Calculating NH 3 Slip for SCR Equipped Cogeneration Units

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York. N.Y GT-105 ^+ The Society shall not be responsible for statements or opinions advanced in papers or in dis- C cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. M Discussion is printed only if the paper is published in an ASME Journal. Papers are available ]^L from ASME for fifteen months after the meeting. Printed in USA. Copyright 1990 by ASME Calculating NH 3 Slip for SCR Equipped Cogeneration Units C. M. ANDERSON and J. A. BILLINGS The Pritchard Corporation Overland Park, Kansas ABSTRACT The NH3 slip pollutant generated by a recently completed cogeneration plant equipped with an SCR system was accurately monitored by a calculation method without the expense of an NH3 analyzer. The method of calculating the NH3 slip developed by the authors is based on measured variables and material balances. Discussed in further detail is the cogeneration plant, SCR characteristics, source of NH3 slip, variable measurements required for the NH3 slip calculation, equation development, accuracy of results, and NH3 analysis methods. NOMENCLATURE CEM - continuous emissions monitoring DCS - distributed control system DeNOx - NO reduction EPA - Environmental Protection Agency GTG - gas turbine generator HRSG - heat recovery steam generator NH3 fed - ammonia injected into the SCR, (lbmole/hr) NH3 reacted - ammonia consumed by the reduction reaction with NOx in the SCR (lbmole/hr) NH3 slip - ammonia injected into the SCR but not consumed by the reduction reaction, (lbmole/hr) NOx - nitrogen oxide mixture, NO and NO2, MW=46 NOx in - NOx in the turbine exhaust gas, (lbmole/hr) NO out - NOx in the gas from the SCR (lbmole/hr) ppm - parts per million, (volume) ppmvd - parts per million by volume, dry SCR - selective catalytic reduction SCFM - standard cubic feet per minute Space velocity - (hr -1), gas quantity (ft3/hr) / catalyst volume (f) UV - ultraviolet INTRODUCTION Government regulations restricting NO x emissions for "New Stationary Sources" has brought about an increased demand for deno x systems. One such system, the Selective Catalytic Reduction System (SCR), uses ammonia in the presence of a catalyst to selectively reduce NOx emissions. Due to inherent inefficiencies of the SCR design, a small amount of NH3 slip becomes an additional exhaust stream pollutant discharged into the atmosphere which requires monitoring. Ammonia is normally listed as a pollutant in the environmental permit to construct when an SCR system is specified for NO x reduction, therefore a method to report NH3 slip will also be required. Although the Code of Federal Regulations 40 CFR 60 does not specifically address NH3 slip, state agencies usually require reporting of NH3 emissions. Typically an ammonia analyzer is installed for the measurement, however, other methods to accurately monitor NH3 slip should not be overlooked. One alternative is to calculate the NH3 slip. A cogeneration plant equipped with SCR utilizing a specific configuration of instrumentation hardware and software will allow the NH3 slip to be calculated accurately by the following expression: NH3 slip = NH3 fed - (NOx in - NOx out) (1) There are several advantages to calculating NH3 slip as an alternative to installing an NH3 analyzer. Capital cost is reduced due to the elimination of hardware and analyzer maintenance is not required. Good repeatability and consistent accuracy are also positive attributes of the calculation method. The difficulty involved with making an accurate dry NH3 measurement with a conventional analyzer is another important reason to consider calculating NH3 slip. 'Presented at the Gas Turbine and Aeroengine Congress and Exposition June 11-14, 1990 Brussels, Belgium

2 PLANT DESCRIPTION The NH3 slip was continuously monitored in the recently completed Mid-Set Cogeneration Plant in Kern County, California. The plant was designed and built by the Pritchard Corporation and was configured as shown in Figure 1. The Mid-Set Cogeneration plant was designed with two functions: to generate commercial power and to generate high pressure, varible quality steam for enhanced oil recovery. The plant was based on a General Electric PG6531 combustion turbine designed to produce 33 megawatts of net commercial power and a heat recovery steam generator designed to generate approximately 215,000 lbs/hr of 80% quality steam at 1500 psi. Other typical operating parameters are shown in Figure 1. Electric power was under contract to be sold to the Pacific Gas and Electric Company. The steam distribution system was designed to supply Texaco Producing, Inc. and one other customer with variable quality saturated steam. The General Electric Frame 6 gas turbine generator (GTG) was controlled by a GE Mark IV SpeedtronicsTM control system. All turbine measurement data was transmitted to a distributed control system (DCS) through the Mark IV, RS-232 data dump. This turbine data was used in displays and EPA reporting, including the NH3 slip calculations. All displays, calculations, and reports were performed by the DCS. The GTG was equipped with water injection to control the turbine NO emissions at 42 ppm. Control of the NOx level below 42 ppm by water injection was impractical due to loss of cycle efficiency in the turbine. To comply with strict emission regulations, the Heat Recovery Steam Generator (HRSG) was equipped with an SCR denox system using a honeycomb type catalyst design. NOx was reduced in the SCR from 42 ppm at the inlet to 8.4 ppm at the outlet, exhibiting an 80% denox efficiency. In the ammonia injection process, liquid ammonia was vaporized, then mixed with air and injected into the exhaust gas upstream of the catalyst., The NH3 injection rate was controlled by a ratio controller that measured the inlet NOx flowrate, which was the uncontrolled flow and the NH3 flowrate, which was the controlled flow. The ratio controller modulated the NH3 control valve to maintain the prescribed ratio setpoint (NH3 molar flow/nox molar flow = 1.0). The Mid-Set display was configured in mass flowrate (lb/hr) to allow operators to monitor the process in more conventional units. Conversion from volumetric flow to mass flow yielded a NOx flowrate of approximately 70 lb/hr and a NH3 flowrate of approximately 26 lb/hr at base load conditions. The mass ratio setpoint was The Continuous Emissions Monitoring (CEM) system provided the SCR inlet NOx measurement for the ratio controller and the raw analysis of the stack gas required for EPA reporting. The CEM system consisted of NOx, CO, and EXHAUST GAS FLOW 1.05 x 10 6 */HR, M.W: ( MMSCFH) ^ HRSG? NOx IN 42 PPM 72 */HR NOx OUT PPM 13.7 */HR STACK L 2. SCR TURBINE 1.13 X iut -/1111 NOX; (#/HR) I I STATIC MIXER AIR 300 SCFM SCR _ 0j_ READING WATER INJECTION i i 26.1#/HR I NH3 NOX CONTROL I VAPORIZER FT I I I --^ I FIG. 1i STACK NO x READING (TRIM SIGNAL) ' I 2

3 HRSG TURBINE EXHAUST NIN `: NH 3 NH 3 N 2 OUT ^ HZ^OUT NOx OUT NH3 SLIP FIG. 2 NH 3 t- I I I I I SCR CATALYST ^I-- Z NH 3 FEED CO2 analyzers. The 02 concentration was calculated from the CO2 measurement by EPA methodsl to correct all readings to 15% 02 as required for reporting, A NOx trim controller was also provided from the stack NOx measurement to accomplish exact NOx control. The stack NO x measurement fed the analyzer controller which trimmed the ratio setpoint to provide stable control during long time-constant changes in NO x stack emissions. The NO trim control was provided because several factors can cause NOx stack emissions to drift even though the ratio controller is tuned properly. Variations in catalyst parameters affecting denox efficiency will cause drift of the final NOx stack emissions. Variable factors which contribute to deno x efficiency are exhaust gas temperature, space velocity, flow profile, bypass leakage, and catalyst activity. The trim controller was included to correct for these slow variations in NO x emissions by automatically adjusting the ratio setpoint within confinedlimits. A small offset between the NOx setpoint and the NO measurement at the stack varied the amount of NH3 injected by automatically adjusting the ratio setpoint. In summary, the cogeneration unit utilized two stages of NO x control. Water injection was used at the turbine to reduce the NOx from 160 ppm to 42 ppm, and SCR control reduced NOx from 42 ppm to 8.4 ppm. The control was designed such that the NOx into the SCR could be varied by adjusting the GTG water injection flowrate. Similarly, the NOx and NH3 stack emissions could be varied by adjusting the NOx trim controller setpoint. These control setpoints were optimized based on economic and environmental considerations. SCR CHARACTERISTICS The SCR converted NO x to N2 and H2O by reacting NH3, NOx, and 02 in the presence of a catalyst, see Figure 2. The SCR is integral to the HRSG located at the GTG exhaust. The SCR catalyst bank is positioned within the HRSG to operate at the optimum temperature of 650 F. The NH3/air mixture was injected into the exhaust gas upstream of the SCR catalyst bank through nozzles which traverse the HRSG cross-section. As the exhaust gas and ammonia passed through the catalyst bank, the NH3 reduced the NOx to N2 and H2O. By design, not all NO x was reduced, therefore some NO x was discharged to the atmosphere. There was also some unreacted NH3 which was emitted to the atmosphere. The graph in Figure 3 depicts the relationship between denox efficiency and the NH3/NO x molar ratio. The amount of catalyst surface area will dictate deno x efficiency with all other variables held constant. More surface area will allow for higher efficiencies and vice versa. The graph shows two possible denox efficiencies, 80% and 90%, and reflects the increased area of installed catalyst required for the 90% efficiency curve. The Mid-Set deno x efficiency was guaranteed at 80% reduction under specified operating conditions with a molar ratio setpoint of 1.0:1 (NH3/NO x), and NH3 slip less 1 Code of Federal Regulations, Title 40 - Protection of Environment, Chapter I, Part 60, Appendix A, Method 19,

4 than 20 ppmvd by the catalyst manufacturer. Optimized installations can produce denox efficiencies greater than guarantees, accompanied by lower molar injection ratios and corresponding lower slip values. DeNO x efficiencies and mole ratio setting can be manipulated within the confines of emissions limits to conserve NH3 usage and reduce NH3 slip. The new condition of the catalyst installed at the Mid-Set site exceeded guarantees as follows: 81% denox efficiency with only 8 ppmvd NH3 slip and a mole ratio setpoint of 0.98 under specified operating conditions. The NH3 slip will increase dramatically for molar ratios higher than 1:1, and the gain in NO x reduction will be negligible. This is due to the fact that once the optimum stoichiometric ratio is attained, the only additional NO x reduction is due to overfeeding NH3 to compensate for less than perfect mixing of the NH3 in the exhaust gas. Catalyst denox efficiency cannot be estimated or derived by empirical equations with enough accuracy to support the calculation method because several fixed and variable factors affect catalyst performance. Fixed factors such as catalyst configuration, injection grid placement, HRSG design and variables such as catalyst activity, gas temperature, exhaust gas velocity profile, bypass leakage, and space velocity all affect SCR performance. EQUATION DEVELOPMENT The equation used to calculate ammonia slip was developed from a molar balance of ammonia around the SCR/HRSG system. NH3 fed = NH3 reacted + NH3 slip (2) The first term in eq. (2), the amount of ammonia fed is a directly measured quantity. The second term, the amount of ammonia reacted is not directly measured however, an expression using directly measured variables can be developed for NH3 reacted based on the stoichiometric amount of NH3 required for complete reaction with the NOx and the SCR NOx reduction efficiency. The following reaction equations describe the stoichiometry of the NO reduction by NH3 based on experimental results. 1 CAT 4NO + 4NH3 +02 > 4N2 + 6H20 (3) CAT NO + NO2 + 2NH3 ---> 2N2 + 3H2O (4) CAT 6NO2 + 8NH3 ---> 7N2 + 12H2O (5) The rate of reaction of the equimolal mixture of NO and NO2, eq. (4) was determined to be much faster than the NO or NO2 reactions, eqs. (3) and (5). 1 The NOx in the GTG exhaust gas is composed of approximately 90% NO and 10% NO2. 2 The NO2 is consumed predominantly by the equimolal reaction, eq. (4) along with the stoichiometric amount of NO, while the excess NO reacts according to eq. (3). The molar ratio of reactants, NH3:NO x for both reaction eqs. (3) and (4) is 1:1. Therefore, the stoichiometric ratio is 1:1 for a NO/NO2 mixture containing 50% or more of NO by volume. 1 Kato, A., et al. 1981, "Reaction Between NO and NH3 On Iron Oxide-Titanium Oxide Catalyst," The Journal of Physical Chemistry, Vol. 85, No. 26, pp General Electric Gas Turbine Multiple-Combustion System, General Electric Gas Turbine Reference Library, GER- 3435A, ACTUAL 0 ^o`. ^ INSTALLED ^ EFFICIENCY 0 `/,i SCR 60 DE-NOX 50 EFFICIENCY 40 NH3SLIP PPMV 30 i, G^QE^^ G^^(^^^i A TUPL 10 0 NNH^- MOLE RATIO NO X IG. 3 SCR CHARACTERICTICS

5 Since the NOx and NH3 react on a 1:1 molar basis, the amount of NH3 reacted is equal to the amount of NO x reduced in the SCR. moles NH3 reacted/moles NOx reduced = 1.0 (6) moles NH3 reacted = moles NO x reduced (7) The amount of NOx reduced can be expressed as the inlet NO times the SCR NO x reduction efficiency. Alternately, the NOx reduced can be defined as the NO x in minus the NO x out. Equation development for the alternate is continued in Appendix A. The end result is the same for both definitions. The following derivation utilizes the SCR efficiency because this term is an important concept in SCR technology. The SCR NOx reduction efficiency is useful in monitoring the operation of the system. In addition, the NH3 slip calculation at the Mid-Set facility was derived based on this expression. NH3 reacted = (NOx in) (SCR efficiency) (8) The SCR efficiency must be monitored on a real time basis because the SCR denox efficiency varies as discussed in the SCR Characteristics section. The actual efficiency of the SCR was available using the NOx concentration in the exhaust gas measured upstream and downstream of the SCR. The upstream NOx concentration is termed NOx in and downstream of the SCR is NOx out. actual SCR NOx reduction efficiency = 1 - (NOx out/no x in) (9) At the Mid-Set plant the NOx was measured in ppmvd. The units of the NOx terms in eq. (9) may be selected for convenience as long as they are consistent in order to cancel out in the ratio. Now the amount of NH3 reacted can be determined using directly measured variables. NH3 reacted = (NOx in) (1 - (NOx out/nox in)) (10) The NH3 slip can be accurately determined using measured variables by combining eqs. (2) and (10) then solving for NH3 slip. NH3 fed = (NOx in)(1 - (NOx out/nox in)) + NH3 slip (11) NH3 slip = NH3 fed - (NOx in)(1 - (NOx out/nox in)) (12) The NH3 slip equation can be further simplified, NH3 slip = NH3 fed - (NOx in - NO out) (1) The units in eq. (1) are molar or volumetric and must be consistent for all terms. They may be selected for convenience such as ppmvd, lbmoles/hr, or SCFM. At the Mid-Set facility the NH3 slip was calculated as ppmvd. The NOx concentration was read in ppmvd and the NH3 fed measurement was measured in lb/hr. The NH3 fed was converted to ppmvd using the exhaust gas flow measurement and the molecular weights of NH3 and exhaust gas. MEASUREMENT ACCURACY When considering the cumulative effects of all possible errors in measurement inputs, it is possible to monitor the NH3 slip measurement to +/-4% of reading (The NH3 orifice measurement is accurate to +/-1.0%, gas turbine flow +/-1%, the NOx measurement +/-1.2%). To ensure the accuracy of all parameters used in the EPA calculations and specifically the NH3 slip calculation, the following configurations were implemented: Turbine Exhaust Flow - The actual mass flow rate was measured as a continuous signal with turbine instrumentation instead of using an inferred flow rate based on the fuel gas flow signal. This measurement was verified to be correct during compliance testing. The turbine exhaust flow was the sum of the inlet air, fuel gas, and the water injection flows. This flowrate was combined with the inlet NOx (ppm) measurement to provide NOx mass flow into the SCR to be compared with the NH3 mass flow for ratio control. SCR Efficiency - The SCR deno x efficiency was continually monitored by measuring the NOx concentration at the SCR inlet and at the stack on a 7.5 minute alternating cycle. A dual range chemiluminescent analyzer was used for this measurement and the range was automatically changed to provide the proper resolution. This design feature allowed one analyzer to be used instead of two independent NO x analyzers and saved approximately $10,000. The calculation (1-(NOx out/nox in)) x 100 yielded denox efficiency on a real time basis and was more accurate than using empirical equations. Empirical equations for SCR efficiency use fixed values for space velocities and gas temperature and usually disregard other pertinent variables. The NOx reduction efficiency measurement is important to the NH3 slip calculation and also allows operations to continuously monitor the SCR performance. Uncontrolled variables described previously such as space velocity and catalyst degradation affect catalyst efficiency and do not permit an accurate empirical determination of SCR efficiency over time. Therefore, measurement of inlet and outlet NOx is a requirement for an accurate NH3 slip calculation, and should be considered for all SCR installations.. NH3 Flow Measurement - This measurement was performed with an orifice plate sized for a fixed operating condition. When the orifice plate was specified, the differential pressure was calculated based on a given gas density which is affected by pressure and temperature. Seldom do the actual operating conditions exactly match design conditions; therefore, pressure and temperature compensation of the measurement must be incorporated. Ambient temperature swings will affect gas temperature to a degree which can introduce significant errors unless the flow measurement is compensated. Erroneous Values - Safeguards were implemented in the DCS algorithms such that intermediate and final calculated values would not produce skewed results if an input variable had a zero value or a bad value. Multiplication and division by zero was avoided by restricting lower limits of measurement inputs in the DCS database. 5

6 NH3 ANALYSIS METHODS Several factors make NH3 measurement at low concentrations unreliable. Transporting the sample is difficult since NH3 is very soluble in water, and NH3 is absorbed on teflon and stainless steel sample lines. Intolerable lags in response time will occur and measurement accuracy will be sacrificed for long sample lines. Current measurement principles employed in NH3 analysis such as second derivative UV absorption are limited by analyzer sensitivity and interferences, however manufacturers are continuing development in an effort to produce a cost effective and reliable low range NH3 analyzer. Ideally, this analyzer would have to be an in-situ device to eliminate sample line concerns. Some NH3 analysis methods incorporate a high temperature converter which converts NH3 and NO2 to NO and compares this measurement to a second sample stream where NO2 is converted to NO. The difference of the two NO measurements yields the NH3 concentration. This measurement method utilizes a chemiluminescent NOx analyzer, and is valid as long as the conversion rate of NH3 and NO2 to NO is constant. It should be noted that the conversion rate is temperature dependent and requires excess air. Another possible problem is the formation of ammonium salts that can plug the converter and sample lines. This analysis method addresses sample line absorption by converting NH3 to NO at the stack, however the converter is subject to maintenance. The installed cost of a stand alone NH3 analyzer will exceed $50,000. Operating and maintenance costs will also be required to keep the analyzer functioning. It has been reported that many NH3 analyzer installations have failed to deliver satisfactory performance when applied to stack monitoring of NH3 slip in the 20 ppmvd range. SUMMARY The basis for calculating NH3 slip incorporates two simple principles: the molar balance of ammonia around the SCR as stated in eq. (2) and the stoichiometric relationship between NH3 and NOx given in eqs. (5) and (6). The NH3 slip can be calculated accurately and consistently when the following process variables are quantified: a. NOx flowrate into the SCR b. NOx flowrate out of the SCR c. NH3 flowrate into the SCR The NOx flowrate out of the SCR and the NH3 flowrate into the SCR are typically measured variables in a cogeneration unit equipped with an SCR. The NO x flowrate into the SCR is not typically measured; however, this NOx measurement can easily be obtained without the addition of a separate analyzer by using the same NO analyzer that is required by the EPA for the NOx stack emissions. method is a viable alternative for environmental monitoring of NH3 slip. The repeatability and reliability of the NH3 calculation method is dependent on the reliability of the NH3 orifice flow measurement and the stability and accuracy of the NOx analyzer. The orifice measurement accuracy is well documented and the NOx analyzer has been tested and certified accurate by an EPA approved testing method. The analyzer is also calibrated automatically with known span gas at least once per day. Calculating NH3 slip is a reliable and economical method of monitoring NH3 slip in cogeneration plants utilizing SCR units for NOx reduction. The Pritchard Corporation is currently evaluating the suitability of applying the NH3 slip calculation method to a selective non-catalytic reduction system utilizing urea injection. Although the chemical reactions are much more complex, it may still be possible to define the main reaction with enough accuracy to calculate the NH3 slip. SI UNIT EQUIVALENTS 1 ft =0.3048m ( F - 32) x 5/9 = C 1 ft3 = m3 1 lb = kg 1 psi = 6,895 Pa APPENDIX A This alternative derivation of the NH3 slip calculation is based on the amount of NO x reduced being expressed as NOx in minus NOx out. The derivation is identical to the Equation Development section up to and including eq. (7). Key equations to this point are the ammonia molar balance around the HRSG, eq. (2) and the definition of NH3 reacted, eq. (7). NH3 fed = NH3 reacted + NH3 slip (2) moles NH3 reacted = moles NO x reduced (7) The amount of NO reduced was available using the NO x concentration in the exhaust gas measured upstream and downstream of the SCR. The upstream NO x concentration is termed NOx in and downstream of the SCR is NOx out. NOx reduced = NOx in - NOx out (8A) At the Mid-Set plant the NO x was measured in ppmvd. The units of the NOx terms in eq. (8A) must be consistent with the units in eq. (1). Now the amount of NH3 reacted can be determined using directly measured variables by combining eqs. (7) and (8A). NH3 reacted = NO in - NO x out (9A) The NH3 slip can be accurately determined using measured variables by combining eqs. (2) and (9A) then solving for NH3 slip. In view of the relatively moderate success with which NH3 fed = (NOx in - NOx out) + NH3 slip (10A) NH3 can be monitored by conventional means, it is the opinion of the authors that the use of the calculation NH3 slip = NH3 fed - (NO x in - NO out) (1)