Design of Combustion Technology for Low- Cost NOx Reduction and Fossil to Biomass Conversion Supported by CFD Modelling

Transcription

1 Design of Combustion Technology for Low- Cost NOx Reduction and Fossil to Biomass Conversion Supported by CFD Modelling Pauli Dernjatin, tel.: , Kati Savolainen, tel , Antti Heinolainen, tel , Jouko Heikkilä, tel , Fortum Nuclear and Thermal Power, P.O. Box 100, FI Fortum, Finland Perttu Jukola, tel.: , Marko Huttunen, tel , Sirpa Kallio, tel , VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI VTT, Finland

2 Abstract Reduction of NOx emission and introduction of biomass based energy are important topics considering retrofitting of existing combustion units inside the EU. Tailored solutions are required almost without exception. Computational Fluid Dynamics (CFD) modelling is a powerful tool to investigate, verify and optimize suggested solutions. Fortum delivers techniques for low-cost NOx reduction in pulverized fuel (mainly coal) and fluidized bed combustion. Techniques are grounded on established low-nox burner technology, air staging and selective non-catalytic NOx reduction (SNCR). Low-NOx burners have been developed also for co-firing applications, lean gas combustion and bio-oil firing. VTT has extensively applied CFD modelling in co-operation with Fortum for 25 years to design and develop the techniques in question. Here we present three examples of retrofitting or conversion projects where modelling studies have played a substantial role. The cases presented are: 1) the retrofitting of Jaworzno pulverized coal power station (6 x 225 MW e ) in Poland with low-nox combustion technology and SNCR. CFD was applied to verify the combustion system performance. 2) Conversion of Vaskiluoto 560 MW fuel coal boiler for indirect biomass co-firing, where 140 MW of coal is replaced by product gas from a biomass gasifier. A tailored low-nox burner was developed for lean product gas combustion in the Vaskiluoto case. Modelling was utilized in burner development, to select the locations for the four lean gas burners to be installed and to analyze the furnace performance. 3) The CFD aided design of specific low- NOx combustion techniques for fluidized bed combustion to be combined with SNCR. Rauhalahti bubbling fluidized bed boiler (300 MW fuel ) in Finland has been retrofitted accordingly resulting in 25% primary and 50 % total NOx reduction. A modelling study of a 235 MW e circulating fluidized bed boiler firing lignite demonstrates the possibility to apply a similar method for CFB boilers as well. 1. Introduction The emission of nitrogen oxides from large combustion plants has to be reduced inside EU from 2016 onwards as stated by the industrial emission directive (IED). Many existing plants need some kind of retrofitting to comply with the new requirements. On the other hand the target to increase the share of renewables in energy production may encourage the plants to seek possibilities for at least partial fossil to biomass conversion depending on the market situation. Reasonable and cost-efficient retrofitting requires tailored case by case solutions. Fortum delivers tailored techniques for low-cost NOx reduction mainly in pulverized fuel and fluidized bed combustion. Techniques are grounded on established and well proven low-nox burner technology, air staging and selective non-catalytic NOx reduction (SNCR). Specific

3 low-nox burners have been developed also for solid biomass co-firing applications, lean gas combustion and bio-oil firing. VTT has since 1989 co-operated with Fortum in combustion technology development by extensively applying computational fluid dynamics modelling (CFD). CFD is a powerful tool to investigate, verify and optimize suggested solutions quickly at low cost. The modelling framework utilized consists of 1) the latest commercial and open source CFD tools, 2) physical and chemical sub-models (user-specified, if needed) based on scientific literature or derived from experimental data and 3) professional experience from industrial scale combustion modelling since the beginning of 1980 s. The models are constantly developed, refined and validated against experimental data available. New modelling capabilities are created to meet future needs. In the following few examples of retrofitting or conversion projects are presented and discussed. In all of those cases modelling studies have played a substantial role. 2. The Jaworzno low-nox project In the on-going project scheduled between Fortum retrofits six 225 MW e hard coal fired boilers in Jaworzno Poland to meet the IED NOx requirement of 200 mg/m 3 n (dry at 6 % O 2 ). The furnaces in question feature front wall firing technology with totally 24 burners in four rows (4 x 6). The latest generation of NR3 type low-nox burners enabling vertical tilting of the flame combined with an advanced, multi-stage over-firing air (OFA) system is used for primary NOx reduction while SNCR with urea injection is used for secondary reduction. In this case CFD modelling was applied prior to the modification to verify the combustion system performance with a preliminary design at full load operation. The result indicated that primary NOx reduction of 20 % compared to a system equipped with more conventional low NOx burners and single stage OFA and an emission below 300 mg/m 3 n would be attainable. Simultaneously fly ash unburned carbon was predicted to increase by a percentage point. In other respects the boiler performance was rather similar in both cases. Later the role of primary air in NOx reduction was investigated with the actually installed combustion system. The simulations showed that decrease in primary air flow, which in this case was remarkably large, leads to reduction in NOx emission. Primary NOx level below 200 mg/m 3 n was predicted to be possible at full load with the three lowest burner levels in operation. This conclusion was confirmed in experiments. The SNCR system in Jaworzno was designed by the technology provider without CFD modelling contribution from VTT. The actual experience from the boilers handed over at the time of writing this paper show that primary NOx levels below 300 mg/m 3 n and emission levels below 190 mg/m 3 n are constantly attained - the latter with SNCR. Measured primary NOx values from a couple of test runs are presented in figure 1. In operation with the three lowest burner levels the primary method keeps NOx emission below 200 mg/m 3 n whereas with the three uppermost levels the

4 emission is more load dependent varying between mg/m 3 n in test conditions. In the latter case the combustion system is likely to be more sensitive to the combination of stoichiometry, residence time and thermal NOx formation at the OFA level. The fly ash unburned carbon (UBC) is generally measured to be at maximum 1.5 percentage points higher than before the modification. CO emission is typically smaller than 100 mg/m 3 n and in all cases below 300 mg/m 3 n. Furnace slagging is not observed to be increased in coal firing and no side effects in oil firing or at boiler start-up are detected. Boiler steam performance and dynamics remained unchanged. The typical daily average for consumption of 32.5 w-% urea solution remains between l/h with temporary variations between l/h. Ammonia concentration in flue gas stays below 5 ppm. Figure 1: Jaworzno, measured primary NOx results without SNCR 3. Vaskiluoto: burner technology for gasifier product gas co-firing with coal Vaskiluoto is a combined heat and power plant with capacity of 560 MW fuel located in western Finland. The plant fires pulverized hard coal as the main fuel in opposite wall firing (4 x 4 burners) mode. A two stage combustion system i.e. low-nox burners combined with OFA is adopted to control the emission of nitrogen oxides. However in 2012 the furnace was converted for indirect biomass co-firing utilizing lean gas as the secondary fuel. Lean gas is produced in a biomass gasifier located at the site next to the main boiler. A quarter of the coal,corresponding to 140 MW, may be replaced with lean gas at full load conditions. In the conversion project Fortum was the lean gas burner supplier and responsible for the commissioning of the combustion system. Lean gas is combusted with four burners placed on both side walls in the bottom part of the furnace as shown in figure 2. The burners operate at sub-stoichiometric conditions similarly

5 to coal burners maintaining the two-stage combustion principle. A tailored low NOx lean gas burner specifically designed for Vaskiluoto was developed for the purpose. The final burner design is presented in figure 3. Figure 2: Vaskiluoto, furnace (showing also the mesh used in CFD simulations) Figure 3: Vaskiluoto, lean gas burner design

6 CFD modelling was utilized in different phases of the work: at first in burner development as geometrical features of the device were refined and the final design was verified supported by modelling. According to the simulation results the designed burner ensures a very stable flame. This proved to be true in reality as the experienced flame stability has been excellent. The simulated and real flame shapes are compared in figure 4. Figure 4: Vaskiluoto, lean gas burner: simulated (top) and real (bottom) flame shapes The whole furnace was also simulated in order to analyze the combustion behavior in cofiring mode and to look for optimal locations for the gas burners. The locations shown in figure 2 were decided based on the results. The whole furnace simulations indicated that the gas burners reshape the furnace flow pattern because of the crossflow they create affecting the coal flames as presented in figure 5. Due to the relatively small volume of the Vaskiluoto furnace the flame interactions are generally more intense than in many other cases. Additionally, the flow pattern is affected as some of the coal burners (at least one row) are switched off in co-firing. Some cooling air is fed through the idle burners. Furnace heat transfer below nose level was predicted to be enhanced by some 5 % since combustion shifts down in the co-firing mode. On the other hand the total flue gas flow rate increases as coal is partially replaced with a more low-calorific fuel. The quantity is dependent on gasified fuel properties such as the moisture content etc. In the simulated case the increase in flue gas flow

7 was around 5%. As a consequence the furnace exit flue gas temperature was simulated to decrease by 30 o C which is good considering the potential superheater corrosion issues related to biomass based fuels. Despite these changes the fly ash UBC and CO as well as NOx emissions were predicted to remain practically at the same level compared to coal combustion. Figure 5. Vaskiluoto: simulated temperature distribution at selected vertical planes in the furnace. Left: coal firing 560 MW, right: coal 420 MW + lean gas 140 MW In reality the lean gas burner designed is identified to provide an extremely stable flame. Thanks to stability the whole furnace has been successfully operated in gas-only mode at load around 140 MW. In general no disturbances are detected in co-firing with coal. The boiler performance has been stable with normal steam parameters. NOx emissions have been reduced by % compared to coal mode an even better outcome than predicted by modelling. 4. NOx reduction methods for fluidized bed combustion 4.1 Bubbling fluidized bed (BFB) furnaces Previously Fortum and VTT have developed a specific low NOx combustion technique for bubbling fluidized bed (BFB) boilers [1]. The development work was mainly conducted by utilizing CFD modelling. The technique named here as enhanced air staging is based on optimization of furnace air distribution with an additional air introduction as presented in figure 6.

8 The enhanced air staging method was applied to retrofit the 300 MW fuel Rauhalahti BFB boiler located in central Finland and firing fuel mixtures consisting of peat and woody biomass. Peat is the main fuel component constituting typically around 70 % of the energy input. Preliminary CFD studies indicated that with the new method combustion may be shifted down in the furnace and NOx reduced from around 400 mg/m 3 n to 250 mg/m 3 n (dry at 6 % O 2 ) as presented in figures 6 and 7. SNCR seemed to be well applicable for secondary reduction. Figure 6. Rauhalahti: simulated temperature distribution at selected planes in the furnace (preliminary CFD study). Left: standard operation mode, right: enhanced air staging Figure 7. Rauhalahti: simulated NO distribution at selected planes in the furnace (preliminary CFD study). Left: standard operation mode, right: enhanced air staging.

9 The furnace was later modified according to the principles suggested by modelling results. In performance tests conducted NOx reduction from around 400 mg/m 3 n to 300 mg/m 3 n (dry at 6 % O 2 ) was achieved in conditions similar to those presumed in preliminary studies. At the same time fuel burnout and CO emission remained unaffected. Increase in recirculated flue gas (FGR) flow to control the fluidized bed temperature was concluded to be the main reason for smaller NOx reduction than what was predicted beforehand. Bed heat load rises as the combustion shifts down in enhanced air staging mode. Additional cooling of the bed provided by FGR may be necessary with relatively dry (moisture content below 50 w-%) fuel mixtures such as the one fired during test runs. Preliminary SNCR tests were conducted as well with urea solution as the reagent. The emission was reduced down to 200 mg/m 3 n (dry at 6% O 2 ) when applying SNCR together with enhanced air staging. The CFD model, particularly the NOx sub-model was validated against test data with and without SNCR. A comparison is presented in figure 8. The NOx model seems to work accurately in the test conditions. Further CFD studies revealed that SNCR performance can be easily improved by enhancing droplet penetration (nozzle type selection) and by relocating the injection nozzles to more suitable positions an example of this is presented in figure 9. Figure 8. Rauhalahti: comparison of measured and simulated NOx emission (mg/m 3 n, dry at 6 % O 2 ). Enhanced air staging with and without SNCR. In future the enhanced air staging method will be optimized further in Rauhalahti case and a full scale SNCR system will designed. CFD results provide a sound basis for the work.

10 Figure 9. Rauhalahti: comparison of the tested, preliminary SNCR installation (left) and an improved SNCR system (right) based on simulation results 4.2 Circulating fluidized bed furnaces Similar ideas that have been proved to work in bubbling fluidized bed combustion were tested in circulating fluidized bed combustion (CFB) as well. For that purpose a comprehensive CFD modelling study of a full scale CFB boiler firing lignite was conducted. A novel closure method [2] developed at VTT enabling efficient time-averaged simulation of gas-solid flows in combination with combustion [3] and NOx emission [4] models for CFB combustion was applied in the simulations. However, due to the multiphase nature of the flow adding a new level of complexity into the modelling, the results are estimated to be more qualitative and less quantitative in comparison to cases presented in previous sections. The furnace simulated is almost similar to a 235 MW e CFB boiler in Turow, Poland although all geometrical details were not extractable from the public sources available for the work. The considered fuel input is 670 MW of lignite. New air staging technique was investigated and compared with the normal mode of operation. The new method features deeper air staging with some of combustion air introduced as over fire air (OFA) in the upper part of the furnace.

11 According to the results over 50 % reduction in NOx emission would be possible with proper staging and positioning of OFA. Additionally, the model predicts no major drawback in CO or fixed carbon burnout. Furnace temperature is predicted to rise due to changes in lower part stoichiometry and fluidization. Fluidization velocity decreases below OFA level leading to reduced solid flow to the cyclones. The main characteristics of the flow are preserved, however. Selected simulation results are presented in figure 10. Figure 10. Results from CFD simulation of air staging in a CFB boiler 5. Summary Low-NOx combustion technology based on novel low-nox burners and proper air staging in combination with well-designed SNCR system has enabled retrofitting of the pulverized fuel fired boilers in Jaworzno and the Rauhalahti fluidized bed boiler to meet the IED requirements. A new lean gas burner provides excellent flame stability together with low emissions in Vaskiluoto case ensuring reliable operation of the boiler in gasifier product gas co-firing with coal. In all cases the system design and technology development were substantially supported by utilization of CFD modelling. Modelling results indicate substantial NOx emission reduction possibilities also for CFB boilers.

12 References [1] Jukola P, Huttunen M., Dernjatin P., Heikkilä J., New methods for NOx emission reduction in fluidised bed combustion: CFD modelling results from a stationary fluidised bed boiler, VGB Powertech Journal 93/11 (2013) p [2] Taivassalo V., Kallio S., Peltola J., On time-averaged CFD modeling of circulating fluidized beds, Int. Journal of Nonlinear Sciences and Numerical Simulation 13 (2012), p [3] Taivassalo V., Peltola J., Kallio S., Time-Averaged Modeling of a Circulating Fludized Bed Combustor, FBC21 (2012), Naples, Italy [4] Jukola P., Kallio S., Dernjatin P., Time-averaged CFD Modelling of NOx Reduction by Over Fire Air in a Full Scale CFB Furnace, FBC22 (2015), Turku, Finland