OXYGEN AND OPACITY STACK EMISSION MONITORING SYSTEMS TO OPTIMIZE COMBUSTION EFFICIENCY AND MEET EPA REQUIREMENTS Robert C. Molloy Dynatron Inc. Wallingford, CT ABSTRACT The combustion efficiency of industrial fuel burning equipment can be optimized through the use of oxygen and opacity stack emission monitoring systems. By providing the operator with the means to obtain accurate, continuous, dynamic feedback of the status of the products of combustion fuel - air - mixture ratios can be maintained at optimum levels resulting in improved combustion efficiency and reduced air pollution in compliance with EPA guidelines. The smoke going up your stack is to a large part unburned fuel. The ever-spiraling cost of fuel over the past few years coupled with the world-wide energy shortage has made it imperative to operate industrial fuel burning equipment at the highest level of operating efficiency possible. Awide variety of fuel burning emission sources can now economically use oxygen and opacity monitoring systems to optimize combustion efficiency and meet EPA requirements. Bark &Black Liquor Boilers Diluent Monitoring Naval Boilers Carbon Black (furnace process) Driers Oil Fired Boilers Catalytic Crackers Gas Fired Boilers Oxygen Enrichment Systems Cement, Lime K"ilns Glass Fiber Refinery Process Heaters Ceramic, Phosphate Kilns Glass Melting Furnaces Regenerative Furnaces Chemical Recovery Boilers Heat Treating Furnaces Turbine Engine Test Cells Coal Fired Utility Boilers Incinerators Wet Scrubbers Coke Ovens, Soaking Pits Kraft Recovery Boilers Windboxes DieselEngines Metallurgical Furnaces BASIC FUNDAMENTALS OF COMBUSTION The combustion of fuel is a chemical reaction in which Most common fuels such as coal, oil, and gas consist fuel is combined with oxygen which results in the primarily of carbon, hydrogen, small amounts of sulfur, formation of the products of combustion and heat. The and trace amounts of other elements. When these fuels primary chemical reactions which occur during combus- are mixed with oxygen at elevated temperatures, comtion are: bustion will occur and the above heat-generating C + 02... C02 + Hea t chemical reactions will resu1 t. In order to insure complete combustion a sufficient amount of both fuel and oxygen must be present. C Carbon In order to achieve ideal "stoichiometric" optimum H Hydrogen CH4 Methane combustion efficiency it is necessary to mix the fuel S Sulfur to be burned with the exact amount of oxygen necessary to insure complete combustion. On a practical basis there are a number of combustion variables which affect CH4 + 202 ~ 2H20 + CO 2 + Heat + Heat combustion. Some of these combustion variables include S + 02... S02 + Heat air and fuel pressure, air and fuel temperature, humidity, type of fuel, BTU content, moisture, specific C02 Carbon Dioxide gravity, viscosity, grindabi1ity, ash content, burner H20 Water S02 Sulfur Dioxide condition, fan condition, linkage wear, fire box 02 Oxygen design, etc. 577
In a typical fuel-burning application virtually complete combustion will occur when there is a sufficient amount of excess oxygen. Once a sufficient amount of excess oxygen is provided to complete combustion the greatest source of inefficiency is the heat loss associated with excess air in the stack emissions. (NOTE: Air is only 20.95% oxygen. As a result, in addition to oxygen, nitrogen and the other components of air also carry heat with them as they are exhausted out the stack exit.) The magnitude of these losses is determined by the stack gas exit temperature (which is a function of boil er heat transfer) and the volume of excess air in the emissions. EXCESS AIR HEAT LOSS IN FLUE GAS FUEUAIR MIXTURE RATIO OPTIMUM COMBUSTION EffiCIENCY EXCESS OPTIMUM EXCESS OXYGEN OXYGEN AND OPACITY OPACITY An insufficient amount of oxygen can cause high smoke opacity conditions in violation of EPA regulations, slag buildup, boiler tube fouling, decreased heat transfer, excessive maintenance, wasted fuel, and potentially dangerous excess fuel conditions. An excessive amount of oxygen can cause costly wasteful loss of energy in the form of sensible heat when excess air, in addition to that required to support combustion, is drawn into the firebox at room temperature, heated, and discharged to the atmosphere at a high temperature with the products of combustion. In addition to saving money by reducing wasteful fuel consumption, an oxygen monitor can be helpful in reducing NO x and S03 emissions due to excess oxygen, identifying burner malfunctions, reducing cold end corrosion of air heaters, and providing the dynamic feedback necessary for closed-loop automatic combustion control systems. Figure 1. In the above chart you can see that the loss of efficiency associated with too much excess fuel is much more severe than in the case of too much excess air. Once an adequate amount of oxygen is present to insure complete combustion and avoid the severe energy losses associated with unburned fuel i't is essential that excess oxygen be kept to a minimum. With today's strict enforcement of EPA opacity regulations there is a tendency to supply too much excess air to avoid high smoke opacity conditions. The magnitude of the energy losses resulting from the presence of excess oxygen is a function of the volume and temperature of the excess air in the flue gas at the stack exit. The volume of the excess air can be reduced by operating with minimum excess oxygen. The temperature of the excess air in the flue gas at the stack exit can also be reduced by operating with minimum oxygen as a result of allowing the hot flue gases to remain in When the fuel/air mixture is too lean and excess oxygen is being provided, a major loss of energy o~curs contact with the heat transfer surfaces for a longer in the form of sensible heat in the exhaust. When period of time for improved heat tr~nsfer efficiency. excess fuel is provided there is incomplete combustion A typical boiler control system consists of a steam and the unburned fuel in the exhaust results in increased smoke opacity. Optimum combustion efficiency pressure transducer which senses load demand and a fuel flow controller which increases or decreases fuel occurs when operating with the minimum amount of flow as a function of load demand. Typically a mechanical linkage interconnects the fuel valve with excess air necessary to insure complete combustion. See Fi gure 1. the air damper. On a practical basis it is impossible to properly "characterize" the air damper to account for fluctuations in process variables such as changes 578
, ~. ESL-IE-80-04-108 '."., in fuel and air quality. Typically the system is set up to operate with excess oxygen to provide for occasional momentary upset conditions. As a result a majority of the time the system is operated with more excess oxygen than is necessary. OXYGEN MON ITORS The concentration of oxygen in the flue gas passing through a stack or duct work is measured by detecting variations in the output of a zirconium oxide fuel cell oxygen sensor. As the oxygen concentration varies, an output signal is generated which can be directly related to the oxygen content of the flue gas. it to instantaneously react to dynamic fluctuations An In Situ Oxygen Analyzer is mounted through the wall in process variables on an automatic closed loop basis of the stack so that the oxygen sensor cell can be without operator intervention. in direct contact with the flue gas sample to be measured. In Situ measurement eliminates the problems associated with extractive systems which require maintenance-prone sample conditioning systems while providing a more representative sample of the gas to be measured. An oxygen monitor is only as accurate as the sample being monitored. It is important to position the In Situ Oxygen Analyzer in a location where a representative sample is present. To avoid stratification, installations at or near junctions or bends in the stack or breech should be avoided. To avoid errors due to infiltration (air leakage) locate the In Situ Oxygen Analyzer as close to the source of combustion as possible. Avoid stagnant air pockets. Choose an In Situ Oxygen Analyzer of sufficient length to project into at least 30% of the cross sectional diameter of the stack or breech at the point of sampling. On large breeches average the output of multiple probes on either a linear or weighted average basis. oxygen analyzer can be used as a feedback element in a closed loop combustion control system which utilizes 02 Trim to optimize the fuel/air mixture ratio. An Automatic 02 Trim System compares the actual stack gas 02 reading with a desired excess 02 set point and generates an appropriate output control signal to the air damper draft control to automatically adjust the amount of combustion air to the burner to ensure that the desired excess 02 set point will be maintained. An Automatic 02 Trim System can provide a fine trim adjustment to the combustion control system to enable The "characterized" Set Point Function Generator utilizes a signal from the boiler master controller proportional to load to compute a corresponding excess 02 set point. The relationship between boiler load and excess 02 set point can be easily programmed in the field. The function generator can be used to maintain the desired fuel/air mixture ratio for each level of load demand such that the air will lead the fuel on increasing fuel-load demand and lag on decreasing fuel-load demand. See Figure 2. Figure 2., Oxygen monitors can be used to meet EPA requirements..~ for normalizing S02 and NO emission levels to The zirconium oxide fuel cell is an ideal oxygen stoichiometric conditions. Oxygen monitors are also sensor for the majority of combustion efficiency an excellent tool for monitoring diluent air leakage applications. For optimum performance the following when determining the effectiveness of bag houses, application limitations should be observed: The electrostatic precipitators, and other types of air pollution control devices. platinum electrodes on the cell may become contaminated or poisoned in heavy reducing atmospheres in which lead, lead compounds, arsenic, or antimony are An oxygen monitoring system can provide the dynamic present, resulting in shortened cell life. The cell feedback necessary to enable the operator of industrial can not be used in process applications to measure fuel-burning equipment to supply the proper amount of oxygen in the presence of high concentrations of oxygen to insure optimum combustion efficiency. An 579
combustible background gas. Stack gas temperature adverse vewing conditions. limits should not be exceeded. Applications involving repeated temperature cycling, mechanical, or thermal An opacity monitor is only as accurate as the sample shock should be avoided. being monitored. It is important to position the transmissometer in a location where a representative Oxygen provides accurate reliable feedback to enable cross-section of the emissions to be emitted from the the operators of industrial fuel burning equipment to exit of the stack can be viewed. To minimize turbuoptimize combustion efficiency, minimize air pollution, lence, installations at or near junctions or bends in reduce wasteful fuel consumption and save money. It is the stack or breech should be avoided. If it is necnot adversely affected by variations in fuel composition essary to mount the transmissometer near a bend, it and it is an excellent cost effective measure should be mounted in the plane of the bend. Trans ment technique for measuring and controlling excess missometer path lengths less than the stack exit air. With an oxygen analyzer the operator can should be avoided. To be sure of viewing a sample measure the oxygen content of the products of combustion on a dynamic basis and adjust the fuel/air mixture ratio to compensate for fluctuations in representative of stack outlet conditions, the trans~ missometer should be mounted down-stream of air pollution control equipment. process variables to insure operation with minimum excess oxygen. OPACITY MON ITORS Opacity is the percentage of visible light attenuated due to the absorption and scattering of light by particulate matter in an optical medium such as flue gas. %Opacity = 100% -% Transmittance Opacity readings have proven to be an effective means for the measurement and control of visible emissions. Key constituents of visible emissions are sub-micron particles which are a major cause of the adverse health effects of air pollution including: respiratory ailments, smog, and gas phase reactions with other air pollutants. In addition, opacity readings can be useful as an aid in optimizing combustion efficiency. As a result of the ability of opacity monitors to In a clear stack 100% of the light is transmitted and demonstrate compliance and to provide data to aid in the opacity is 0%. In a stack with an opaque effluent operating control equipment,it is now mandatory on a 0% of the light is transmitted and the opacity is 100%.national basis for certain categories of emission sources to install and maintain approved opacity monitoring The opacity of the flue gas passing through a stack systems. The latest Federal Environmental or ductwork is measured by detecting variations in Protection Agency (EPA) compliance requirements for light transmittance. As the opacity of the effluent stationary emission sources are outlined in the increases, a greater percentage of the light transmitted October 6, 1975 Federal Register. In addition many by the light source is deflected by the par states and local municipalities have implemented ticulate matter in the smoke and the intensity of similar legislation requiring opacity monitors on the light incident on the photo detector is reduced. even broader categories of emission sources. The output signal generated by the photo electric light detector is directly related to effluent opacity. The smoke opacity monitoring system can be used to This signal is converted in the monitor into input reduce wasteful excess fuel consumption by giving signals compatible with the display, the alarm control the operator an accurate dynamic reading of the circuits, the counter timer, and the strip chart current opacity of the products of combustion. This recorder. information is useful for the operator to lower smoke An opacity monitor can accurately duplicate the readings of a trained human observer watching the outlet of a stack with the advantage of being able to operate on a 24 hour basis without being affected by night time, bad weather, cloud cover, or other opacity levels and avoid violating EPA smoke opacity regulations. Optimum combustion efficiency can be obtained on a long term basis by operating at minimum opacity. By using the smoke opacity monitoring system to optimize combustion efficiency, carbon buildup on 580
The boiler tubes can be reduced, thereby improving CONCLUSION internal heat transfer within the boiler and minimizing Significant fuel savings coupled with reductions in costly downtime for cleaning and maintenance. (Not air polluting emissions in compliance with EPA guideapplicable to natural gas.) lines can be achieved by operating industrial fuel FUEL SAVING POTENTIAL ESL-IE-80-04-108 burning equipment at the optimum fuel/air mixture ratio through the use of the dynamic feedback provided by oxygen and opacity stack emission monitoring systems. I ' I. i v ~ $ Z 8, I 10, IUDUCTION IN IXCIU OXVQIII_ 'lftclnt Figure 3. The amount of money that a specific customer can expect to save by the installation of an oxygen and opacity stack emission monitoring system varies from installation to installation. The key factors to be considered are: 1. Annual fuel consumption, type of fuel, and cost of fuel. 2. The degree of efficiency at which the plant is currently operating. 3. The capability and willingness of operating personnel to utilize the oxygen monitor in their standard operating'procedures. REFERENCES 1. KVB, Inc. Report to FEA, Contract No. C-04-50085-00. Assessment of the Potential for Energy Conservation through Improved Industrial Boiler Efficiency, Final Report, Volume I. 2. Everything you Need to Know About Smoke Opacity Monitoring Systems, Dynatron Inc.,1978,Volume 1* 3. Everything You Need To Know About Oxygen Monitoring Systems, Dynatron Inc., 1979, Vol ume 1* *Available Free of Charge from Dynatron Inc. Reprinted with permission. From Advances In Instrumentation Volume 34 ~ Instrument Society of America 1979. The real possibility of future fuel shortages, allocations, rationing, and major cost 'increases may dramatically change the rate of "return on investment" analysis for a particular installation. Consider the cost of shutting down the plant for one day next winter. The cost of the fuel saved becomes insignificant compared to the cost of lost production if you are faced to operate with less fuel than you have, historically used. i On a practical basis, savings of as much as 3 to 5% have been obtained on many small to medium size plants. 581