Energy Fuels 2010, 24, 2162–2169 Published on Web 01/15/2010
: DOI:10.1021/ef9012399
Process Evaluation of Oxy-fuel Combustion with Flue Gas Recycle in a Conventional Utility Boiler Wei Zhou* and David Moyeda General Electric Energy, 1831 East Carnegie Avenue, Santa Ana, California 92705 Received October 28, 2009. Revised Manuscript Received December 24, 2009
In recent years, extensive studies were conducted to understand the potential implementation of oxy-fuel combustion with flue gas recycle in conventional boilers as an effort of mitigating CO2 emissions. The benefits of the technology include reduction of NOx and CO emissions by recycling more than 60% of the flue gas back into the boiler; generation of a flue gas stream with CO2 concentration higher than 90% on a dry basis, which can be easily recovered and stored; and utilization of the conventional boiler with minimum modifications. However, a practical implementation of the technology needs a thorough analysis of the mass and energy balance and their impacts on boiler thermal performance. This paper conducts a detailed process analysis on the oxy-fuel combustion with flue gas recycle in a conventional boiler. Key parameters such as the flue gas recycle ratio, boiler exit oxygen, and flue gas velocity and temperature distributions are investigated. Computational fluid dynamics (CFD) simulations are performed to help the understanding of the oxy-fuel combustion characteristics in a conventional boiler and in a single coal fired low NOx burner. The study shows that the ideal flue gas recycle ratio depends on boiler exit O2 and fuel properties and is generally around 0.7-0.75. Dry flue gas recycle helps to maintain the furnace temperature.
The technology of oxy-fuel combustion with FGR was first proposed by Berry and Wolsky6 in 1986. The concept allows coal to be burned in a mixture of oxygen and recycled flue gas (RFG) to produce an exhaust flue gas stream with high CO2 concentration. Recycled flue gas is mixed with oxygen for controlling of flame temperature to baseline level obtained in conventional boilers. Wolsky et al.7 conducted both pilot scale testing and heat transfer modeling to understand the boiler thermal characteristics in the oxy-fuel/FGR combustion mode. The study indicated that the flue gas recycle is a viable means of controlling the combustion and heat transfer characteristics comparable to operation on air. Wilkinson et al.8 did a study on optimization of a retrofit design concept for oxy-fuel firing in a refinery power station boiler. The study conducted a detailed CFD and heat transfer modeling to evaluate the retrofit impact on boiler thermal performance. Discussions are made on several issues related to oxy-fuel combustion retrofit, including oxygen purity, boiler turndown, flame luminosity, and boiler ingress, etc. The study increased the confidence on converting a conventional boiler to oxy-fuel operation for CO2 capture, without change in the costly steam pressure parts and without loss of duty. Assessment was also performed by Tigges et al.9 to evaluate the oxy-fuel combustion retrofit for the existing power stations.
1. Introduction Fossil fuels are the main energy resource used in utility and industrial boilers worldwide as of today. However, their combustion contributes significantly to the increased green house gas (carbon dioxide (CO2) gas) emissions, which is over 7000 million metric ton in the United States alone in 2007.1,2 Several approaches have been evaluated and reviewed for capturing CO2 for the utility industry.3-5 These approaches can be summarized in the following three categories: precombustion capture or fuel decarbonisation; oxy-fuel combustion; and postcombustion capture with CO2 separation from flue gas. This paper focuses on understanding oxy-fuel combustion with flue gas recycle (FGR), a potential technology for CO2 capture in the conventional boilers with modifications of boiler upstream and downstream auxiliary systems including addition of an air separation unit and a flue gas recycle system. However, a key issue for this technology is to understand the impact of conversion from air to oxygen/FGR mixture on boiler thermal performance and burner flame stabilities. *To whom correspondence should be addressed. E-mail: wei.zhou@ ge.com. (1) National Oceanic And Atmospheric Adminstration. Greenhouse Gases, Carbon Dioxide And Methane, Rise Sharply In 2007. In ScienceDaily; April 24, 2008;http://www.sciencedaily.com-/releases/2008/04/ 080423181652.htm (accessed May 12, 2009). (2) U.S. Department of Energy. U.S. Greenhouse Gas Emissions Still Increasing. In ScienceDaily; Dec. 5, 2008; http://www.sciencedaily.com/ releases-/2008/12/081204093041.htm (accessed May 12, 2009). (3) Tan, R.; Corragio, G.; Santos, S.; IFRF Doc. No. G 23/y/l. (4) VDZ Research Institute of the Cement Industry and PENTA Engineering Group. Carbon Dioxide Control Technology; Portland Cement Association, PCA R&D SN 3001, 2008. (5) Wall, T. F. Proc. Combust. Inst. 2007, 31-47. (6) Berry, G.; Wolsky, A. Modeling Heat Transfer in an Experimental Coal-Fired Furnace When CO2/O2 Mixture Replace Air. ASME 1986 Winter Annual Meeting, Paper 86-WA/HT51, December 1986. r 2010 American Chemical Society
(7) Payne, R.; Chen, S. L.; Wolsky, A. M.; Richter, W. F. Combust. Sci. Technol. 1989, 67, 1–16. (8) Wilkinson, M. B.; Boden, J. C.; Panesar, R. S.; Allam, R. J. CO2 Capture via Oxyfuel Firing: Optimisation of a Retrofit Design Concept for a Refinery Power Station Boiler, First National Conference on Carbon Sequestration, May 15-17, 2001, Washington DC ( http://www. netl.doe.gov/publications/proceedings/01/carbon_seq/1b3.pdf ). (9) Tigges, K. D.; Klauke, F.; Bergins, C.; Busekrus, K.; Niesbach, J.; Ehmann, M.; Kuhr, C.; Hoffmeister, F.; Vollmer, B.; Buddenberg, T.; Wu, S.; Kukoski, A. Energy Procedia 2009, 1 (1), 549-556.
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Energy Fuels 2010, 24, 2162–2169
: DOI:10.1021/ef9012399
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the boiler exit O2 in oxy-fuel/FGR combustion is calculated by MWFG;oxy ðmO2 - 0:233Rmcoal Þ ð1Þ O2;exit ¼ MWO2 ðmO2 þ mcoal ð1 - %ashÞÞ where O2, exit is the boiler exit oxygen (vol%); MWFG,oxy and MWO2 are the molecular weights of flue gas and oxygen, respectively. Since the flue gas mainly contains CO2, its molecular weight is about 40 and is higher than that of the flue gas produced in the conventional firing boiler. R is the air-to-fuel demand; m is the mass flow rate, kg/s; and %ash is coal ash content. Therefore,
Figure 1. Schematic diagram and key parameters of oxy-fuel combustion with FGR.
In the study, computational fluid dynamics simulation and heat transfer analysis were applied to compare the furnace temperature distributions for coal air conventional combustion and oxy-fuel/FGR combustion in an 820 MWe unit. The burner flame structures were also examined in the oxy-fuel combustion condition. The study shows that the retrofit will have small impact on boiler thermal performance with a slight boiler efficiency reduction due to increased flue gas flow rate and flue gas exit temperature. This paper develops key process requirements for an oxyfuel/FGR combustion retrofit via a comprehensive mass and energy balance calculations. CFD models for a full-scale wallfired unit and a low NOx burner were developed to verify the derived process requirements.
mO2 ¼ mcoal
O2;exit MWO2 ð1 - %ashÞ þ 0:233RMWFG;oxy MWFG;oxy - O2;exit MWO2 ð2Þ
To maintain the same flue gas flow rate and convective heat transfer characteristics in the boiler, the recycled flue gas flow rate should be equal to the difference between the airflow rate and the oxygen flow rate, that is, ð3Þ mRFG ¼ mair - mO2 In conventional coal/air combustion, the following relationships exist, ð4Þ mcoal ð1 - %ashÞ þ mair ¼ mFG
2. Oxy-fuel Combustion with Wet or Dry Flue Gas Recycle A schematic diagram of oxy-fuel combustion with wet or dry flue gas recycle is shown in Figure 1.7 The diagram also denotes the key process parameters such as: air flow rate (mair), oxygen flow rate (mO2), fuel flow rate (mfuel), flue gas flow rate (mFG), recycle flue gas flow rate (mRFG), and boiler exit O2 (O2,exit), etc. As shown in the diagram, the process separates oxygen from air in an air separation unit (ASU) and mixes oxygen with recycled flue gas (RFG) prior to it entering the boiler and combusting with fuel. Flue gas (FG), when it leaves the boiler, mainly consists of CO2 and water. Once dried, the CO2enriched flue gas stream can be stored or used. The flue gas can be recycled back to the furnace in either a wet or a dry stream. In order to minimize the retrofit impact on existing boiler performance, the following design criteria are proposed as a basis for the analysis conducted in the rest of the paper: (1) Oxy-fuel/FGR boiler has the same total heat input as that of the conventional boiler. (2) Oxy-fuel/FGR combustion yields the same boiler exit O2 as that of air/fuel combustion. (3) Oxy-fuel/FGR combustion yields the same flue gas flow rate as that of air/fuel combustion. (4) Oxy-fuel/FGR combustion yields the similar adiabatic flame temperature as that of air/fuel combustion. When the above criteria are satisfied, the boiler thermal and emission performance after oxy-fuel/FGR combustion retrofit can be expected to be the same as that of the air/fuel combustion baseline condition. Deviations from these criteria are likely to happen in the practical application. However, the analysis shown in this paper provides some insight on how these process parameters will impact on the boiler performance.
O2;exit ¼
0:233MWFG ðmair - Rmcoal Þ MWO2 ðmair þ mcoal ð1 - %ashÞÞ
ð5Þ
and mair ¼ mcoal
0:233RMWFG þ O2;exit MWO2 ð1 - %ashÞ ð6Þ 0:233MWFG - O2;exit MWO2
In eqs 4-6, MWFG is the molecular weight of the conventional flue gas and is around 29. From eqs 3, 4, and 6, the total flue gas flow rate and the recycled flue gas flow rate can be expressed as: 0:233ð1 - %ash þ RÞMWFG ð7Þ mFG ¼ mcoal 0:233MWFG - O2;exit MWO2 and 0:233RMWFG þ O2;exit MWO2 ð1 - %ashÞ 0:233MWFG - O2;exit MWO2 ! O2;exit MWO2 ð1 - %ashÞ þ 0:233RMWFG;oxy ð8Þ MWFG;oxy - O2;exit MWO2
mRFG ¼ mcoal
The flue gas recycle ratio then can be calculated by 0:233RMWFG þ O2;exit MWO2 ð1 - %ashÞ 0:233MWFG - O2;exit MWO2 ! 0:233RMWFG;oxy þ O2;exit MWO2 ð1 - %ashÞ MWFG;oxy - O2;exit MWO2 ! 0:233ð1 - %ash þ RÞMWFG ð9Þ 0:233MWFG - O2;exit MWO2
mRFG ¼ mFG
3. Wet Flue Gas Recycle
Equation 9 shows that the flue gas recycle ratio depends on the existing boiler exit O2 and fuel properties, such as air-to-fuel ratio and ash content. Figure 2 plots the flue gas recycle ratio as a function of the boiler exit O2 for an ash content of 0.08 at two different air-to-fuel ratios. It
A key parameter for the design of oxy-fuel/FGR combustion is the flue gas recycle ratio. It can be derived from the design criteria defined in the above section. When assuming complete combustion and 100% oxygen supply, 2163
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: DOI:10.1021/ef9012399
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Figure 3. CO2 content in CO2-enriched product.
Figure 2. Wet flue gas recycle ratio.
indicates that for a retrofit that satisfies the design criteria, the FGR ratio is a linear function of the boiler exit O2 and increases slightly with air-to-fuel ratio. In general, the flue gas recycle ratio should be around 0.7-0.75, which is consistent with the conclusion drew by Woycenko et al.10 The CO2 content in the flue gas can also be calculated. The CO2 mass fraction in the flue gas is derived as follows: mcoal %C MWCO2 X CO2, oxy ¼ ðmcoal ð1 - %ashÞ þ mO2 ÞMWC ¼
mcoal %C MWCO2 ðmcoal ð1 - %ash þ 0:233RÞ þ O2;exit mFG ÞMWC ð10Þ
Figure 4. Specific heats as a function of gas temperature.
where XCO2 is the mass fraction of CO2 in the flue gas; %C is the fixed carbon content in the fuel; and MWC and MWCO2 are the molecular weight of carbon and carbon
XCO2, oxy ¼
dioxide, respectively. Applying eq 7, the flue gas CO2 and H2O concentrations in oxy-fuel/FGR combustion can be expressed as
MWCO2 ð0:233MWFG;oxy - O2;exit MWO2 Þ%C MWC ½ð0:233MWFG;oxy - O2;exit MWO2 Þð1 - %ash þ 0:233RÞ þ 0:233ð1 - %ash þ RÞMWFG;oxy O2;exit
and
XH2 Ooxy ¼
MWH2 O %H þ %H2 O mcoal 2MWH ðmcoal ð1 - %ashÞ þ mO2 Þ
ð11Þ
If one compares the flue gas moisture content in the oxyfuel/FGR combustion (eq 13) with that in the conventional air/fuel combustion calculated by eq 14, he/she would find out that the flue gas moisture content in the oxy-fuel combustion is about 3.5 times of that in the conventional flue gas. MWH2 O %H þ %H2 O XH2 Oconv ¼ 2MWH
ð12Þ
where XH2O is the mass fraction of H2O in the flue gas, %H and %H2O are coal hydrogen and moisture contents, respectively. Applying eq 2, eq 12 becomes MWH2 O %H þ %H2 O 2MWH ! XH2 Ooxy ¼ ð13Þ MWFG;oxy ð1 - %ash þ 0:233RÞ MWFG;oxy - O2;exit MWO2
0:233MWFG - O2;exit MWO2 0:233MWFG ð1 - %ash þ RÞ
ð14Þ
The increased moisture content causes an impact on the adiabatic flame temperature. Figure 4 plots the specific heats for CO2, N2, O2, and H2O as a function of gas temperature. The plots indicate that the specific heats of CO2 and N2 are similar in the temperature range shown in the figure. However, at a typical flame temperature, that is, 2000 K, the specific heat of H2O is almost twice those of CO2 and N2. If assuming that (1) the total heat inputs for the oxy-fuel/FGR combustion is the same as that of the conventional combustion; (2) the specific heat of water is twice those for CO2 and N2; (3) the difference of oxygen sensible heat in the flue gas for the two conditions is negligible; and (4) the moisture content in the oxy-fuel combustion is 3.3 times of that in the
Similar to flue gas recycle ratio, the flue gas CO2 and moisture contents also depend on the boiler exit O2 and fuel properties, such as air-to-fuel ratio; fixed carbon content, %C; moisture content, %H2O; and hydrogen content, %H. Figure 3 plots the flue gas CO2 content in wet and dry CO2-enriched product as a function of exit O2 for a sample coal analysis. (10) Woycenko, D. M.; Ikeda, I.; de Kamp, W. L. IFRF Document No. F98/y/l.
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Figure 5. Ratio of flame temperatures as a function of flue gas moisture.
Figure 6. Dry flue gas recycle ratio with A/F of 7.76, coal ash content of 13.4%, coal H content of 4%, and H2O content of 15.45%.
conventional combustion (Figure 5), the adiabatic flame temperature ratio for the two conditions can be calculated as ð%CO2, oxy CpCO2 þ %N2, oxy CpN2 þ %H2 Ooxy CpH2 O Þ ðTf , oxy - T0 Þ ¼ ð%CO2, conv CpCO2 þ %N2, conv CpN2 þ %H2 Oconv CpH2 O ÞðTf , conv - T0 Þ
ð15Þ
where T0 is the reference temperature. Since %H2 Ooxy ≈ 3:5 %H2 Oconv and CpH2 O ≈ 2CpN2 Equation 15 becomes ðð1 - 3:5 %H2 Oconv Þ CpN2 þ 3:5 %H2 Oconv 2CpN2 ÞðTf , oxy - T0 Þ ≈ ðð1 - %H2 Oconv Þ CpN2 þ %H2 Oconv CpH2 O ÞðTf , conv - T0 Þ
ð16Þ
that is, ð1 þ 3:5 %H2 Oconv Þ CpN2 ðTf , oxy - T0 Þ ≈ ð1 þ %H2 Oconv Þ CpN2 ðTf , conv - T0 Þ
Figure 7. Wall-fired boiler CFD model.
shows that about 75% of dry flue gas recycle is required for optimal performance.
ð17Þ
5. Boiler Performance in Oxy-fuel/FGR Combustion
or Tf , oxy - T0 1 þ %H2 Oconv ≈ Tf , conv - T0 1 þ 3:5 %H2 Oconv
To verify the above analysis and calculations, computational fluid dynamics (CFD) simulation is used in this paper to show the flow and temperature profiles in a conventional wall fired boiler in oxy-fuel/FGR combustion operation. The wallfired boiler has three by four low NOx burners firing a bituminous coal and generates 280 t/h steam at full load. A CFD modeling domain for this boiler is shown in Figure 7 and consists of a hopper, the burners, and the convective pass. The CFD model, which was originally developed for the conventional coal combustion, was modified with flow inputs following the analysis in the above sections for describing the oxyfuel/FGR combustion phenomena. In the CFD model, the eddy-dissipation model, discrete ordinate (DO) model, k-ε turbulence model, discrete phase model, and porous media model were applied to account for the turbulent coal combustion, radiative heat transfer, and pressure resistance from the convective pass.11-13 The eddy-dissipation model consists of two-step global volatile and oxygen mechanism and assumes turbulence mixing dominates the combustion rate. Further study on the validity of this assumption in the oxy-fuel
ð18Þ
One way to remedy the flame temperature reduction is to use dry flue gas recycle, which will be discussed in the next section. 4. Dry Flue Gas Recycle The dry flue gas recycle system recycles the dehydrate flue gas as shown in Figure 1. In this process, the following equations exist: mFG, oxy ¼ mO2 þ mcoal þ mRFG - mash ¼ mRFG þ mH2 O þ mCO2 ð19Þ The flow rate of the water stream can be calculated approximately by MWH2 O %H mH2 O ¼ mcoal þ %H2 O ð20Þ 2MWH Therefore, the dry recycle flue gas ratio after the removal of the moisture is mRFG mRFG ¼ mRFG þ mCO2 mFG, oxy - mH2 O
ð21Þ
(11) Zhou, W.; Moyeda, D. K.; Payne, R.; Berg, M. Combust. Theory Model. 2009, 13 (6), 1053-1070. (12) Zhou, W.; , Moyeda, D.; Payne, R.; Nguyen, Q. Comprehensive Process Design of Layered-NOX Control in a Tangentially Coal Fired Boiler, in press AIChE, 2009(http://www3.interscience.wiley.com/journal/ 122598195/abstract). (13) FLUENT Theory Guide; ANSYS Inc.; www.ansys.com.
To satisfy design criteria one and two, eq 7 and eq 9 are still approximately valid and can be used to derive the dry flue gas recycle ratio. The derived ratio is plotted in Figure 6, which 2165
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Figure 8. Flue gas temperature distribution in the boiler.
Figure 9. Average flue gas temperature distribution in the boiler.
Figure 10. Average flue gas velocity distribution in the boiler.
combustion environment may be needed. The boiler and burner dimensions as well as the boiler wall temperatures and convective pass heat duties that represents the steam side heat absorptions remain the same for the model boundary conditions as those in the conventional coal combustion operation. The gas absorption coefficient is calculated by using the weighted sum of gray gases model (WSGGM).13 In the model, the total emissivity over the distance s is presented as X ε¼ aε, i ðTÞð1 - e - Ki ps Þ
indicate that wet flue gas recycle in oxy-fuel combustion produce lower flame temperature, whereas dry flue gas recycle in oxy-fuel combustion can significantly improve the furnace temperature. However, since the specific heat of CO2 is slightly larger than that of nitrogen, and the flue gas moisture content is still higher than that in the conventional combustion, the furnace temperature in the dry FGR mode is slightly lower than that in the conventional combustion operation. Furnace temperature is one factor that impacts the boiler heat transfer characteristics. Another factor is the flue gas flow velocity. Figure 10 plots the average flue gas velocity in the furnace. The flue gas velocity in oxy-fuel/wet FGR combustion is reduced due to reduced gas temperature and therefore increased density. The flue gas velocity in oxy-fuel/ dry FGR combustion is reduced in a more significant level, since the flue gas density in the dry recycle mode is larger than that of the flue gas produced in the conventional combustion. The total heat transfer rates in the lower furnace, when assuming that the wall temperatures remain the same for all conditions, are compared for the coal/air, oxy-fuel/wet FGR, and oxy-fuel/dry FGR cases and are plotted in Figure 11. The results indicate that the heat transfer rate in the lower furnace is reduced by 6% for the dry FGR oxy-fuel combustion and by 14% for the wet FGR oxy-fuel combustion. The reduced lower furnace heat transfer rate is expected to reduce the steam
where aε,i is the emissivity weighting factor for the ith gray gas, κi is the absorption coefficient of the ith gray gas, p is the sum of the partial pressure of all absorbing gases, and s is the path length. In the paper, the path length is calculated by using the characteristic cell size in the computational domain. Figure 8 shows the predicted temperature distributions in the furnace for three operation modes: coal/air, oxy-fuel/wet FGR, and oxy-fuel/dry FGR. The temperature contours indicate that the oxy-fuel/FRG combustion has similar thermal characteristics in the boiler as the conventional coal/air combustion, that is, similar flame shape and temperature distribution profiles. To further compare the temperature distributions for the three combustion modes, the average flue gas temperatures are plotted in Figure 9. The plots 2166
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Figure 11. Total heat transfer rate in lower furnace.
Figure 13. Average flue gas CO2 concentration in the boiler. Table 1. Heat Loss Efficiency Comparison conventional boilera heat loss due to dry gas heat loss due to fuel moisture heat loss due to combustion of fuel h2 heat loss due to moisture in air heat loss due to combustible in air heat loss due to radiation unmeasured losses total losses boiler efficiency a
Figure 12. Average flue gas H2O concentration in the boiler.
flow rate and increase the steam temperature and the flue gas exit temperature. Figure 12 and Figure 13 show the flue gas H2O and CO2 concentrations in the furnace. The oxy-fuel/FGR combustion whether in a dry recycle or wet recycle mode, generates flue gas with significantly increased H2O and CO2 concentration. It should be noted that all analysis conducted so far does not consider any air leakage in the boiler. Boiler in-furnace leakage will increase the oxygen content and reduce the CO2 concentration in flue gas, which may cause more challenge to downstream CO2 storage and usage. The purity of the oxygen stream is another consideration in the oxy-fuel/FGR combustion. The oxygen concentration at burner inlet has an effect on the burner zone stoichiometric ratio (SR) and therefore NOx emissions. Its impact is expected to be similar to that in the conventional combustion environment. However, a recent study14 found that the soot formation is shown to be dependent on the oxygen concentration in oxy-fuel combustion environment. More studies are warranted in this area.
oxy-fuel/FGR boilerb
3.93 1.47 4
1.97 1.47 4
0.09 0 0.25 1 10.74 89.26
0 0 0.25 n 7.69 þ n 92.31 - n
Calculated. b Estimated.
loss, L, consists of the following main items: (1) Heat loss due to dry gas, L1. (2) Heat loss due to moisture in fuel, L2. (3) Heat loss due to H2O from combust of H2, L3. (4) Heat loss due to combustible in refuse, L4. (5) Heat loss due to radiation, L5. (6) Unmeasured loss, L6. In an oxy-fuel/FGR combustion environment and assuming that O2, CO, and N2 concentrations in flue gas are negligible, the heat loss due to dry gas can be approximated as following %S ð23Þ L1, oxy ¼ 3:7 kC þ C p ðTFG - Tair Þ 267 where, kC is 1 lb of carbon burned in 1 lb of fuel, %S is coal sulfur content, CP is the specific heat of flue gas, and TFG is the temperature of flue gas leaving the boiler (air heater or economizer). Tair is the combustion air temperature. For a typical boiler application shown in the previous section where flue gas consists of 15% CO2 and 74% N2, L1, conv 15:2Cp, conv ¼ ≈2 ð24Þ L1, oxy 3:7Cp, oxy
6. Boiler Heat Loss Efficiency in Oxy-fuel/FGR Combustion
assuming the flue gas temperature at boiler exit is similar at both firing conditions and CP of oxyfuel flue gas is twice of that of the conventional flue gas. Equation 24 indicates that the dry gas loss, which contributes about one-third of the total heat loss, reduces significantly in the oxy-fuel combustion firing condition. The heat loss due to combustion in refuse may slightly reduce as well due to the recycling of 70% of flue gas. The unmeasured loss will increase due to power required for flue gas recycling and air separation, etc. The other heat losses remain similar since the fuel moisture and hydrogen contents
ASME boiler heat loss efficiency14 is calculated by the following equation: L 100 ð22Þ HL ¼ 1 Hf þ B where HL is the heat loss efficiency, L is the heat loss, Hf is the fuel chemical heat input, and B is the heat credits. The heat (14) ASME PTC 4.1
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Figure 14. Burner flames.
a significant increase of CO2 and H2O concentrations in the flue gas. The dry CO2 concentration reaches 95% in the oxy-fuel/wet FGR combustion environment. NOx formation is reduced as well due to the reduced local flame temperature and thermal NOx formation from nitrogen. In a practical application, more analysis and design are needed to address retrofit requirements for the upstream system, such as mill operation and oxygen/RFG temperature control etc.
Table 2. Comparison of Burner Model Outputs parameter
conventional
oxy-fuel/FGR
O2 (%) CO2 (%) H2O (%) NOx (ppm)
3.2 14.65 5.85 154
3.1 69 27.5 82
remain the same, and the radiative heat transfer loss depends on the design and is assumed to be unchanged. Table 1 compares the calculated heat loss coefficient for the wallfired boiler discussed in the previous section versus the estimated boiler efficiency for the oxy-fuel/FGR combustion. The boiler efficiency for an oxy-fuel/FGR combustion application may vary depending on the energy requirement for flue gas recycle and air separation, which are taken into account in the unmeasured losses. If the increased losses are lower than the loss reduction from dry gas, the overall boiler efficiency can increase or be maintained.
8. Conclusions Process analysis and calculations are performed in this paper to evaluate the potentials of converting a conventional boiler to an oxy-fuel/FGR boiler. The process requirements for a retrofit with minimum impact on boiler thermal and emission performance are discussed. The analysis, however, is performed under an ideal condition where boiler leakage and the auxiliary system modifications are not considered. The study indicates that the optimal wet flue gas recycle ratio depends on the existing boiler exit O2 and fuel properties and is in general around 0.7-0.75. If the boiler exit O2 and flue gas flow rate remain the same as those in the conventional boiler, oxyfuel/wet FGR combustion generates the adiabatic flame temperature lower than the conventional approach due to high specific heat associated with higher moisture content. To increase the flame temperature, dry flue gas recycle can be used. However, dry flue gas approach will lower the flue gas velocity and therefore impact on convective heat transfer characteristics. CFD modeling is conducted to verify the process analysis. The CFD results indicate that the furnace combustion characteristics and burner flame characteristics are similar between the conventional operation and the oxy-fuel/FGR operation. Further design analysis is needed to understand the impacts of reduced flame temperature or flue gas velocity on boiler thermal performance.
7. Oxy-fuel/FGR Combustion in Conventional Burners For a successful implementation of the oxy-fuel/FGR technology in the conventional boiler, a good understanding on burner performance is required. One of the questions is whether the conventional burner can be used for oxy-fuel/ FGR combustion and any modification is needed to convert the conventional air/fuel combustion to oxy-fuel/FGR combustion. The design criteria defined in this paper has provided a possibility to use the conventional burner without or with minimum modifications to the burner front for the new application. A single burner CFD model has been used to evaluate this possibility. The model was initially used to study the low-NOx burner combustion characteristics.11 The CFD model flow inputs are redefined to account for the oxy-fuel/ wet FGR application. A comparison of the temperature distribution is shown in Figure 14. The modeling study was done by assuming that primary air, secondary air and tertiary air are replaced with the mixture of oxygen and recycled flue gas. The temperatures of each stream remain the same as those in the conventional coal and air combustion. Table 2 compares the model outputs for the conventional coal/air and oxy-fuel/FGR firing conditions. The results show
Nomenclature R = air-to-fuel demand %ash = fuel ash content, mass fraction %C = fuel carbon content, mass fraction %H = fuel hydrogen content, mass fraction %H2O = fuel moisture content, mass fraction 2168
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FGR = flue gas recycle RFG = recycled flue gas mO2 = oxygen mass flow rate, kg/s mair = air mass flow rate, kg/s mFG = flue gas mass flow rate, kg/s mRFG = recycle flue gas mass flow rate, kg/s
MW = molecular weight kg/kmol O2,exit = boiler exit O2, molar fraction T0 = referene temperature Tf = adiabatic flame temperature oxy = oxy-fuel/FGR combustion conv = conventional air/fuel combustion
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