John Goldring, RJM International, UK
Wholesale replacement of outdated equipment to meet emissions regulation is often not an option because of cost and lengthy downtimes (meaning loss of earnings on top of the additional cost), so many site managers are now looking to emissions control specialists to help them achieve compliance without having to resort to major component replacement.
Through a combination of complex monitoring of every critical stage of the combustion process and Computational Fluid Dynamic (CFD) analysis it is possible to identify and make sufficient changes in just a handful of key areas to ensure not just compliance, but achieving standards at times far exceeding current regulations.
CFD helped to upgrade the burners on Mersin Soda’s three boilers to both oil and gas firing
RJM International (RJMI), one of the emissions control specialists, has recently applied its experience and CFD modelling know-how to determine the most suitable technologies to achieve reductions in nitrogen dioxides (NOx) at three sites in Europe: Mersin Soda in Turkey, AES Tisza II in Hungary and AES Kilroot in Northern Ireland. Significantly, two of these projects included the upgrade of existing low-NOx systems and showed how they may be further improved to allow ongoing LCPD compliance at minimal cost and downtime.
Dual firing system achieved
Mersin Soda in Turkey is an industrial plant manufacturing chemicals for the glass industry, and is located 160 km southwest of Adana on the Mediterranean Sea. The scope of this project was to upgrade the burners on all three boilers to offer the plant operators the option of natural gas firing, while maintaining the original oil firing capability.
On one of the boilers, a Schelde unit fitted with Hamworthy low-NOx burners, the client had concerns about the firing system change because even though the oil atomizers had been modified in the lower rows to keep the flames away from the floor, it had suffered from flame impingement and boiler vibration ever since it was first commissioned. To resolve this, RJMI employed CFD modelling to replicate the existing system and determine what changes were needed to solve the ongoing issue of flame impingement. The model showed the concentration of carbon monoxide (CO) inside the burner when firing oil.
In assessing the problem, the combustion and CFD engineers could see from the model that the swirl direction of the existing burners was pulling the fires onto the floor. To solve the problem, new counter-rotating swirl directions to the burners were imposed.
Following installation, combustion was optimized by balancing air and fuel flows. The optimization process resulted in a final boiler excess O2 of < two per cent from a starting point of > 4.5 per cent, thereby improving boiler efficiency. Flame impingement and furnace vibration were also eliminated. The RJMI burners on all boilers met all performance guarantees for both oil and gas firing and additionally, NOx was 180 mg/mO3 at three per cent O2 at NG firing, and 380 mg/mO3 at three per cent O2 for HFO firing.
Hitting NOx target
When Hungary joined the European Union (EU) in 2004, plant managers were faced with having to implement a step-change in emissions performance to meet the more stringent requirements of the LCPD.
One such plant was the AES Tisza II plant (built in 1977), which is located near the city of Tisza, some 160 km east of Budapest. To help it meet the new NOx targets AES appointed RJMI. Its existing furnace and burner geometries were entered into the baseline CFD and the model was run until convergence was achieved. The data were then analyzed in terms of fluid flow and emissions, paying close attention to thermal behaviour, NOx profiles and CO profiles.
Figure 1: Baseline NOx contours
Figure 1 shows the baseline NOx contours, as predicted by the CFD analysis, and it is clear that that the convergent effect of the flames has a detrimental effect on the emissions profile of the furnace. Since there is a large region of high peak flame temperature, the majority of the NOx is produced close to the burners.
Figure 2: Predicted NOx contours following upgrade
Figure 2 shows the predicted NOx contours for the upgrade and should be contrasted with Figure 1 for the baseline. It can be seen that the high NOx region has been significantly reduced, primarily because of the reduction in peak flame temperature.
The temperature contours in the vertical plane of the furnace (through the burner centre line) are shown in Figure 4.
Figure 3: Furnace baseline temperature
Compared with Figure 3 (the baseline) the peak flame temperatures have been reduced from 1927 ºC in the baseline to 1816 ºC in the upgrade and the region of peak flame temperature is significantly more diffuse, resulting in improved heat transfer in the furnace, and then through the convective heat exchangers in the boiler.
Figure 4: Furnace temperature following upgrade
Once the CFD analysis had confirmed what burner upgrades were required, RJMI set about designing and commissioning the fabrication of new low-NOx gas burner assemblies, low-NOx stabilizers and low-NOx oil atomizers. These components were manufactured for RJMI at specialist fabricators in the UK. Following the RJM upgrade, NOx reductions of 80 per cent with gas and 72 per cent with oil were achieved. Boiler efficiency at full load under performance test conditions was also increased by 1.1 per cent (from 93.8 per cent to 94.9 per cent) resulting in major savings in fuel and boiler running costs.
Modelling of carbon oxidation
AES Kilroot power station is a 520 MW plant, located at Carrickfergus, northeast of Belfast. There are two 260 MW coal and oil fired NEI boilers. The boilers are tangential fired and are currently fitted with a low-NOx concentric firing system (LNCFS II).
Current NOx emissions are 650 mg/mO3 on coal and 430 mg/mO3 on oil. The target NOx is less than 500 mg/mO3 on coal, and under 400 mg/mO3 on oil. RJMI was brought onboard to install burner modifications, overfire air (OFA) modifications and mill classifier upgrades. Critically, the upgrade had to achieve minimal changes to the carbon-in-ash (CIA), while achieving the desired NOx emission levels.
The real challenge for the Kilroot project was to contain the impact on CIA while achieving the desired NOx reduction. RJMI was able to successfully develop a method that provided excellent results and insights into the char reaction process (carbon burnout). This method was used extensively during the engineering phase of this project.
Visual diagnostics were used to identify carbon-making streams. This showed where the coal was going in the furnace, and where it was being oxidized. Another diagnostic identified high carbon concentrations during OFA optimization and showed how the coal particles interacted with supplied air though either the offset or OFA ports.
A systematic optimization path was used and the table summarizes the key results.
The table also gives the CIA from the baseline case as a function of injection port. It was found that 52 per cent of the CIA came from a lower mill. By increasing the amount of air to the lower furnace, the CFD model showed CIA was reduced. Several OFA iterations were investigated until an optimal configuration was determined predicting a 33 per cent NOx reduction with a good reduction in CIA.
The CFD modelling predicts that the emission guarantees will be met and has provided a valuable insight into how the combustion system should be optimized to control both NOx and CIA.
Versatility of the virtual world
These three cases highlight that when CFD is used with experience and in-depth knowledge of combustion and boiler performance it contributes to the delivery of a fast-track, low-cost route to establishing detailed plant performance data as it relates to the physical combustion process.
Once this virtual picture has been aligned with the plant’s actual performance characteristics, then the CFD model can be run again and again, changing settings as required, until the desired emissions reduction and other plant performance improvements are achieved. All this takes place off-site, which means the plant can operate normally.
When it is time to convert the CFD settings into the physical upgrades, they can be achieved with minimum capital outlay and, by their very nature, keep costs and downtime to a minimum. So, from a plant manager’s point of view, CFD combined with real experience in the field, really is the secret to banishing those LCPD blues!