Boiler modification. The injection of biomass or RDF into the furnace of a pulverised coal-fired boiler can have a significant impact on plant performance and integrity. Depending on the co-firing ratio and the nature of the biomass, the installed equipment will require modification. For low co-firing ratios, major modifications are not required. This has been demonstrated at Fusina and Kardia PP, with 5-10% thermal input. For higher co-firing ratios, significant modifications to the fuel handling and firing system are normally required, e.g. at Rodenhuize PP, with 50- 100% thermal input. Within the DEBCO project, CFD modelling has proved to be of value for the assessment of the modifications to the boiler and firing systems. The activities have included calculations of velocity and temperature fields, as well as the flue gas composition. The CFD simulations should preferably be validated with experimental measurements, e.g. velocity, temperature, chemical species concentrations. As stated above, at high biomass co-firing levels and 100% biomass firing significant modification of the fuel handling and firing systems are normally required, as at Rodenhuize power plant to 50% co-firing (advanced green) and 100% wood pellet combustion (xxx xxxxx). At Rodenhuize, the existing burners have been modified significantly for biomass firing. The inlet section of the primary air annulus has been redesigned with a hexagonal cross section with the internal surfaces completely covered with ceramic tiles. It is considered that this shape has several advantages: - The kinetic energy of the wood dust particles at the inlet is partly absorbed by the side of the hexagon which is perpendicular to the wood dust flow, - The hexagonal shape contributes to the generation of an even distribution of the wood dust all around the primary air tube, - The primary air tube can be turned in order to extend its lifetime by presenting another side to the inlet wood dust flow. The wood dust is transported in dense phase independently from the primary air and is injected in a concentric way inside the primary pipe about one meter upstream of the inlet to the burner. Another challenge was to ensure efficient combustion of the milled biomass without any support fuel. During the operation at 50% biomass/coal co-firing, biomass was injected together with coal through a number of the burners. In this operating condition, the coal combustion was well controlled, and the biomass combustion was supported by the pulveris...
Boiler modification. Rodenhuize Unit 4 also serves as back-up for a new, dedicated Blast Furnace Gas (BFG) unit located on the Arcelor-Mittal site. The boiler was retrofitted to operate on the following fuels and capacities: - 560 MWth on BFG - 200 MWth on natural gas (only during start-up) - 560 MWth on biomass (industrial wood pellets) The emission requirements (at 11% O2, dry) are: - NOx: <60 mg/Nm³ (BAT) - SOx: <50 mg/Nm³ - Dust: <10 mg/Nm³ Light Green: Co-firing of pulverised wood pellets at row 2 (40 t/h) Advanced Green: Co-firing of pulverised wood pellets at row 2+3 (80 t/h) Start-up fuel changed from HFO to NG Xxx Xxxxx: Co-firing of pulverised wood pellets at row 1+2+3 (120 t/h) BFG burners level 5 to level 2. The major system modifications, in the framework of Xxx Xxxxx, have been: - NOx abatement system in the flue gas line to meet the BAT emission limits for NOx - Retrofit of the ESP to increase its lifetime and to meet the dust emission limit: - Renewing electrical motors for the ID fans - Modifications to CW pump to improve efficiency - New FW pump (incl. Motor and VSD) with higher efficiency - New DCS/Control system Figure 5: a) after conversion to coal; b) Advanced Green, wood dust firing @ rows 2 & 3; c) Xxx Xxxxx xxxx dust firing @ rows 1,2 & 3. BFG burners/NG burners The BFG burners are out of service most of the time, and the cooling of the large furnace openings, of greater than 1.2 m in diameter),has to be ensured properly, while minimising the negative impact of the cooling air flow on the biomass combustion zone. As shown in Figure 5, a number of the BFG burners are located at the same level as the biomass burners. It was decided, after several CFD studies, to use low temperature re-circulated flue gas, taken after air heater and ESP, as the cooling medium, because of its lower oxygen content compared to combustion air. This tended to limit the disturbance of the biomass flame controlled by undesired air entrance into the boiler. Thereby, the re-circulated flue gas circuit has been completely modified to ensure the effective cooling of the BFG burners. To create the optimal conditions for the biomass combustion, i.e. ensuring good flame stability without support fuel and minimising the NO and CO emissions, it is necessary to match the combustion air supply closely to the fuel flow. The NG burners are combined with the biomass burners on levels 1 and 3 to reduce the number of new furnace penetrations, and hence minimise air in-leakage. The position of t...
Boiler modification. CFD simulations were realised with commercial software. The scope of the activities was to identify the optimum size of the biomass particles for the co-firing ratio to be used. One of the most important properties of the biomass particles is the non-roundness. Therefore, updated models for biomass combustion and equation of motion were implemented in the CFD model. The investigations were mostly focused on the effect of the biomass particle size on the fuel burnout. Different options of biomass feeding were investigated to identify the optimum positions for biomass injection. Based on the reference case (lignite feeding at nominal load), differences were estimated in the combustion performance when adding biomass. The results include calculations on velocity and temperature fields. Other parameters, such as NOx emissions, were also considered and heat calculations result for the various co-firing scenarios. and mass balance Figure 7: Boiler geometry of Kardia PP and schematic representation of simulated burner geometry. Operating Scenarios CFD simulations were performed based on current full-load operating conditions for lignite combustion and heat and scenarios. mass balance calculations results for the various co-firing The biomass particle size is a key parameter in the evaluation of CFD results. Thereby, a number of cases were considered. Two biomass diameters are examined (1 mm and 5 mm). The smaller particle size corresponds to typical biomass size for co-firing applications and can be achieved through a separate size-reduction step. The larger particle size, which is somewhat higher than the typical size ranges for co-firing, is achieved directly at the field when the collection of cardoon is performed with forage harvesters. Results of the CFD modeling - Overall, several important operational parameters are unaffected by co-firing biomass at low thermal loading: the furnace exit temperature is only slightly increased, so there is no expectancy of intensifying slagging/fouling phenomena related to the lignite particles in comparison with the reference operational scenario. The total heat transfer to the furnace wall is likewise unaffected. CFD analysis suggests that a potential benefit of co-firing conditions is a decrease of NOx emissions up to 10%, mostly due to the lower nitrogen content of the biomass fuel and the respective mechanism for fuel NOx formation. This trend is well attested by literature and pilot scale testing, but not by short ter...
