how the experienced use of cfd analysis ......rjm corporation – ipg conference 06 1 of 10...

RJM Corporation – IPG Conference 06 1 of 10 www.rjm-international.com

HOW THE EXPERIENCED USE OF CFD ANALYSIS ALLOWS GENE RATORS TO MEET STRICT LCPD NOX (EMISSION) REQUIREMENTS WITH PROVEN TECHNOLOGY AT LOW COST Author: John Goldring, RJM Corporation (EC) Ltd Co-Author: Larry Berg, RJM Corporation (EC) Ltd This paper will show how RJM applies its experience and CFD modelling to determine the most suitable technology for NOx reduction, and allows three power plants in the EU to meet the strict requirements of the LCPD and their contracted power purchase agreements. The three plants are AES Tisza II, Essent Energie Clauscentrale and AES Kilroot. CFD Analysis and combustion experience was used as an important and integral part of the engineering to confirm the expected performance. Significantly two of these projects include the upgrade of existing low NOx systems and show how these may be further improved to allow compliance with the LCPD at minimal cost and downtime. 1.0 AES TISZA II AES Tisza II is an 860MWe gas and oil fired power plant situated in Hungary. The original boilermaker was SES Tlmace with burners by Deutsche Babcock mounted in the boiler floor. There are four boilers each with a capacity: 215MWe. The original NOx for gas firing was 1000mg/m

03 and AES needed to reduce this below 350mg/m

03 . The original NOx for oil firing was

850mg/m03 and AES needed to reduce this below 400mg/m

03 . Following RJM burner

modifications final NOx figures achieved were 199mg/m03 on gas, and 250mg/m

03 oil.

RJM Corporation (RJM) designed burner upgrades to reduce NOx emissions from these boilers. A CFD modeling study was requested to verify that the desired NOx reduction could be achieved. 1.1 Baseline Furnace Model For the baseline furnace model, the original burner design and configuration was modeled. As mentioned above, this model serves as a baseline from which relative NOx reduction is measured. Existing furnace and burner geometries were used in baseline CFD. The CFD model was run until convergence and then analyzed in terms of fluid flow, and emissions. Specifically, RJM examined thermal behavior, NOx profiles and CO profiles. 1.2 Temperature Profile - Baseline

The temperature contours in the vertical plane of the furnace are shown in Figure 1. A characteristic feature of burners that fire upwards directly into the outlet is that flames tend to converge together, towards the center.


1.3 NOx Emissions Baseline

The predicted NOx contours for the baseline are shown in Figure 3. From figure 3 it is clear that that the convergent effect of the flames has a detrimental effect on the emissions profile of the furnace.

This NOx chart corresponds to the region of high peak flame temperature visible in Figure 1.

Since there is a large region of high peak flame temperature, the majority of the NOx is produced close to the burners.

1.4 CO Baseline The contours of CO are shown in Figure 4 (across the burner center line) and in Figure 5 (iso-surface at 15,000 ppm). From the figures also, the elongated flame shape is clearly visible, as is the convergent behavior of the flames within the furnace combustion space.

1.5 Furnace Models with Burner upgrades For the burner upgrade furnace model, the RJM burner upgrade design and configuration were modeled. The horizontal burner pitch was maintained at 2600mm and the vertical burner pitch was maintained at 2250 mm.

Figure 1

Figure 3

Figure 2

Figure 4 Figure 5


As with the baseline case, the upgrade full-furnace CFD model analyzed flow, thermal patterns and emissions. The model predictions for temperature, NOx and CO will be presented here.

1.6 Temperature Profiles - Upgrade

The temperature contours in the vertical plane of the furnace (through the burner center line) are shown in Figure 6. Compared to Figure 1 for the baseline, we see that peak flame temperatures have been reduced from 1927oC in the baseline to 1816oC in the upgrade and that the region of peak flame temperature is significantly more diffuse.

The improved, stabilized flame is not as elongated as the baseline and the overall volume of the high temperature region has been reduced, contributing to the NOx reduction. 1.7 NOx Profiles - Upgrade The predicted NOx contours for the upgrade are shown in Figure 7 and the NOx iso-surface contour at 150 ppm is shown in Figure 8. Figure 7 should be contrasted with Figure 3 for the baseline. Both figures are set to the same scale. We see that the high NOx region has been significantly reduced, mainly because of the reduction in peak flame temperature.

Another feature of the upgrade that is immediately apparent is that the overall NOx level in the combustion space has been reduced in the upper region of the furnace (green in the baseline – Figure 3, and blue in the upgrade – Figure 7) and almost eliminated in the lower region of the furnace.

Confirmation of the NOx reduction can be seen by looking at Figure 8.

1.8 CO Emissions - Upgrade

The contours of CO are shown in Figure 9 (across the burner center line). From Figure 9 and by comparison with Figure 4 (the baseline CO contour) we see that the staged flame has significantly reduced the flame elongation which contributes to reduced CO at the nose. By

Figure 6 Figure 1

Figure 7 Figure 8


staging the flame and increasing lateral diffusion of the fuel-rich region, the high CO region is restricted to the lower half of the furnace.

1.9 CONCLUSIONS – AES TISZA II

• The CFD predicted a 63% NOx reduction from the baseline with upgraded burners. • Following the installation 80% NOx reduction was achieved with gas and 72% with

oil. • The CFD model confirmed the expected emissions reduction before installation of the

hardware and allowed the furnace firing injection pattern to be optimized for better temperature distribution and low CO emissions.

• Project objectives were met for €2/kWe 2.0 ESSENT ENERGIE - CLAUSCENTRALE Essent Energie Clauscentrale is a 1280MWe power plant situated in The Netherlands. There are two 640MWe Stork supercritical boilers each firing Bio-oil and natural gas through 18 opposed fired burners. The original burner make was Lentjes low NOx burners and each burner is rated at 110MWth. The original NOx was ~400mg/m

03 on gas and the target NOx; <200mg/m

03 . Oil fired

emissions for oil must be kept below 400mg/m03 . RJM low NOx burner modifications and

increased FGR rates were applied and have been firing since November 2005. One of the major problems was that the units suffered from flame instability and severe furnace vibrations at higher excess oxygen and FGR rates with the original burners. The solving of this problem was a key objective of the upgrade. A Computational Fluid Dynamics Analysis (CFD) investigation was conducted to determine the following affects from a proposed burner modification prior to installation of the hardware:

1. Stability 2. NOx reductions due to mechanical changes 3. NOx reductions due to increased FGR flow rates 4. Changes to furnace exit gas temperatures (FEGT) at higher FGR flows.

A single burner CFD model was employed to investigate stability, mechanical changes and increased FGR rates. A full furnace model was used for investigation of FEGT.

Figure 9 Figure 4


2.1 Model Summary Single Burner Models

A model of a single burner was constructed so that the geometry of the flame stabilizers, new gas pokers and spray patterns could be examined and compared in detail to the existing burner. Single burner models allow RJM to carefully investigate burner design details, and quickly optimize components.

In addition to the existing configuration, a model of the proposed modifications was developed. In order to ensure accurate flow representation, exact poker drillings were utilized.

Full Furnace Models

In addition to the burner models that were developed, a full furnace model of the upgrade configuration was constructed. Since the furnace is fully symmetrical front to back, a symmetry plane was imposed, and only half of the furnace modeled. Due to the complexity and non-uniform poker arrangement, a full furnace model of the existing configuration was not developed.

2.2 Modeling Effort

Stability Study

Using the simplified geometry established from the modeling effort, a combustion model is utilized to calculate temperature and flow fields for both the existing and proposed upgrade geometries.

The 0% FGR figure compares the temperature contours (degrees C) for the existing geometry and proposed upgrade. In both cases, high temperature zones are shown near the burner outlet. This is as expected, as a stable burner will have a flame “stabilized” by some aspect of

23% FGR

15% FGR

6% FGR

0% FGR

Baseline Upgrade

Figure 10 – Existing Burner Model Geometry

Figure 11


the burner geometry. This stabilized flame is by definition near the burner. It should be pointed out, that temperature contours will be examined in this study for this reason, it is anticipated that instability will be first noticeable by examination of the temperature contours.

For the 6% FGR case there is no difference between what was seen at 0% FGR except for modest reductions in temperature (the same scale will be kept for the entire study).

For 15% FGR, significant variations in the baseline flame are observed. The existing geometry shows no high temperature region near the burner. The entire high temperature region is located 3 to 4 burner diameters downstream (3 to 4 meters). In this configuration, reliable combustion seems to be occurring, but because there is no point where the flame always exists, the type of flame will have large-scale movements which were observed in the field as furnace “rumble” or combustion driven vibration.

Contrary to the existing geometry, the proposed modification maintains a high temperature region near the burner, which will ensure the combustion driven vibration will not occur.

For 23% FGR the existing configuration still reliably combusts the fuel and oxygen, but the flame is detached from the burner (large scale vibration would be anticipated) and the proposed modification still has a high temperature region near the burner.

2.3 Summary of Model Results

Table 1 - NOx Results

Estimated Upgrade NOx – All Fuels Gas Palm Oil % Reduction % Reduction Upgrade 0% FGR 74.7% 71.10% Upgrade 6% FGR 80.10% Upgrade 15% FGR 85.80% 74.50%

Figure 12 below compares baseline NO with the upgrade for gas and oil.

Full Furnace Models

Full furnace simulations were accomplished for gas and oil fuels at 0% and 15% FGR rates. This was accomplished to compare Furnace Exit Gas Temperatures (FEGTs) with the boiler performance modeling that was accomplished. In addition, absolute NOx estimations from the models were compared to single burner predictions as an independent check.

Baseline NO Gas Baseline NO Oil

Upgrade NO Gas Upgrade NO Oil

Figure 12


2.4 CONCLUSIONS – ESSENT ENERGIE CLAUSCENTRALE

• Burner modifications, which include new flame stabilizers and atomizers are shown to produce a stable gas flame.

• Burner stability is demonstrated on gas from 0% FGR to 23% FGR flow rates. • Baseline CFD compares well with the Baseline Field Test Data • Furnace Exit Gas Temperatures are calculated for the various conditions. Good

comparison exists with the boiler performance evaluation, carried out to ensure boiler performance at higher FGR rates.

• CFD model confirms that emissions objectives of the project are met: NOx Natural Gas <200 mg/Nm3 Palm Oil <400 mg/Nm3

Both achieved while operating with CO emissions <200 ppm (@3%O2 dry) • RJM expects to demonstrate the performance objectives before the end of 2006 • Project objectives will be met for a cost of €2.5/kWe

3.0 AES KILROOT POWER AES Kilroot Power Station is a 520MWe plant situated in Northern Ireland. There are two 260MWe coal and oil fired NEI boilers. The boilers are tangential fired and are currently fitted with an Alstom Low NOx Concentric Firing System (LNCFS II). Current NOx emissions are 650mg/m

03 on coal and 430mg/m

03 on oil. The target NOx is <500mg/m

03 on coal, and <400mg/m

03

on oil. RJM will install burner modifications, SOFA modifications and Mill Classifier upgrades. Critically the upgrade must achieve minimal changes to the Carbon-in-Ash (CIA) while achieving the desired NOx emission. In carrying out the CFD operation 3 Mills out of 4 had to be simulated so that the plant could maintain full load generation with spare Mill capacity. Given the stringent plant availability requirements of AES’s PPA this is an important criterion to maintain. Two firing configurations were modeled to ensure this requirement was covered – one with the upper Mills in service and the other with the lower Mills in service. The CFD analysis completed by RJM showed that the modifications proposed would achieve compliant operation for both Carbon in Ash (CIA) and NOx. 3.1 MODELING OF CARBON OXIDATION The real challenge for the Kilroot project is to contain the impact on CIA while achieving the desired NOx reduction. RJM was able to successfully develop a method that provides excellent results and insights into the char reaction process and this method was used extensively during the engineering phase of this project. From the literature, there are three significant carbon oxidation reactions: C + O2 => CO2 C + CO2 => 2 CO C + H2O => H2 + CO


a1 2.5% a2 6.7% a3 3.1% a4 14.1% b1 2.8% b2 8.8% b3 2.0% b4 8.1% c1 6.8% c2 16.4% c3 10.3% c4 18.5%

Table 2 Upper Mills Baseline CIA by Coal Injection

There is some disagreement about the kinetic parameters for the CO2 and H2O reactions, but it is generally agreed that they only occur at high temperatures. The O2 reaction has been studied extensively, and there is fairly good agreement between research groups about the rate parameters.

Identification of Carbon in Ash Sources The model allows for identification of carbon sources from individual injections. Table 2 shows the predicted Upper Mills Baseline CIA, broken down by coal injection. In this case, the first letter is for the mill (a, b, c, or d mill), and the number is for the corner. So injection “c1” means corner 1, c mill injection. Interestingly, 51% of the predicted baseline Upper Mills CIA comes from the lowest level – “C” mill.

Identifies “C” concentration Visual diagnostics are also available to supplement the quantitative information. Figures 13 and 14 give examples of this type of diagnostic. Figure 13 shows coal particle trajectories coloured by carbon concentration. The injection (B4 in this case) shown starts off with maximum carbon concentration (red), and as the carbon is oxidized, the paths become more blue, with dark blue being nearly 0% carbon. This type of diagnostic not only shows where the coal is going in the furnace, but where it is being oxidized. Figure 14 shows carbon concentration as a filled contour. This allows for identification of high “C” concentrations during SOFA optimization.

Shows interaction of particles with SOFA’s and part icle interaction Figure 15 combines a 10% iso-surface of oxygen with coal paths colored by carbon concentration. This unique type of view shows how the coal particles interact with supplied air though either the offset or SOFA ports. Figure 16 shows two injections - B4 & C4. Even though the two injections exit over a large region of the furnace exit, the unburned carbon part of the injections combine, and exit at a common location.

Figure 13: Carbon burnout – red is high C, blue is no C

Figure 14: Carbon burnout – red is high C, blue is no C – SOFA level


3.2 COMPUTATIONAL FLUID DYNAMICS ANALYSIS

• Upper Mill Operation A systematic optimization path was used and Table 3 gives a brief overview of the important cases and results.

Case CIA NOx Reduction Base 6.45% - Modify Coal Injections 16.9% 25% SOFA Optimization 4.65% 33%

Table 3

Summary of Upper Mills Optimization Table 3 gives the CIA from the baseline case as a function of injection port. As pointed out in the text, 52% of the CIA comes from “C” mills. By increasing the amount of air to the lower furnace, the CFD model showed that “C” mill CIA was reduced. Several SOFA iterations were accomplished until an optimal configuration was determined. Optimization was aided by the insights provided from the CIA model and diagnostics (see above). Currently the optimal configuration is predicting a 33% Nox reduction, with a good reduction in CIA. Figure 17 compares the 10,000 ppm iso-surface of the base configuration to the final optimized configuration. As can be seen, final SOFA orientation has mostly eliminated CO prior to furnace exit.

Two Streams Figure 15

Figure 16: One exit for two fuel injections

Figure 17: CO plumes for base case (left) and upgrade (right).


• Lower Mill Operation Table 4 gives a summary of the optimization paths. Case O2 CIA NOx\

(mg) NOx Red.

Base Case – Lower Mills 3.4% 5.98% 554 - Modest Sofa 4.34% 6.2% 410 26% Modest SOFA + Opt. Lower Furnace Air 4.34% 3.45% 41 6 25%

Table 4

Summary of Lower Mills Optimization Lower Mills optimization started off with modeling of the baseline data, and then modeling a NOx compliant upgrade. As per the upper Mills firing case the CFD highlighted that >60% of the CIA was being produced by D Mill, the lowest firing Mill. Starting from the NOx compliant case and no SOFA air, one level of SOFA ports was utilized. In order to keep consistency with the Upper Mills model and knowing that SOFA injection angles would be fixed for all operational modes the same SOFA angles as determined from Upper Mills optimization were used for the Lower Mills optimization. A marginally compliant CIA case was obtained. Then both SOFA levels were utilized and final results predict a good NOx reduction and a reduction to the baseline CIA.

3.3 CONCLUSIONS – AES KILROOT

• Upper Mills operation is predicted to achieve compliant operation, but will require injection angle changes to the SOFA ports to ensure the furnace doesn’t stall.

• Lower Mills operation is predicted to achieve compliant operation with the same injection angle changes to the SOFA ports with good margin on the CIA emission.

• CFD modeling predicts the emission guarantees will be met and has provided an invaluable insight into how the combustion system should be optimized to control both NOx and CIA.

• Cost for meeting the project objectives is estimated at €8/kWe 4.0 PAPER SUMMARY & ACKNOWLEDGEMENTS This paper has shown how the emissions targets for three different styles of boiler, firing oil, gas, coal and bio-oil each fitted with different firing systems have been optimized using combustion experience and sophisticated CFD modeling. In two cases the emissions have been improved from an existing low NOx baseline. The modeling program in each case faced additional challenges including the elimination of furnace vibration, flame instability, uneven temperatures and Carbon-in-Ash containment. Of these plants two have the hardware upgrades installed and are fully operational and firing successfully. RJM employs CFD modeling as a standard in all emissions reduction projects and Generators benefit from low cost solutions being proven before the hardware is installed. The CFD model provides the added benefit of allowing optimal operational settings to be predicted which in turn reduces commissioning periods and outage times. RJM would like to thank AES and Essent Energie for their kind permission to publish this paper.

how the experienced use of cfd analysis ......rjm corporation – ipg conference 06 1 of 10...

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