into the existing boiler. Again, a 10% NOx reduction was predicted compared to the baseline case. CFD simulations can be used to improve the performance of systems throughout a power plant. Improvements in efficiency and reductions in emissions achieved have been shown to be real and have given significant economic benefits to the operating utility. The true power of computer simulation lies in its ability to analyse trade-offs. Comparison of cost versus performance is one of the key advantages of these simulation studies. Using CFD it is possible to evaluate dozens of different options and obtain hard data on which to base decisions, without risking substantial capital outlay.

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