Modeling Mercury Control with Powdered Activated Carbon

Fulland pilot-scale data has been collected on the effectiveness of powdered activated carbon (PAC) injection for control of mercury emissions from flue gas of coal-fired power plants. However, there has been limited modeling work accounting for the removal of mercury by existing equipment as shown by Information Collection Request (ICR) data and by PAC injection, independently of one another. The mathematical model presented in this paper accounts for both components of total mercury removal at a plant equipped with PAC injection. Algorithms based on recent full-scale demonstrations were developed for PAC injection in plants: (1) firing subbituminous coal and having cold-side electrostatic precipitator (C-ESP), (2) firing bituminous coal and having hot-side ESP (H-ESP) followed by a pulse jet fabric filter (PJFF), and (3) firing bituminous coal and having cold-side ESP (C-ESP). INTRODUCTION Injection of Powdered Activated Carbon (PAC) is an approach for controlling mercury emissions that has been developed and tested at the full scale on coal-fired utility boilers. Test programs have been performed on a utility boiler firing subbituminous coal with a downstream cold-side electrostatic precipitator (ESP), on utility boilers firing bituminous coal with a downstream cold-side ESP, and on a utility boiler firing bituminous coal with a Compact Hybrid Particle Collector (COHPAC) arrangement (upstream hot-side ESP with downstream baghouse after the air preheater). Because of these test programs, there are data available for developing performance models of this technology for applications that are similar to these. In this research, performance models were developed. These models are in a form where they can be updated as new information is developed on these applications and for other boiler applications. www.AndoverTechnology.com MERCURY REMOVAL MODELS EPA’s Information Collection Request (ICR) showed that mercury released from coal combustion may be partly removed from the exhaust gases by existing equipment without additional retrofit technology. The existing equipment may be one or more units that contribute to mercury removal. If fequipment is equal to the fraction of mercury removed from the boiler gases by a piece of equipment, then (1 – fequipment) equals the fraction of mercury remaining in the gases after that piece of equipment. The fraction of mercury remaining after n pieces of equipment is equal to Fraction of mercury remaining after n pieces of equipment is [(1 – fequipment 1) × (1-fequipment 2) × (1 – fequipment 3) × . . . × (1 – fequipment n)] Eq. 1 Therefore, the total mercury removal fraction, fTotal, is fTotal = 1 – [(1 – fequipment 1) × (1-fequipment 2) × (1 – fequipment 3) × . . . × (1 – fequipment n)] Eq. 2 If one of the pieces of equipment is PAC injection, then the total mercury removal fraction is fTotal = 1 – [(1 – fequipment 1) × (1-fequipment 2) × (1 – fequipment 3) × . . . × (1 – f PAC injection) × . . . × (1 – fequipment n)] Eq. 3 where f PAC injection is the fraction of mercury removed by PAC injection. If PAC injection is simply added to existing equipment and the removal effects of the existing equipment are combined into one term, then we can represent Equation 3 as fTotal = 1 – [(1 – fexisting equipment) × (1 – f PAC injection)] Eq. 4 where f existing equipment is the removal fraction of the existing equipment. In this research, data from full-scale tests of mercury reduction were used to formulate models for mercury reduction from existing equipment and from PAC injection. Fullscale data for mercury removal by existing equipment are available from the ICR data. Full-scale testing results of mercury reduction from PAC injection are available from the Department of Energy’s field testing programs at Southern Company’s Gaston Plant, Wisconsin Electric Power Company’s Pleasant Prairie Power Plant (PPPP), and at PG&E; Corp. National Generating Group’s Brayton Point and Salem Harbor Plants. 1 MERCURY REMOVAL BY EXISTING EQUIPMENT, fexisting equipment Through statistical analysis of the ICR data, Reference 2 shows that mercury reduction is a function of emission control equipment configuration, of chlorine content of the coal, and (in some cases) of the SO2 emissions level from the boiler. Reference 2 provides algorithms to estimate mercury capture as a function of the plant configuration, the coal chlorine content, and the SO2 emissions. These algorithms are: www.AndoverTechnology.com 2 Algorithm 1 (cold-side ESP): fexisting equipment = C1 × ln [(coal Cl, ppm)/(SO2, in lb/MMBtu)] + C2 Eq. 5 Algorithm 2 (all other categories): fexisting equipment = C1 × ln (coal Cl, ppm) + C2 Eq. 6 There are minimum and maximum allowable values that set the allowable range for the results of Equations 5 and 6. These are shown with C1 and C2 in Table 1 for hotand cold-side ESP conditions. According to this model (Eqs. 5 and 6), the mercury reduction efficiencies of existing equipment for conditions at Gaston, PPPP, Brayton Point, and Salem Harbor are estimated in Table 1. Table 1. Collection of mercury by air pollution control equipment, predictions using correlation of Reference 2. Gaston PPPP Brayton Point Salem Harbor Chlorine, % by weight in coal 0.03 0.0015 0.08 0.03 Coal Chlorine, ppm 300 15 800 300 Flue Gas SO2, lb/MMBTU 0.650 0.360 0.820 0.500 C1 C2 Min Max ESPc 0.1233 -0.3885 0.0% 55.0% 7.1% 46.0% 40.0% ESPh 0.0927 -0.4024 0.0% 27.0% 12.6% ESPc = cold-side ESP ESPh = hot-side ESP Gaston fires bituminous coal and has a hot-side ESP followed by an air preheater and then a low-pressure pulse-jet fabric filter (PJFF) for a COHPAC arrangement. Reference 2 did not include algorithms for facilities with this arrangement. One might expect that the mercury reduction without PAC might correspond approximately to the predicted mercury reduction in Table 1 for a hot ESP (ESPh). Under the conditions at Gaston, this equals 12.6%. However, tests at Gaston showed negligible mercury removal, but the difference may be reasonable considering the range of variability in the possible results. However, this demonstrates that this algorithm will give reasonable estimates but not precise values. At the PPPP, a facility firing PRB coal with a cold-side ESP, the test results showed about 5% actual mercury removal from existing equipment compared to about 7% as estimated by the algorithm of Reference 2 (Eq. 5) for the same conditions at PPPP, and shown in Table 1. Therefore, this is approximately in the same range. The chlorine content of the coal used at PPPP (15 ppm, which is much lower than those of most other PRB sites) probably contributes to the low removal by existing equipment. With chlorine content more typical of a PRB coal, around 100 ppm or more, the algorithm predicts that mercury would be reduced by a greater amount. www.AndoverTechnology.com 3 For Brayton Point, a facility firing bituminous coal and equipped with a cold-side ESP, the algorithm of Reference 2 (Eq. 5) produces an estimated mercury reduction by existing equipment of about 46% (see Table 1) versus an actual measured removal efficiency of 32%. These values, which are in about the same range, further illustrate that the algorithm of Reference 2 is not exact, but approximate, at estimating mercury removal by existing equipment. At Salem Harbor, a facility firing bituminous coal and equipped with a cold-side ESP, 87% mercury reduction from existing equipment was measured. This compares to about 40% estimated from the algorithm of Reference 2 (Eq. 5) and shown in Table 1. Salem Harbor operates with fly ash loss-on-ignition (LOI) in the range of 25%-35%. According to Reference 6, this is approximately equivalent to a carbon loading of 60-84 lb/MMacf in the exhaust stream – a much higher carbon loading than one would typically inject PAC. So, the carbon in the ash likely contributed to the very high intrinsic capture of mercury. The capacity of PAC to absorb mercury is so great that it should not be limiting except at temperatures of about 350 oF or more, which is greater than the gas temperature at the exit of most air preheaters. So, cooling usually has little or no beneficial effect on mercury absorption by PAC. However, the ability of fly ash and unburned carbon in the fly ash to absorb mercury is far less than that of PAC and may be enhanced by cooling. Therefore, while spray cooling may enhance mercury absorption by fly ash and downstream capture in the ESP or fabric filter, spray cooling is not expected to enhance mercury capture by PAC. The importance of temperature on intrinsic capture is demonstrated by test results at Salem Harbor. Because Salem Harbor has the ability to increase its ESP inlet temperature through operation of steam heaters, parametric tests of intrinsic mercury removal as a function of temperature could be performed. Figure 1 shows the results of that testing under various firing conditions along with data taken from another test using low sulfur (LS) bituminous coal (not the baseline coal). The trend is quite clear that increasing temperature reduces intrinsic mercury capture from as much as 90% down to around 10%. Because a facility’s mercury reduction by existing equipment may be significantly different than what the algorithm of Reference 1 determines, this algorithm should be used with care and only for making estimates. As the measurements at Salem Harbor clearly indicate, LOI (or other ash qualities) and gas temperature can have a very significant impact on the level of mercury being removed by existing equipment and may be worth including as parameters in this algorithm at some future date when more information is available. Therefore, the algorithm of Equation 6 and Reference 1 may provide reasonable estimates in many cases. But, there is a chance that actual mercury capture may differ significantly from what Equation 6 predicts. For any specific facility, actual measurements of mercury removal, if available, should be used. www.AndoverTechnology.com 4 Figure 1. Salem Harbor Mercury Romoval Without PAC Injection (Ref. 6) 0 10 20 30 40 50 60 70 80 90 100 270 290 310 330 350 370 Temperature (degrees F) H g R em ov al (% V) 17-19% LOI (45 lb/M Macf) 20-24% LOI (55 lb/M Macf) 25-29% LOI (68 lb/M Macf) >30% LOI 30-35% LOI, C1, High Load 21-27% LOI, C2, High Load LS bitum coal MERCURY REDUCTION BY PAC INJECTION, fPAC injection Reference 7 has algorithms developed from pilot-scale data for mercury reduction on boilers equipped with PAC injection. In this work, we have made the following model improvements: 1. The algorithms of Reference 7 were developed from pilot-scale tests and characterize total mercury reduction from both PAC injection and existing equipment as a function of PAC injection concentration. When using the algorithms of Reference 7, it is necessary to have a different PAC injection algorithm for each type of equipment configuration, including upstream equipment. These PAC injection algorithms may have to be updated as new information regarding mercury control from existing equipment becomes available. In the research described in this paper, the mercury reduction from PAC injection was isolated from that of the other equipment. Therefore, as we gain more information on reduction of mercury from equipment other than PAC injection, it should not be necessary to perform new regressions on the PAC injection models, and it will also be possible to assess the fate of mercury in equipment that is either upstream or downstream of the PAC injection system. 2. The algorithms of Reference 7 are of a form in which it is possible to approach 100% mercury removal by injection of very high concentrations of PAC. As will be shown, experience at PPPP showed that, under some circumstances, it is not www.AndoverTechnology.com 5 possible to achieve such extremely high reduction of mercury emissions with PAC injection. Therefore, the algorithm for mercury reduction from PAC injection was modified to permit an upper limit to mercury removal that may be less than 100%. 3. Because the algorithms of Reference 2 are based on the full-scale ICR data, it is desirable to use them to characterize mercury reduction from existing equipment. However, it is not possible to integrate the algorithms of Reference 2 into the approach used in Reference 7. By treating the mercury reduction from PAC injection independently from mercury reduction from other equipment, it is possible to use the algorithms of Reference 2 to characterize mercury reduction from existing equipment. In the case of PPPP, PAC injection test results demonstrated that mercury reduction behaved asymptotically with a maximum achievable mercury reduction from PAC that is well below 100%, regardless of PAC injection rate. For this reason, the equation that is used in Reference 7 to characterize the relationship between mercury reduction and PAC injection % reduction = η = 100 × ffrom PAC injection = 100-[A/(M+B)] Eq. 7 where M is the mass injection rate of PAC (in lb/MMacf) so that M = {[A/(100 – η)]} – B Eq. 8 has been modified to be M = {[A/((100*D) – η)]} – B Eq. 9 Where D is the fraction of mercury reduction that is asymptotically approached. A set of constants A, B, C, and D are specified for a given plant configuration and gastemperature. At this time, these constants can only be derived for full-scale applicationssimilar to the conditions where full-scale data exists. For some other boilerconfigurations, there are test data available from pilot-scale tests that can be used untilfull-scale data become available. For configurations where neither full-scale nor pilot-scale data exists, the constants can be developed as data becomes available from futuretests. PAC INJECTION MODELS DEVELOPED FROM FULL-SCALEDATAFor the purpose of modeling, we are interested in estimating the necessary PAC injectionrate to achieve a specified level of mercury control. Therefore, we developed algorithmsof PAC injection rate as a function of desired mercury reduction by PAC. So, rather thanplotting mercury reduction versus PAC injection concentration, as is done in References5, 3, and 4, we have reversed the axes from what is shown in these references. www.AndoverTechnology.com6 In these tests several different PAC sorbents were tested. The different PAC sorbents willbe designated on the legends of the figures. Since specific information regarding theproperties of the tested sorbents was not available, the impact of these properties onmercury removal performance was not evaluated. However, effects of sorbent choices onmercury capture performance are noted. GastonFigure 2a shows mercury collection results measured from an on-line mercury analyzerduring testing conducted at Gaston. Data is plotted as PAC injection concentration versusmercury reduction percent. Data includes results with several different sorbent types.Figure 2a also shows a curve developed in the form of Equation 9 to approximatelycorrespond to the results achieved at Gaston. The coefficients for the algorithm are listedin Table 2. It is notable that, at Gaston, the choice in sorbent appeared to have little or noimpact on performance. It is also notable that, at mercury removal rates in the range of92%-96%, mercury reduction is less sensitive to changes in PAC injection rate. Figure 2bshows the data for 92%-96% mercury reduction in greater detail. The enclosed region onFigure 2b includes the estimated 95% confidence range for this mercury reduction data.Figure 3, a plot of deviation of the predicted and measured PAC injection rate,demonstrates this in another way. For most mercury reduction levels, the deviationbetween model and actual PAC injection rates is only about 10%. For mercury reductionin excess of 90%, however, the deviation is higher on a percentage basis. While Figure 3shows that, expressed as a percent of predicted level, at high removal rates the deviationbetween the model and measured value is –30% to +40%, in fact this only corresponds toa range of under ±1 lb/MMacf. The high percentage in the deviation is due to the actualvalues being relatively small at Gaston. Table 2. Coefficients For Curvefit Algorithms Plant Algorithm ABCDGaston530.121.00a150510.72b140110.69PPPP c145310.705a30030.81.13b30000.81.05Brayton c3001.50.81.09 a. 95% confidence range for Hg reduction and for PAC injection concentration are determined by ±2standard deviations from the arithmetic mean, with correction for sample size.b. Calculated as (actual rate – predicted rate) / predicted rate and expressed in percent www.AndoverTechnology.com7 Figure 2a. Gaston TestingData from reference 8 0.00.51.01.52.02.53.03.54.04.5 0 10 20 30 40 50 60 70 80 90 100 110Percent Hg RemovalInjectionConcentration(lb/MMacf) FGDPAC20InsulHydroCalgorithmR-squared of algorithm 68% 95% confidence area for all dataover 90% reduction of mercury. Figure 2b. Gaston TestingData from reference 8 0.00.51.01.52.02.53.03.54.04.55.0 90 91 92 93 94 95 96 97 98 99 100Percent Hg RemovalInjectionConcentration(lb/MMacf) FGDPAC20InsulHydroCalgorithm Includes the 95% confidence area forall data over 90% reduction of mercury. www.AndoverTechnology.com8 Figure 3. Deviation of the Gaston PAC Algorithmdeviation = (actual PAC rate minus predicted PAC rate) divided by predicted PAC rate -50%-40%-30%-20%-10%0%10%20%30%40%50% 30405060708090100 Percent Hg Reduction from PACDeviationaspercentofpredictedPACrate Pleasant Prairie Power Plant (PPPP)Figure 4 shows mercury collection results measured from an on-line mercury analyzerduring testing conducted at PPPP. Data includes results with several different sorbenttypes. Figure 4 also shows a data point for the total mercury removal as measured by theOntario Hydro method. The Ontario Hydro method shows a somewhat higher, butnevertheless similar, mercury removal as the on-line mercury analyzer used for thetesting. Curves were developed in the form of Equation 9 to correspond to specific sets ofdata and are plotted on Figure 4. The coefficients of these algorithms (A, B, C, D) arelisted in Table 2. Unlike the results at Gaston, at PPPP the choice of sorbent has asignificant effect. This may be a result of the fact that, at Gaston, there is a downstreamfabric filter, which provides improved sorbent-gas contact, while at PPPP all of themercury absorption had to occur in the duct. Figure 5 is a plot of deviation of thepredicted and measured PAC injection rate. Had one algorithm been used for all of thesorbents, the deviations would have been very high in some cases. Nevertheless, there isenough scatter in some of the data that, even with different algorithms for each sorbent,deviation can be on the order of 40%. Note that the one data point with very high percentdeviation (over 70%) was actually at a low removal rate, and the absolute differencebetween the algorithm results and measured results was quite small. It is recommendedthat, for other plants with conditions similar to those at PPPP, some consideration shouldbe made for the sorbent type. Since the choice of sorbent affects performance, a differentalgorithm may be necessary to accurately model the performance for each sorbent. c. Calculated as (actual rate – predicted rate) / predicted rate www.AndoverTechnology.com9 Figure 4. PPPP TestingData from reference 8 051015202530354045 020406080% Hg removal from PAC injectionInjectionConcentration(lb/MMacf)FGDFGD (14 microns)FGLInsul (7 micron)algorithm aalgorithm balgorithm cOntario HydroR-squared of algorithmsalborithm b and FGL, 99%algorithm c and Insul (7 micron), 98%algorithm a and FGD (14 micron) data, 85% excluded fromr-squared calculation Figure 5. Deviation from the PPPP PAC Algorithmsdeviation = (actual PAC rate minus predicted PAC rate) divided by predicted PAC rate -40%-20%0%20%40%60%80%100% 30 35 40 45 50 55 60 65 70 Percent Hg Reduction from PACDeviationaspercentofpredictedPACrateFGD (14 microns), algorithm aFGL, algorithm bInsul (7 micron)the absolute difference was under 0.5 lb/MMacf www.AndoverTechnology.com10 Brayton PointFigure 6 shows results of testing at Brayton Point. Data includes results with severaldifferent sorbent types. Figure 6 also shows curves developed in the form of Equation 9that correspond to specific sorbent types. The coefficients of these algorithms are listed inTable 2. Like PPPP and unlike the results at Gaston, at Brayton Point the choice ofsorbent appears to have a significant effect on performance. When considered with thePPPP results, this provides further evidence that the sorbent choice may have a greaterimpact when a downstream fabric filter is not installed. While good correlation ispossible for all data with algorithm c (R = 77%), improved correlation was possible byusing different correlations for different sorbents, as demonstrated by the highercorrelations of algorithms a and b with the sorbents indicated on Figure 6. Figure 7 showsthat the predictive accuracy of the algorithms across a broad mercury removal range doesnot change much. However, Figure 7 shows that improved accuracy will result if thealgorithm is tailored to the sorbent. For algorithm c, maximum deviation ranges –60% to+50%. But, by tailoring the algorithm to the sorbent, as shown for Alt Sorbent 1 withalgorithm b and FGD1 with algorithm a, the deviation is reduced sharply. Figure 6. Brayton Point TestingData from reference 8 051015202530 020406080100% Hg removal from PAC injectionInjectionConcentration(lb/MMacf)FGD1Alt. Sorbent 1FGD (SO3 off)FGDalgorithm aalgorithm balgorithm cR-squared of algorithmalgorithm c and all data, 77%algorithm b and Alt. Sorbent 1 data, 86%algorithm a and FGD, FGD1 & FGD (SO3 off), 85% www.AndoverTechnology.com11 Figure 7. Deviation from the Brayton Point PAC Algorithmsdeviation = (actual PAC rate minus predicted PAC rate) divided by predicted PAC rate -100%-80%-60%-40%-20%0%20%40%60%80%100% 0 10 20 30 40 50 60 70 80 90 100 Percent Hg Reduction from PACDeviationaspercentofpredictedPACrateAll data, algorithm cAlt Sorbent 1, algorithm bFGD1, algorithm anote the reduction in deviation that is possible byusing an algorithm tailored for the sorbent Salem HarborAccording to Reference 6, long-term testing of PAC injection with baseline coalindicated about 90% reduction of mercury without any PAC injection as well as with 10lb/MMacf of PAC injection. Because of the high level of intrinsic mercury reduction atSalem Harbor and the sensitivity of the measuring methods, the increased mercuryreduction from PAC is difficult to assess. Therefore, it was not analyzed in this effort.However, Salem Harbor information provided useful insights to the effects of unburnedcarbon and gas temperature on intrinsic levels of mercury reduction as previouslydiscussed in this paper. CONCLUSIONSIn this program, correlations for mercury removal from coal-fired power plants have beendeveloped. The model incorporates information on mercury removal from existingequipment that was developed from the ICR data in Reference 2. It also incorporatesmercury removal from injection of PAC, as developed from full-scale demonstrations ofPAC injection where data is available. These algorithms should be updated and modifiedas more information becomes available on experience with mercury removal. www.AndoverTechnology.com12 www.AndoverTechnology.com13The following summarize some important findings that affect modeling mercuryremoval: • Models that permit isolation of the effects of different air pollution controlequipment on the fate of mercury will facilitate modeling combined effects withPAC injection over a wide range of boiler configurations and scenarios withoutthe need for new regressions of PAC injection test data. Impact of a specific pieceof equipment can be estimated with models best suited for that equipment. Themodels here are a step in that direction in that they isolate the effects of PACinjection from the effects of other air pollution control equipment. • PAC injection followed by a PJFF results in much lower injection concentrationsbeing necessary for a given level of mercury reduction than for PAC injectionfollowed by a cold-side ESP. Thus, economic modeling may show that in somecases the additional capital cost of a FF may be justified by reduced operatingcosts associated with PAC consumption. • Sorbent selection appears to have little effect on performance when PAC injectionis followed by a FF, but it appears to have a significant effect when PAC injectionis followed by an ESP. • As demonstrated by the Salem Harbor test results, LOI and temperature can havea significant effect on the mercury removal by existing equipment. For thisreason, the correlations of Reference 2, which do not include these effects, do notalways provide an accurate indication of mercury removal by existing equipment. • In some cases PAC injection without a downstream FF may not be able to achievemercury removal rates of 90% or more regardless of PAC injection concentration.