Gas measurements in FFC unit catalytic regeneration and flue gases are essential for efficient and safe operation.

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1 INGOLD Leading Process Analytics White Paper Gas measurements in FFC unit catalytic regeneration and flue gases are essential for efficient and safe operation. But most gas analysis technologies are unreliable due to the extreme process conditions. A series of tunable diode laser analyzers that operate in situ deliver dependable measurements, long analyzer lifetime, and are simple to install and maintain. Effective, Reliable and Easy TDLs in Refinery FCCs Introduction Fluidized Catalytic Cracking (FCC) is an essential operation in most modern refineries. FCC units break (crack) the long chain hydrocarbons in heavy gas oils into short chain hydrocarbons to produce gasoline and fuel oils. Flue gases produced in the catalyst regeneration process must be analyzed to prevent the risk of explosion and the venting of nonregulatory-compliant exhausts. Conditions in FCC units are challenging for many gas analysis technologies. Probe-type, in situ, tunable diode laser (TDL) gas analyzers offer measurement solutions that are not only dependable in FCC unit applications, but are also easy to install and are low in maintenance. FCC unit process The cracking of long chain hydrocarbons in FCC units is achieved by mixing vaporized feedstock with catalyst beads at a very high temperature. This mixture behaves like a fluid (hence fluidized ) and can be pumped around the FCC unit. When in the FCC unit s reactor, the catalyst beads become coated with carbon (catalyst coke) from the cracking activity. This build-up reduces the catalyst s effectiveness and so must be removed. The removal process, catalytic regeneration, comprises burning off the coke using very hot air. This is an exothermic reaction and the heat produced is absorbed by the regenerated catalyst and is also used to vaporize the feedstock. As well as heat, catalytic regeneration produces large amounts of CO and CO2. It is critical to measure the levels of these gases in

2 White Paper Flue gas Catalyst fines separator Catalyst fines hoppers Measurement point GPro 500 Catalyst fines Flue gas CO boiler Flue gas Gas analysis measurement points in FCC unit Flue gas Combustion air 715 C 2.41 barg Regenerator the regenerator flue gas as well as the residual oxygen level, as all of these are indicative of the rate and temperature of regeneration. If the regeneration rate is too fast and the temperature too high, the catalyst will become overheated and unwanted sintering may occur, with consequent reduction in efficiency and potential catalyst destruction. If the rate of regeneration is too low, then there will be insufficient oxidation of the carbon, leading to poor catalyst activity and inefficient cracking. The CO-rich flue gas from the regeneration step is routed through a steam-generating boiler (CO furnace) where the CO is burned as a fuel to generate steam for use in the refinery and also to comply with environmental regulations on flue gas exhaust. Fuel (e.g., natural gas) may be added to the flue gas prior to the boiler to increase combustion efficiency. Before it is vented to atmosphere, the flue gas may pass through an electrostatic precipitator (ESP) which removes particulates Stm Total feed 535 C 1.72 barg Reactor through the use of high voltage static electric fields between a series of metal pipes and plates that charge and then strip the entrained particles from the gas. Gas and organic compounds measurement is important in FCC units to monitor catalytic regeneration, and in the flue gas from the CO boiler to the ESP, both for environmental reasons and to prevent explosions in the ESP caused by the CO and the high voltages across the ESP s plates. CO furnace Since there are a variety of complex chemicals in the petroleum feed, often including organic sulfur and nitrogen compounds, the flue gas from an FCC unit usually contains a wide range of contaminants. These can include not only CO but also trace levels of aldehydes, cyanides, ammonia, sulfur dioxide (SO2), plus oxides of nitrogen (NOx), metals and other particulates. Typical legislation therefore usually specifies a wide range of emissions measurements for these flue gases, often including oxygen, CO and possibly CO2. The exhaust gas from the regenerator unit (before the CO furnace) has the following typical composition: Carbon dioxide % Carbon monoxide % Oxygen < 1 % Water vapor 1 % Sulfur dioxide < 0.5 % Nitrogen oxides < 0.1 % Nitrogen balance 2 METTLER TOLEDO

3 White Paper The sample is very hot (circa 600 C) and has a dust loading which is very high and includes many fine particles (typically down to 70 µm). Due to the sample conditions, careful thought must be given to the process adaptor type and materials of construction used. Continuous emissions monitoring As mentioned, the flue gases from the CO boiler (or directly from the catalytic regenerator) are vented to the atmosphere after suitable scrubbing to remove dust and pollutants. The gas composition will be similar to that of a typical incinerator flue gas: Carbon dioxide % Carbon monoxide < 1000 ppm(v) Oxygen 1 5% Water vapor % Sulfur dioxide < 1000 ppm(v) Nitrogen oxides < 500 ppm(v) Nitrogen balance The sample at this stage will have a moderate dust loading. Emissions monitoring will be required which will typically include CO, oxygen and again possibly SO2, NOx and opacity measurements. The choice of analyzer technologies and types will depend on user or regulation preferences, and the required hazardous area rating (refineries are invariably Zone 1 or Zone 2 {Class 1 Division 1 and Division 2} areas). Issues with gas analyzers A number of analyzer technologies are in use on FCCs for gas measurement. Paramagnetic and non-dispersive infrared (NDIR) analyzers were initially employed but measurement response was slow due to sample conditioning and transportation times. Also, narrow tubing could easily become blocked with particulates and extractive sample cells fouled. This made these analyzer types high in maintenance if measurement reliability was to be preserved. Many oxygen analyzers based on the paramagnetic principle have a measuring cell which is assembled using epoxy resins. These are susceptible to attack by traces of corrosives in the sample, eventually leading to premature failure of the cell. Oxygen analyzers using electrochemical cells should not be considered for these applications. The electrochemical cell uses an alkaline electrolyte which will be rapidly neutralized by the acidic contaminants in the sample gas, leading to cell exhaustion. If an electrochemical cell falls to zero output (i.e. 0% oxygen) this would constitute failure to danger on any safety application. The majority of CO combustion sensors in common use are based on pellistor technology, but these are also not without their issues. They are prone to measurement errors due to the two pellistor beads not being perfectly matched, radiative heat loss from each bead being different, and dirt in the gas building up on the sample pellistor but not on the reference. But their biggest issue is poor selectivity to CO, which can make measurements from such sensors highly unreliable. A gas analyzer technology that is fit for purpose Tunable diode laser (TDL) analyzers offer a modern approach to gas measurement in FCCs that overcomes the drawbacks of the above sensors. TDL analyzers work on the principle of laser absorption spectroscopy. A focused and tunable laser beam is used to analyze absorption lines that are characteristic of the particular gas species to be measured. TDLs usually measure in situ or directly from the gas stream without any sampling or conditioning. TDL analyzers for process applications have two basic design types, namely cross-stack and probe-type. In the cross-stack design, the laser source is placed on one side of the pipe or duct and the receiver on the other. The wider the pipe diameter, the more difficult it is to align the laser source and receiver. In METTLER TOLEDO 3

4 White Paper probe-type TDL analyzers, the defining feature is the sensor probe that protrudes into the process gas stream. The laser source and the detector are contained in a single unit, requiring a single flange connection. Compact, reliable, accurate and low maintenance TDLs METTLER TOLEDO s GPro 500 is a series of compact, probe-type TDL O2, CO CO2 and water vapor gas analyzers. They are highly suited to the conditions found in FCC units as they can be used with sample gases which contain corrosive traces and which are highly flammable. The range carries Optical Intrinsic Safety (OPIS) certification, meaning that the analyzers do not create optical energy of a sufficient magnitude to cause an explosion. The measurement performance offered on GPro 500s is also superior to other technologies, particularly in stability and rejection of measurement interferences. The series is available with a variety of adaptions that enable a very wide range in both application use and point of insertion. These adaptions include a sintered metal filter and baffle for hot and dusty applications such as catalytic regeneration, and a wafer cell for use in pipes as narrow as DIN 50. Where an extractive solution is preferred or required (due to more extreme process conditions) the GPro 500 can be configured with an extractive cell. Unlike other gas analyzer technologies, GPro 500 analyzers have no moving parts, and other than annual verification, require no maintenance. Conclusion Fluidized Catalytic Cracking units are found in most modern refineries and effective FCC unit operations are reliant on dependable gas measurements from resilient analyzers. METTLER TOLEDO s GPro 500 range of robust, in situ gas analyzers deliver highly stable O2, CO, and CO2 measurements in regenerators and flue gas. Their compactness, modularity and extremely low maintenance make them the ideal choice for FCC unit applications. c Mettler-Toledo AG Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland Visit for more information 04/ 2 015

5 INGOLD Leading Process Analytics White Paper Don t Get Burned in Combustion High Efficiency TDL Analyzers Once almost the exclusive domain of zirconium oxide and rudimentary catalytic combustion sensors, tunable diode lasers (TDLs) are quickly becoming established as the best available technique (BAT) for O2 and specific CO analysis for the optimization of combustion processes. Whereas traditional technologies have suffered from variable performance and short sensor lifetime, the inherent reliability, superior performance, and speed of response of TDLs combine to offer an increasingly attractive alternative. Introduction There are literally tens of thousands of combustion processes in operation globally, ranging from small to medium scale waste incinerators right the way up to large scale power plants. Industries as diverse as refining and petrochemicals, chemical intermediates, power generation, and iron and steel production utilize a variety of furnaces, boilers, kilns, process fired heaters, incinerators, and thermal oxidizers in their operations. Each of these combustion processes requires reliable oxygen measurements to ensure operating safety and to maximize efficiency. In addition, tight oxygen control leads to significantly reduced fuel costs and reduction in atmospheric emissions of undesirable combustible and greenhouse gases. Achieving these goals requires a totally reliable, accurate and low maintenance analyzer that is able to operate continuously with minimal user intervention.

6 High Efficiency TDL Analyzers Combustion efficiency Efficient combustion is essential for ensuring plant safety and to minimize atmospheric emissions. It also offers enormous potential for cost savings by ensuring neither a fuel rich nor excessively air rich condition exists in the burner. A fuel rich condition is wasteful of fuel, creates additional safety issues, and also creates increased CO emissions. While an air rich condition leads to excess cooling, which results in inefficient combustion and the increased generation of NOx and SOx emissions. Figure 1 demonstrates an idealized combustion efficiency curve. It can be seen that maximum combustion efficiency is achieved when just sufficient excess air is present to ensure complete combustion. The objective is therefore to achieve the lowest excess air value possible, but with a safety margin to ensure that the combustion can never move over the crossover point to become fuel rich, indicated by a breakthrough of CO. CO Fuel rich Air rich NOx & SOx Combustion efficiency Maximum efficiency Fuel rich incomplete combustion Air rich complete combustion NOx & SOx Combustion efficiency Combustibles Figure 2: Changing combustion conditions Oxygen only control When the combustion process utilizes an oxygen-only measurement approach, the ideal excess air level of the system is usually determined first by modelling, or from data supplied by the manufacturer of the burner or furnace. The operator then runs the process at a slightly higher level of excess air (slightly air rich) to ensure a safety margin. The amount by which the excess air is increased beyond the ideal value will be largely dependent on the confidence that the operator has in the oxygen analysis. Therefore, having a reliable and accurate oxygen measurement is crucial in order to ensure highest efficiency while maintaining safety. If there is uncertainty in the oxygen measurement, it is typical to run the process excessively air rich, with significantly increased cost implications due to the requirement for additional fuel % Excess air Figure 1: Combustion efficiency curve (static system) The above situation would apply in a static system, i.e., where the fuel type / quality, the ambient conditions, and the loading of the burner never vary. Of course, in reality, some or all of these parameters will change, with the result that the crossover point and therefore the optimum level of excess air to ensure maximum efficiency, can dramatically change. Figure 2 demonstrates how changing parameters affect the combustion efficiency curve, effectively shifting the point of maximum efficiency and the crossover point. Factors affecting the crossover point include: Changes to fuel composition / type and heating value Density changes of the fuel Load variation Changes in atmospheric conditions, particularly humidity, affecting the air used for combustion Condition of the burners (fouling) General wear of the entire combustion system To highlight the costs involved in running an inefficient combustion process, the following estimate can be used as a general guide. For every 1.5 % excess O2 ~ 1 % added fuel cost For every 0.2 % excess CO ~ 1 % added fuel cost For large scale combustion processes, these costs can be enormous. Running the operation at maximum efficiency means the initial purchase and installation costs of the analysis equipment can be recovered in just a few months of operation. Oxygen and CO measurement (CO trim control) Increasingly, combustible gas measurements are also being added in combination with oxygen measurement, to further improve efficiency and reduce atmospheric emissions by providing the option of CO trim control. Again, with reference to figure 1, it can be seen that the ideal control point (maximum efficiency) is just above the level where CO breakthrough begins to occur as the conditions begin to 2 METTLER TOLEDO White Paper

7 High Efficiency TDL Analyzers CO e Raeading (ppm) Process stable. Oxygen level controlled at approx 5 %. CO e at low background levels. 2. COe breakthrough event. Oxygen level drops 2 % CO e level increases quickly to>1500 ppm 3. Oxygen level returns to excess air. CO e reading drops quickly to base level Time (hours) CO e Reading (ppm) Reading (%) Reading (%) Figure 3: Combustible gas breakthrough showing sharp CO peaks move into the fuel rich region. This CO breakthrough is very rapid, as demonstrated in figure 3 which shows sharp CO peaks occurring as the oxygen level (excess air) is reduced. Therefore, the addition of a fast responding and accurate CO measurement can be used to determine and control the set point for increasing or decreasing the excess air level. For many modern, high efficiency, low NOx burners CO levels can rise very quickly, from typically less than 10 ppm(v) to greater than several 100 s ppm(v) for just a few thousand ppm(v) change in the O2 value. This highlights again the need for fast responding CO analysis if CO trim control is to be successful. A slow CO analysis, or one that is out of phase with the oxygen measurement, will lead to inaccurate trim control and over- or under-compensation of excess air, resulting in continuously unstable combustion conditions. Early measurement techniques In the early days of combustion optimization and control, extractive technologies such as paramagnetic O2 and non-dispersive infrared (NDIR) CO analyzers were sometimes utilized. While both technologies offer excellent measurement performance and in the case of NDIR, specific CO measurement, extractive systems of this type are not ideal for combustion applications. Reasons for this include: Slow response due to sample conditioning and transportation times O2 and CO signals are usually out of phase because of differences in sampling times or instrument response, even when the tapping point is at the same location High maintenance requirement for the sample conditioning systems Blockage of sample transport lines Failure of heated transport lines Fouling of extractive sample cells Cost of purchase of suitable extractive analyzers with heated sample cells Cost of sampling systems and heated lines These constraints and the associated costs involved led to the development of zirconium oxide O2 combustion analyzers in the 1970 s and 1980 s. This technology offered an in situ measurement of oxygen and although not as accurate as paramagnetic O2 analyzers, provided adequate performance in most applications. Driven by increasing fuel costs and burgeoning environmental concerns and emission standards, interest in CO trim control increased, so zirconium oxide analyzers with the option of a simple catalytic combustion sensor (usually based on pellistor technology, see Catalytic combustion sensors below) became 3 METTLER TOLEDO White Paper

8 High Efficiency TDL Analyzers available. This combination gave the advantage of O2 and combustion measurements that were in-phase and therefore such sensors became suitable for use in a trim control configuration for combustion optimization. The problem with these simple combustion sensors is that they are generally very non-specific; in fact, they are often referred to as CO equivalent (COe) or simply combustion detectors. In effect, they are generally non-specific to carbon monoxide due to the very generic catalysts they use. Some manufacturers have attempted to improve sensor selectivity to CO, but at best they still offer relatively poor performance, which is explored in more detail below. Traditional sensor technology and its drawbacks Zirconium oxide sensors It is easiest to consider a zirconium oxide measuring cell as an oxygen balance. Zirconium oxide is a ceramic material and as such, is a good electrical insulator. For the cell to work, the inherent resistance to electrical current flow must first be reduced. This is achieved by heating the zirconia to a high temperature. To accomplish this, the cell is fitted with an electrical heater, typically operating at a temperature of between 500 and 750 C (932 and 1382 F), depending on the application and type of electrode / catalyst used. The heater is normally controlled by an embedded thermistor. Zirconia disc Sample side Heater Reference side e e e Temperature sensor Sample side O 2 O 2 O2 Stabilized zirconium oxide Cell output Electrodes Figure 5: Zirconium sensor electrical circuit Cell heater Reference side The necessity for high temperature heating of the sensor and the electrode design are the reasons for the inherent weakness of these sensors. The high temperature requirement creates multiple failure modes, including: Heater failure Thermistor failure Cracking (due to thermal shock) of the zirconia disc Electrode peeling, i.e., the electrode detaches from the zirconia disc. This usually occurs under reducing conditions and particularly when the sensor is exposed to corrosive gases such a sulphur compounds In addition to these catastrophic failure modes, the catalysts used on the disc surface can be readily poisoned or inhibited, causing the sensor to lose sensitivity and response, and normally requiring sensor replacement. Due to all these drawbacks, a typical zirconia oxygen sensor can be expected to last about three years maximum in operation, before replacement will be required. On more aggressive applications, this lifetime can be even shorter. e e e Electrode Figure 4: Typical zirconium oxygen sensor To create the electrical circuit, two electrodes are bonded to the zirconium surface, one on each side, to allow connection of the cell output wires (see figure 5). In addition, the surface of the zirconium is coated with a suitable catalytic material to enable tunnelling of oxygen ions through the zirconium. The catalyst is often combined with the electrode in a single coating material. This coating is critical, as it must provide a reliable electrical connection and permanent bond to the zirconium, while remaining porous to oxygen ions. Catalytic combustion sensors The majority of combustion sensors in common use are based on pellistor technology. A pellistor, also known as a catalytic or heated bead sensor, consists of either a single, or more commonly a pair of matched precision resistors onto which two different coating are applied (see figure 6). The first pellistor bead is coated with a catalyst that creates an exothermic reaction when exposed to combustible gases, principally, but not exclusively carbon monoxide. The second bead is covered with an inert (non-reactive) coating and is used as a reference to reduce temperature variations (due mainly to process flow) from generating errors in the measured value. In order to increase the reaction rate on the catalyst, the pellistors are typically heated to about 500 C (930 F). The pellistor pair is typically configured into a Wheatstone bridge. 4 METTLER TOLEDO White Paper

9 High Efficiency TDL Analyzers Figure 6: Typical pellistor matched-pair arrangement Pellistor limitations Pellistors are quite crude sensors, providing typical measurement ranges of 0 2,500 ppm(v) ± 125 ppm and suffer from a number of limitations, including: Pellistors not being perfectly matched Radiative heat losses from each bead being different Dirt in sample building up on sample pellistor but not on reference, causing sensor drift Simple catalyst which is easily poisoned or inhibited and not selective to CO, leading to measurement errors TDL analyzers Tunable diode laser analyzers work on the principle of laser absorption spectroscopy: A focused and tunable laser beam passes through the gas sample to a receiver. The laser scans a very narrow part of the electromagnetic spectrum where absorption lines that are characteristic of the gas species to be measured, exist. Analysis of the surface area of the absorption lines (or peaks), allows determination of the concentration of the target gas. In situ TDL analyzers first entered the market in the late 1990 s. Initially, they were used for atmospheric research, followed by their implementation in environmental emission measurements. They then became accepted in process and now combustion measurement applications. Over this time, they have progressively been adopted, replacing earlier extractive technologies such as paramagnetic O2 and optical IR analyzers, in more and more situations. The driving force behind their uptake has had a lot to do with industry s requirement for less maintenance. TDLs do not need sample conditioning systems, or frequent and expensive calibration routines, leading to as much as a 60 % reduction in cost of ownership over the lifetime of the analyzer. In addition, the very selective nature of the TDL measurement technique and its fast response time has brought the technology from a niche market, to the center of current gas measurement methodology in a very short time. However, most current TDLs are based around cross-stack designs, which are not without their problems. These systems typically use large diameter housings and optical assemblies, and require considerable volumes of nitrogen purge gas (typically 20 to as much as 120 liters / min per side) to keep the optical windows free from dust. Further, cross-stack installation demands precise alignment of the laser and receiver units to ensure adequate performance. Installation on hot incinerator or burner walls can be challenging, as changes in process temperature can lead to walls flexing resulting in poor alignment. In worse cases, temperature changes can cause total loss of the transmission signal and the requirement for costly re-alignment. Many TDLs also use 2nd harmonic (2f) signal processing techniques, a method which generally is not well suited to measuring gas streams where the composition changes substantially. Molecular interactions affect the absorption peaks which can generate large measurement errors. Latest generation, probe-style TDLs To overcome the drawbacks of earlier designs, a new generation of TDLs with a probe design has come to the forefront. Developed by METTLER TOLEDO, the GPro 500 series utilizes fast signal processing and the latest in optical design. The sensor consists of a combined, detachable laser source and TDL spectrometer, and a probe that is installed directly in the gas stream. The laser beam passes through the probe to an optical retro reflector (corner cube) and is directed back up the probe to the spectrometer (see figure 7). This folded optical path design means no alignment is necessary, and effectively doubles the measuring path length. Due to the unique design of the sensor s purge nozzle, the nitrogen requirements are a fraction of other designs (typically 1 to 2 liters / min); therefore, installation and operating costs are significantly lower. 5 METTLER TOLEDO White Paper

10 High Efficiency TDL Analyzers Figure 7: Modern, probe-style TDL, GPro 500 The direct absorption spectroscopy (DAS) measurement technique that the GPro 500 series uses does not suffer from the same background gas errors as seen with 2f systems. This powerful signal processing method is combined with an in-built spectral database, enabling a real-time reference of the measurement gas peaks, and ensuring that the laser is always locked to the correct part of the spectrum. With SpectraID technology, three consecutive absorption peaks are analyzed in height, relative position and area. These results are then compared with a physical model of the absorption lines stored in the spectral database. If there is a a positive correlation between the two sets of data, then there is a perfect DNA match and it can be concluded that the observed absorption peaks are fully identified. This unique approach provides absolute confidence in measurement integrity. Conclusion Tunable diode laser analyzers are now at the forefront of gas analysis and are increasingly the first choice for a growing number of applications which were once the domain of extractive gas analyzers. TDLs are increasingly challenging zirconium oxide and catalytic technologies on combustion applications, where their low cost of installation and maintenance, fast response time, and reliability have cemented their reputation. A new generation of probe-style TDLs are available, which take the core benefits of TDL technology, but overcome the earlier drawbacks of high purge gas flow and alignment difficulties to provide a truly flexible, easy to install, compact, and reliable solution. 4www.mt.com/o2-gas GPro 500 TDL oxgen sensor Mettler-Toledo AG Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland For more information 04/2013

11 INGOLD Leading Process Analytics White Paper No More Purge Gas New TDLs for Combustion Processes Combustion processes are an integral operation in refineries, petrochemical, fertilizer, and power plants across the globe. Rising fuel costs and increasing competition are driving plants to adopt new techniques for measuring combustible gases with the goal of optimizing their combustion processes. Leading the pack is tunable diode laser (TDL) technology. A new generation of TDLs not only offers exceptional fuel costs savings, but eliminates the need for process side purge gas. Introduction Refineries, petrochemical, fertilizer and power plants worldwide have large numbers of combustion processes in operation, from process heaters and fired heaters to package boilers and large steam turbines. Whenever a process fluid needs to be heated as part of a chemical reaction, or there is a requirement for steam generation, there will be a combustion process at its heart. This paper presents a review of the general anatomy of a typical combustion process; describing each zone and its function, before discussing some of the challenges of combustion measurement and how modern TDL analyzers such as METTLER TOLEDO s GPro 500 and its range of process adaptions offer many advantages over typical combustion analysis technologies. Finally, a selection of specific combustion applications will be described, highlighting the typical process conditions. These applications are ideal candidates for superior TDL combustion measurement.

12 New TDLs for Combustion Processes Heating Zone Combustion Zone Super Heater Economizer Figure 1: Diagram of a generic combustion process Anatomy of a general combustion process In the above diagram we have seven general zones that follow a linear procession from the combustion zone to the emission stack. 1. The combustion zone is where the burners are located and as the name suggests, this is where the highest temperatures are to be found. For some applications temperatures can reach 1,200 1,500 C (2,191 2,732 F). Depending on the capacity, there may be multiple burners. 2. The heating zone or radiant section (sometimes called the firebox) is the region directly above the burners. Temperatures here can be typically 700 1,200 C (1,292 2,191 F). In the case of fired heaters, in this section there will be radiant tubes where the compound to be heated will be exposed to the highest temperatures. 3. The super heater is a device that superheats steam and will be typically found in steam reformers. Its main purpose is to increase the temperature of saturated steam without raising its pressure. 4. Economizers are used to recover some of this heat from the combustion. Stack economizers are utilized to increase efficiency when large amounts of makeup water are used (e.g. when not all condensate is returned to the boiler or large amounts of steam is consumed in the process, so there is no condensate to return) or there is a simultaneous need for large quantities of hot water for some other use. Savings potential is based on the existing stack temperature, the volume of makeup water needed, and the hours of operation. Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers. CO Electrostatic Precipitators 5. Electrostatic precipitators remove particles from the gas stream using electrostatically charged plates. The operation of this device is covered in more detailed in our white paper covering Stack non-combustion CO applications for the GPro 500 TDL. To recap, fast response ID Fan CO measurement is often a requirement to ensure that flammable gases do not reach the electrostatic plates, and therefore reduce explosion risk. 6. The ID fan is used to ensure there is a forced velocity of cleaned gas fed to the stack to ensure good dispersion. 7. Finally, the stack ensures that the cleaned flue gas is discharged well above ground level for atmospheric dispersion. Understanding the combustion process The whole purpose of measuring combustion gases is to optimize the efficiency of the combustion process and therefore reduce fuel costs and also reduce wear to plant and equipment. To understand efficient operation, the process of combustion should first be understood. Stable combustion conditions require the right amounts of fuel and oxygen. The combustion prod- Fuel ucts are heat energy, carbon dioxide, water vapor, nitrogen, and other gases (excluding oxygen). In theory, there is a specific amount of oxygen needed to completely burn a given amount of fuel. Oxygen Heat The famous combustion triangle tells us that for combustion to occur we need three things: Heat Fuel (hydrocarbon) + oxidant (oxygen) C + water If we have complete combustion, a chemical equation to describe this process would be: Heat Methane (CH 4 ) + (oxygen) Carbon dioxide (C ) + water (H 2 O) 2 METTLER TOLEDO White Paper

13 New TDLs for Combustion Processes In practice, combustion conditions are never perfect. Therefore, more air Fuel Rich than necessary has to be supplied to burn all fuel entirely. To determine the amount CO of excess air which will be required for any particular combustion system, we have to start with the stoichiometric air-fuel ratio, known as the perfect or ideal fuel ratio, or the stoichiometric combustion. During stoichiometric combustion there is a chemically correct mixing proportion between the air and the fuel. During such a process no fuel or air will be left over. For total combustion the stoichiometric equation would be: CH =C + 2H 2 O Process heating equipment almost never runs in stoichiometric balance. Even so-called on-ratio combustion, used in boilers and high temperature process furnaces, incorporates a modest amount of excess air 10 to 20 % more than needed to burn the fuel completely. If insufficient air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide are exhausted from the boiler. This results in surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosions due to excess combustible gas generation. In general, an equation for incomplete combustion would be: Heat Hydrocarbon + oxygen carbon monoxide + carbon + water To avoid these inefficient and unsafe conditions, combustion processes have traditionally operated at a high excess air level. The excess of air also provides protection from insufficient oxygen conditions caused by variations in fuel composition and operating variations in the fuel-air control system. To increase the efficiency and safety of the process, it is desirable to reduce the amount of excess air, while ensuring that conditions of incomplete combustion and high levels of combustible gases are not reached. This is difficult to achieve and control consistently without good feedback of the current and CO concentrations post combustion. % Excess Air Air Rich Combustion Efficiency Maximum Efficiency Figure 2: Effect on combustion efficiency caused by changes in excess air levels ID Fan NO x & SO x CO breakthrough point indicating incomplete This is where the use of highly reliable and accurate measurement combined with a combustible gas analysis can provide the critical real-time and in-phase analysis data required. Whereas, the measurement of oxygen provides a good understanding of the amount of excess air being utilized, the accurate measurement of the combustibles, in the form of CO, provides the fine control signal to indicate when the excess air has been reduced too much and incomplete combustion is occurring. This is because at the point where incomplete combustion begins to occur, there will be a very sudden rise in the level of CO. When this point is reached the operator will open the dampers. This is typically automated on larger process heaters and power generation plant equipment. Challenges of combustion measurement Oxygen and CO measurement (CO trim control) Again with reference to Figure 2, it can be seen that the ideal control point (maximum efficiency) is just above the level where CO breakthrough begins to occur, as the conditions begin to move into the fuel rich region. This CO breakthrough is very rapid indeed with sharp CO peaks occurring as the oxygen level (excess air) is reduced. It can therefore be appreciated that the addition of a fast responding and accurate CO measurement is essential for precise control of the combustion conditions by increasing or decreasing the excess air level to the furnace. This technique is known as CO trim control. There are therefore two distinct approaches in combustion measurement: Combustion monitoring: Only oxygen is measured for combustion monitoring. Therefore, the maximum efficiency point cannot be precisely controlled and is only determined by burner design information or modelling, meaning the burner is 3 METTLER TOLEDO White Paper

14 New TDLs for Combustion Processes normally operated with more excess air than necessary, as a safety margin. Combustion control: Where CO measurement allows more precise control of the process by monitoring for the CO breakthrough point to provide adjustment of the excess air to account for changes in fuel composition, loading and atmospheric conditions, etc. For many modern high efficiency, low NO x burners, the CO levels can rise very quickly indeed, from typically less than 10 ppm(v) to greater than several 100s ppm(v) for just a few thousand ppm(v) change in the value. This highlights again the need for a fast response CO analysis if CO trim control is to be successful. A slow CO analysis, or one that is out of phase with the oxygen measurement, will lead to inaccurate trim control and over or under compensation of excess air leading to continuously unstable combustion conditions. Typical excess air values and dust loading considerations Large power plant boilers and process heaters can run with as much as 10 to 20 percent excess air, while some natural gas-fired boilers and low NO x burners may run as low as 5 percent excess air. Pulverized coal-fired boilers typically run with 20 percent excess air with some solid fuel coal-fired plants operating at much greater levels. Typical values of excess air for some common fuels are shown in Table 1. As had been discussed, the goal is always to reduce the level of excess air as far as possible, while maintaining complete combustion. These figures only provide broad guidance and actual values will depend on the design and age of the equipment as well as the type and origin of the fuel used: Fuel Excess air (%) Anthracite 40 Coke oven gas 5 10 Natural gas 5 10 Coal, pulverized Coal, stoker Oil (No. 2 and No. 6) 10 to 20 Semi anthracite, hand firing 70 to 100 Semi anthracite, with stoker 40 to 70 Semi anthracite, with traveling grate 30 to 60 Table 1: Common fuels and excess air levels Note: From the above table it can be seen that gas or oil-fired combustion processes require considerably less excess air for efficient combustion and will generate much lower particulate concentrations. All combustion applications will have variations in particulates or fly ash loading. Due to the heavy particulate (fly ash) present in the gas stream from some coal-fired installations, special care should be exercised to investigate and understand the concentration and type of particulates that may be present under these circumstances, to ensure that sufficient filtration of the sample is provided. The economics of combustion control Example of typical savings on a typical battery of 6 gas-fired heaters: Fuel costs for the production of 200 m BTU/hour energy per heater: with unit price of 4 $ per m BTU/hour, the total amount is 7 m $ per heater per year. If excess air can be reduced by 1.5 %, the estimated savings on fuel will be 1 %, or 70,000 $ per heater/year. Considering total costs of ownership (equipment, installation, engineering, and spares for an estimated analyzer lifetime of 5 years) of 75,000 $ for one and one CO analyzer per heater, The investment will be cash positive in 13 months, with a total 5-year return on investment of 275,000 $ per heater, or 1,65 m $ for the whole heater battery. Additional, collateral benefits are cost savings for significantly lower NO x and CO emissions. The above example clearly demonstrates the significant economic drivers that focus the desire to optimize efficiency of combustion processes throughout the plant. Catalytic combustion sensors and their limitations for precise combustion control The majority of combustion sensors in common use are based on pellistor technology. A pellistor, also known as a catalytic or heated bead sensor, consists of either a single, or more commonly a pair of matched precision resistors onto which two different coatings are applied (see figure 3). The first pellistor bead is coated with a catalyst that creates an exothermic reaction when exposed to combustible gases, principally but not exclusively carbon monoxide. The second bead is covered with an inert (non-reactive) coating and is used as a reference to reduce temperature variations (due mainly to process flow) from generating errors in the measured value. In order to increase the reaction rate on the catalyst, the pellistors are typically heated to about 500 C (930 F). The pellistor pair is typically configured into a Wheatstone bridge. 4 METTLER TOLEDO White Paper

15 New TDLs for Combustion Processes Figure 3: Typical pellistor matched-pair arrangement These sensors are non-co specific. As mentioned above, they operate by detecting a temperature change due to combustion occurring on a catalytic surface. This means, however, that all combustible gases will react, and consequently they will report a false CO measurement if other combustible gases are present. In addition, they typically have poor accuracy, sensitivity, and response time, resulting in inaccurate or delayed detection of the CO breakthrough point. As these are catalytic contact sensors, the process gas is in direct contact with the sensor itself and can poison the catalyst, which limits sensor lifetime and affects measurement reliability. Pellistor limitations Pellistors are quite crude sensors, providing typical measurement ranges of 0 2,500 ppm(v) ± 125 ppm, and suffer from a number of limitations, including: Pellistors not being perfectly matched (balanced), which creates measurement offsets Radiative heat losses from each bead being different, again creating imbalance and measurement errors Dirt in sample building up on sample pellistor but not on reference, causing sensor drift Use of a simple catalyst which is easily poisoned or inhibited and not selective to CO, leading to measurement errors. Together, these limitations result in, at best, a fairly crude and unreliable CO e (CO equivalent) reading. This is hardly ideal for effective combustion trim control. Other CO measurement techniques In the past, nondispersive infrared (NDIR) CO analyzers have sometimes been used for CO combustion measurement applications. While they can offer excellent measurement performance and CO specific determination, most rely on extractive sample handling systems which can result in measurement and maintenance issues, including: Slow response due to sample conditioning and transportation times and CO signals being out of phase due to differences in sampling times or instrument response, even when the sample is drawn from the same location High maintenance requirements for the sample conditioning system Blockage of sample transport lines caused by particulate loading Failure of heated transport lines Fouling of extractive sample cells Cost of purchase of suitable extractive analyzers with heated sample cells Cost of sampling system and heated lines. The above limitations have largely resulted in the demise of extractive NDIR analysis in the majority of combustion applications. The ideal CO combustion analyzer To guarantee the maximum integrity for the CO measurement, several things should be considered. These include: In situ measurement not requiring a sample handling system CO specific measurement versus a non-specific total combustibles measurement. Accuracy of the combustibles measurement Speed of response In-phase measurement Reliability Sensor lifetime In comparison with the many drawbacks and technical compromises of catalytic sensors and extractive NDIR technology, a modern, in situ, probe-type TDL, such as METTLER TOLEDO s GPro 500 series, offers considerable measurement, operational, 5 METTLER TOLEDO White Paper

16 New TDLs for Combustion Processes and cost benefits. Table 2 highlights the advantages that a high integrity, CO-specific TDL measurement provides for accurate and reliable combustion control measurement. Catalytic com- In situ probe type bustion sensor TDL CO analyzer CO specific no yes Accuracy poor high Sensitivity low high Speed of response average fast Poisoning of sensor possible no Reliability average extremely high Sensor lifetime 1 2 years 10+ years Table 2: Comparison of catalytic combustion sensor with TDL analyzer One significant challenge for TDL analyzers, particularly for combustion monitoring or control applications, has been the significant consumption of purge gas to protect the analyzer s optical windows. Even though probe-type TDLs reduce purge gas consumption considerably compared with earlier cross-stack designs (and eliminate the need to align the sender and receiver units), this can still be a constraint, particularly for retrofit installations. This limitation has been overcome by the release of a range of innovative process adaptions designed to augment the already high performance measurement of the GPro 500 TDL. ing a fully flexible measurement solution, and allowing successful installation of TDLs into processes and locations once thought impractical, or even impossible. In the case of combustion monitoring and control, the most appropriate process adaption is the non-purged (NP) filter probe, either the standard design (Figure 5) or with filter blowback facility (Figure 6). The filter provides protection for the analyzer s optical surfaces without the need for the traditional process side purge; simplifying installation and reducing long-term operating costs. Figure 5 - GPro 500 TDL with non-purged (NP) filter probe Figure 6: Non-purged (NP) filter probe with blowback Figures 4, 5, and 6 illustrate the design of the GPro 500 and the variety of available process adaptions. These process adaptions offer an interface solution for a wide range of applications, creat- These probes have been designed specifically with combustion processes in mind and provide a reliable fit and forget solution. Figure 4: The range of process adaptions available for the GPro 500 TDL series. 6 METTLER TOLEDO White Paper

17 New TDLs for Combustion Processes Water 350 C Burner Figure 7: Diagram of typical package boiler Steam space 3rd pass (tubes) 2nd pass (tubes) 1st pass [furnance tube(s)] Combustion process systems 1) Package boilers A package boiler refers typically to relatively small scale, predesigned or off the shelf boilers which are available in a large number of types and capacities. Due to their optimized designs they are very efficient and typically use less fuel and electric power to operate than non-integrated designs. They are therefore commonly used in a large variety of applications in the food, light industrial, pharmaceutical, food, ceramic, and associated industries. Just as with other combustion process units, a package boiler operates more efficiently when the excess air concentration in the flue gas is reduced while always ensuring that incomplete combustion is avoided. Optimizing air intake for boiler operation requires continuous measurement of the oxygen concentration in the flue gas. The typical package boiler is a water tube boiler or flue and smoke tube boiler with a capacity of 5 to 20 t/h (average steam generation capacity). The most widely used fuels are heavy oil, light oil, and gas. Gas temperature 150 to 300 C (302 to 572 F) Gas pressure ± 0.5 kpa ( 0.07 to 0.07 psi) Dust loading 1 g / Nm 3 Fuel Fuel oil, kerosene or gas Table 3: Typical process conditions in a package boiler Steam at 150 C Chimney 200 C for in situ, probe-type TDL analyzers, where their compact size, single flange entry, and probe configuration allow direct installation in place of traditional Zr /combustion analyzers. The typical installation point will be at the boiler or economizer outlet. Water 2) Process heaters The term process heater commonly refers to any process in the plant which directly employs hot combustion gases to raise the temperature of a gas or liquid process stream. Process heaters are, in effect, heat exchangers and are used extensively throughout refineries and petrochemical plants. They are the main consumers of fuel on site and are therefore a major focus for combustion efficiency optimization. Process heaters consist of multiple coils of tubes inside of which the process fluid passes. Typically, in a refinery this will be a liquid hydrocarbon stream which needs to be heated to a set temperature before entering a refining stage of the plant. The stream is heated by heat exchange with the hot flue gases as these rise through the heater, and also directly by heat from the burners in the radiant section of the heater. Damper Draft Gauge Feed Stock in Draft Gauge Radiant Tubes CO Stack Temp Crossover Bridgewall It can be seen from Table 3 that the typical flue gas conditions encountered in the average package boiler are not extreme, with flue temperatures circa 300 C. This is an excellent application Feed Stock out Burners Figure 8: Diagram of a typical process heater 7 METTLER TOLEDO White Paper

18 New TDLs for Combustion Processes Although there are many types of process heaters and thermal crackers, the general analyzer application requirements are similar for all of them. The typical example shown in Figure 7 is a natural draught unit, fired by arrays of burners using natural gas, sour gas, or waste oil as fuel. Gas temperature 300 to 500 C (572 to 932 F) Gas pressure ± 0.5 kpa ( 0.07 to 0.07 psi) Dust loading 1 g / Nm 3 Fuel Fuel oil, kerosene or gas Table 4: Typical process conditions in a process heater The fuel flow and air supply to the process heater burners are controlled by the rate of process production. The combustion air flow is controlled by adjusting the position of the air damper(s). Without a flue gas analysis of the excess air in the flue, the process operator has to assess the position of the dampers manually, based on experience and/or heater manufacturer estimates, and will therefore take the safest approach, i.e., typically introducing a significantly increased level of excess air to ensure that CO all fuel is burnt under all load and fuel conditions. This decreases the efficiency of the heater due to Furnace flame temperature loss and heat loss to the stack, and increases the risk of NO x production. Flue gas measurements are specified on a wet basis (i.e., directly in the flue gas) and therefore have long been based on zirconia technology as this eliminates the need for sampling systems and minimizes measurement response times. However, zirconiabased sensors have a limited life, their catalytic surfaces can be poisoned, and flame out conditions in the heater can present an explosion hazard due to the hot internal surfaces of the sensor (unless expensive flame arrestors are utilized). On the other hand, probe-type TDLs such as the GPro 500 offer a purely optical measurement technique and are immune to these drawbacks. They provide an extremely long lifetime without raising concerns of sensor damage or safety issues, and represent a more reliable, accurate, and cost-effective solution. In addition to combustion control analysis there may also be requirements to measure the stack emissions from process heaters and thermal crackers. The analysis required will be governed by local legislation and the fuels being used, but will typically include oxygen, carbon monoxide, and nitrogen oxides. 3) Refinery process-fired heaters Air Preheater Stack Fast analysis of the oxygen concentration in the Window Box flue gas allows the excess air levels to be reduced, improving efficiency and safety while at the same time providing an adequate air margin to ensure complete combustion. If this is also combined with a combustibles (CO) analyzer and trim system, this enables the excess air levels to be controlled precisely and offers detection of the onset of incomplete combustion, thus giving the optimal combustion efficiency for every fuel. Figure 9: Diagram of a typical process fired heater Draft Blower Exhaust Blower Gas temperature 300 to 500 C (572 to 932 F) Gas pressure ± 0.5 kpa ( 0.07 to 0.07 psi) Dust loading 1 g / Nm 3 Fuel Fuel oil, kerosene or gas Table 5: Typical process conditions in a fired heater Fired heaters share the same characteristics as process heaters and indeed their names are largely interchangeable. For gas or oil-fired heaters the measurement requirements and process conditions are shown in Table 5. Larger fired heaters may consist of many cells, each of which contain multiple burners, and for these, multiple analyzers may be necessary. This approach 8 METTLER TOLEDO White Paper

19 New TDLs for Combustion Processes ensures that an analysis is dedicated to each bank of burners and their burner control system. The critical performance parameters for both oxygen and combustibles measurement are speed of response (typically 10 s total for T90 is required), analyzer integrity, and measurement repeatability. In situ TDL analyzers are highly suited for use in fired heaters as they offer fast speed of response (< 2 sec), precision long life measurement, specific CO determination, and are unaffected by catalyst poisons in the gas stream that can damage other sensors. 4) Heavy oil or gas-fired power generation boiler Generator Cooling tower Steam generator Turbine Air, fuel supply Condenser Figure 10: Diagram of typical power generation boiler. Gas temperature 300 to 500 C (572 to 932 F) Gas pressure ± 0.5 kpa ( 0.07 to 0.07 psi) Dust loading 1 g / Nm 3 Fuel Fuel oil, kerosene or gas Table 6: Typical process conditions in a power generation boiler Power generation boilers are usually large to very large facilities. Single or /CO measurement may be utilized on smaller cogeneration plants, but on larger plants multiple measurement points may be configured in the flue. For gas-fired plants the particulate loading will be reasonably low, increasing somewhat on oil-fired units. Typical flue gas temperature after the reheater are in the region of 300 C and this presents another ideal application for TDL measurement of both and CO. Again, for this application, TDL technology offers the benefit of fast speed of response, specific CO measurement, immunity to background gases and catalytic inhibitors in the process stream, and superior sensitivity and lifetime, reducing ongoing cost of ownership. Conclusion Tunable diode laser analyzers are at the forefront CO of gas analysis and are increasingly the first Filter choice for an ever growing number of applications which were once the province of extractive gas ana- Stack lyzers. Now they are also challenging zirconium oxide and catalytic technologies throughout a ID Fan range of combustion applications, where their measurement precision, lower cost of installation and operation, minimal maintenance, fast response times, and reliability has cemented their reputation as the technology of choice. A new generation of TDLs with innovative process adaptions, takes the core benefits of TDL technology, but overcomes earlier drawbacks of alignment difficulties and the requirement of optical process purge gas, to provide a truly flexible, easy to install, compact, and reliable TDL solution for combustion measurement. 4www.mt.com/GPro500 Mettler-Toledo AG Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland For more information 01/2015

20 INGOLD Leading Process Analytics White Paper HCl Monitoring in Stacks and Scrubbers Now Has a Dependable Solution Due to its polluting effects, hydrogen chloride (HCl) levels in industrial exhausts are of growing concern. To ensure levels are within required limits, HCl monitoring equipment is installed on stacks and scrubbers where the gas is present. However, measuring the gas can be very challenging with commonly used technologies. In situ, probe-type analyzers using tunable diode laser spectroscopy offer a cost-effective, low maintenance and reliable alternative. The requirement for hydrogen chloride analysis The most common non-sulfurous acid gas emitted from industrial processes is hydrogen chloride. It is a significant atmospheric pollutant which is harmful to human health and the environment. HCl contributes to acid rain and consequential damage to both infrastructure and agriculture. Common sources of HCl in industrial exhausts include waste incineration and ethylene dichloride production. These processes generate significant volumes of HCl which must be reduced to very low levels before off-gases can be released to the atmosphere. HCl is a pungent gas with a low odor threshold of 0.26 ppmv and has an Immediately Dangerous to Life and Health concentration of 50 ppmv. It has a National Institute for Occupational Safety and Health and Occupational Safety and Health Administration Permissible Exposure Limit of just 5 ppmv. Due to these hazards HCl release from fixed emission sources are increasingly monitored and controlled, and legislative emission levels apply in many countries. The challenges of extractive measurement technologies There are several emission monitoring system technologies available that can be considered for the measurement of HCl, including extractive techniques such as non-dispersive infrared, gas filter correlation, fourier transform infrared and cavity ringdown spectroscopy. Each of these has their relative merits but all

21 HCl Monitoring in Stacks and Scrubbers 30 _ 25 _ 20 _ 15 _ 10 _ 5 _ 0 _ Figure 1: An example of reversible retention. The actual HCl concentration is shown in red with the analyzer response shown in yellow. It is clear that there is not only a significant response time delay, but in addition, the analyzer never reaches the true peak HCl reading. share the same requirement: extraction and conditioning of a gas sample from the process to allow determination of HCl levels. Consequently, one of the first challenges encountered when considering any of these methods is the provision of suitable sample extraction and conditioning equipment. Removing a gas sample containing HCl for analysis via off-line techniques can be problematic and expensive. This is largely due to the solubility and sticky nature of HCl gas, whereby any sample-wetted surfaces (pipes, regulators, rotameters, filters, etc.) will absorb and desorb the gas, a process often referred to as reversible retention, which can lead to delayed and inaccurate results (see Figure 1). Also, any contamination in the sample conditioning system can contribute significantly to the retention of HCl (dirt in the regulators or tubing is a typical example). Indeed, a dirty sample transport line can result in T90 response times of 30 minutes or more. Such a delay in analysis is, of course, impractical and therefore, in an attempt to address the issue, conditioning systems require very careful material selection and design to reduce the internal surface area and minimize the effects of reversible retention. Various techniques can be used to decrease response times, including running high concentrations of HCl through the system to passivate surfaces, followed by reduction in concentration and regular cleaning of any in-line filters which could be sites of HCl retention. Moist HCl passivation gas can also be effective. In addition, it is essential to maintain the sample system at a high temperature (typically 180 C or greater) when measuring low level HCl to prevent loss of sample through the system which would lead to false low readings. Therefore, what is termed a hot/wet system is normally required. The key components of a hot/wet sample system are: Sample probe (depending on flue gas temperature) Sample probe filter Sample line Heated head pump Fittings Measurement cell All of the above mentioned constraints add additional expense to the purchase of the HCl measurement system, and ongoing operational servicing and frequent maintenance required on such equipment can be very costly. Problems in sustaining reliable HCl measurements can ultimately reduce confidence in the analysis itself. In addition to the requirement for monitoring pollutants from fixed emission sources, as part of the mitigation process to reduce HCl levels some processes use gas scrubbers to wash HCl from the carrier gas stream. 2 METTLER TOLEDO White Paper

22 HCl Monitoring in Stacks and Scrubbers Acid Scrubber Raw gas Fan HCl Clean gas Droplet separator Scrubbing liquid Pump Figure 2: With sufficient height in the tower, HCl scrubbers can have efficiencies as high as 99.9 %. Wet gas scrubbers Many industrial and chemical processes create large quantities of waste gas. Some of these waste gas streams are acidic in nature and require treatment to neutralize the acid prior to further treatment. This prevents downstream damage to plant infrastructure or processes and ultimately reduces plant emissions. To achieve this neutralization, wet scrubbing towers (or columns) are used. Inside these towers, water or liquid chemicals are typically sprayed in a counter-flow to the waste gas stream to chemically absorb (scrub) the acidic components from the gas (Figure 2). HCl is one such acidic gas and can be present for example in raw and clean gas streams from thermal oxidizers, pulp and paper mills, and cement plants. Alkali Pump Suppletion water Sluiced scrubbing liquid To ensure efficiency, the internal structure of the scrubbers are designed to provide the maximum contact area between the waste gas and the scrubbing medium, and designs integrate a variety of gas/liquid contacting methods, including spray chambers, water jets and packed beds. Depending on the levels of HCl, in some scrubbers mist removal systems are required to prevent acid mist formation at the exit of the unit. Measurement of HCl on the scrubber tower outlet using extractive technologies has very similar requirements and constraints to stack emissions as discussed above, therefore extraction and conditioning equipment, with its inherent high maintenance and ongoing operation costs, remains a constraint. To control efficiency in these scrubbers HCl is monitored at the scrubber outlet to detect breakthrough (sometimes inlet HCl levels may also be monitored). These measurements are typically considered as a process control measurement and are not directly subject to legislative controls. Plain recycled water may be used to absorb the HCl in scrubbers, resulting in wastewater comprising a weak hydrochloric acid solution (hydrochloric acid is formed when HCl gas is in contact with water), which may be a valuable by-product for use elsewhere on the plant. However, as HCl is highly acidic, scrubber efficiency is significantly improved if a strongly alkali scrubbing medium is used, such as sodium hydroxide (NaOH). The resulting by-product in this case is sodium chloride (NaCl). HCl + NaOH NaCl + H2O HCl measurement directly in the process A better solution to the problems of extracting and conditioning gas samples for HCl measurement was found in tunable diode laser (TDL) spectroscopy. This technology uses a laser beam that is of the same frequency as the absorption frequency of the gas to be measured. The laser source is mounted directly in the gas stream and a detector is installed directly opposite. As the laser beam passes through the gas, some of the light is absorbed by the target gas and analysis of the quantity of light received at the detector specifies the level of the target gas in the stream. As TDL analyzers are installed in situ, sample extraction and conditioning equipment is not required. This also means that measurements are almost instantaneous. Because of these and other advantages TDL solutions for measuring O2, CO2, CO, HCl, moisture and other gases are becoming the standard measurement technology in many chemical and petrochemical processes. 3 METTLER TOLEDO White Paper

23 HCl Monitoring in Stacks and Scrubbers Commonly, TDL analyzers are of a cross-stack configuration as described above, i.e. separate laser source and detector installed on opposite sides of the stack, pipe or vessel. Such an arrangement requires the mounting of a flange on both sides of the stack and careful alignment of laser source and detector, plus there must be access to the two flanges. Unfortunately, this line of sight configuration can have its issues. When the stack walls flex due to thermal expansion and contraction, the laser source and detector can become misaligned and in extreme cases the entire signal can be lost. The solution is to realign the beam or move the installation to a point where thermal conditions are less severe, but perhaps to a location where the gas stream is less representative of the target gas. In addition, high consumption of purge gas is required to keep the optical windows free of dust, particles and sticky hydrocarbons. Alignment-free TDLs are the solution The answer to these concerns is TDLs such as METTLER TOLEDO s GPro 500 series, that do not require alignment. These analyzers contain both laser source and detector in one unit. The probe attached to the analyzer head has a corner cube (retroreflector) at the far end which directs the laser beam from the source back up the probe to the detector. This means mounting in only one side of the stack or vessel is required, and even if high process temperatures warp the stack/ vessel the corner cube will still direct the laser beam to the detector. The design of the probe is such that purge gas consumption is much lower than with cross-stack TDLs, and as the GPro 500 has no moving parts, maintenance is very low, amounting only to annual verification and periodic cleaning of the optics. The absence of alignment issues, compact dimensions of probe and detector/analyzer head unit, plus a range of process interfaces ensure that the GPro 500 can be installed precisely where the measurement is required, even in tight spaces, without the compromises that often have to be made when operating with bulky cross-stack designs. GPro 500 analyzers provide all the benefits of in situ analysis but without the alignment and purge gas consumption concerns of the majority of cross-stack TDLs. This makes the GPro 500 HCl analyzer a highly suited and economical solution to monitoring stack emissions and gas scrubber outlets. Conclusion To help minimize pollution, hydrogen chloride levels in industrial waste gases must be curtailed, but the practicalities of measuring HCl using extractive technologies can be very cumbersome and costly. Tunable diode laser analyzers are at the forefront of gas analysis and are increasingly the first choice for a growing number of applications which were once the province of extractive gas analyzers. The measurement precision, lower cost of installation and operation, minimal maintenance, fast response times and reliability of TDLs has cemented their reputation as a technology of choice. A new generation of probe-type TDLs takes the core benefits of the technology, but overcomes earlier drawbacks of alignment difficulties and the requirement for significant optical process purge gas, to provide a truly flexible, easy to install, compact and reliable solution for HCl measurement. GPro is a registered trademark of Mettler-Toledo AG in Switzerland, the USA, the European Union and a further five countries. Mettler-Toledo GmbH Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland Visit for more information 04/ 2 016