Trimming NOx From Furnaces

Originally appeared in: CHEMICAL ENGINEERING

November 1992 Issue, pgs 122-128. Reprinted with publisher’s permission.

TRIMMING NOx FROM FURNACES Ashutosh Garg, Furnace Improvements Each passing year seems to bring about increasingly stringent pollutant-emission laws governing combustion equipment. In the future, one can expect even-stricter emissions limits to be imposed on the chemical process industries (CPI). As regulations tighten, the necessity to consider environmental concerns in the operation of furnaces is also mounting. Of the various environmental laws now affecting the CPI, laws covering nitrogen oxides (NOx) are among the most sweeping. For example, in the U.S., the South Coast Air Quality Management District, or SCAQMD, which covers the Los Angeles basin, already has one of the strictest

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standards. Under SCAQMD’s standard, furnaces with capacities of less than 40 million Btu/h must release less than 40 ppm of NOx by September 1991. For furnaces larger than 40 million Btu/h, the limit is less than 25 ppm by December 1995 (NOx emissions from 36 % of the units greater than 40 million Btu/h must be cut down to 25 ppm by September, 1992). Finding the means for limiting NOx from fired heaters has become a major thrust of many sectors in the CPI. The utility industry – the first industrial sector in the U.S. to be affected by NOx controls – has been the spawning ground for many of the new technologies now being used to stem NOx from CPI furnaces. Other

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Emissions-control technologies need not limit a fired heater’s performance

FIGURE 1. CPI furnaces, available in box or cylindrical designs, can be fitted with a number of coil configurations.

Trimming NOx from Furnaces sectors that have also been strongly affected by NOx standards are petroleum refining and petrochemicals. The stricter NOx limits means that it is increasingly important to understand both the capabilities of these new emissioncontrol technologies, and how they affect a fired heater’s overall performance, reliability and operating flexibility. This is especially significant when retrofitting combustion equipment with new emission controls. The NOx Dilemma Vertical heaters used in the CPI fall broadly into two categories: cylindrical and box heaters. In both types, the tubes are laid out on the walls of the radiant section. In cylindrical heaters, the tubes are installed vertically, while in box heaters, the tubes are arranged horizontally. In both designs, the burners are installed on the floor, and fire vertically upwards. Most of burners employ a naturaldraft design, in which the stack provides the draft for drawing air into the furnace for combustion. Newer units are equipped with forced-draft firing systems and air preheaters to improve fuel efficiency. The


convection section consists of bare and extended-surface tubes to recover heat from the flue gases before they exit from the stack. Figure 1 shows the typical heater types used in the CPI. The pollutants generated by burning fuel fall into three primary categories: carbon monoxide, unburned hydrocarbons, and partially oxidized organic materials and soot that result from incomplete combustion; sulfur oxides and ash directly attributable to fuel composition; and nitrogen oxides formed at firebox temperatures by the reaction of the oxygen and nitrogen present in the air and fuel. Incomplete combustion products can usually be held to tolerable minimums by the proper operation of modern burner equipment, while sulfur oxide and ash emissions can be cut by using the right fuel. However, nitrogen oxide concentrations are primarily functions of fuel composition, burner design and firebox temperature, and so have to be controlled by choosing the right operating conditions. There are several ways that NOx is formed in a furnace. Thermal fixation of atmospheric nitrogen and oxygen in the combustion air produces “thermal NOx”

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while the conversion of chemically bound nitrogen in the fuel produces “fuel NOx”. For natural-gas and light-distillate-oil firing, nearly all NOx emissions result from thermal fixation. With residual fuel oil, the contribution from fuel-bound nitrogen can be significant and, in certain cases, predominant. This is because the nitrogen content in residual fuel oil can be as high as 0.3% N2, and conversion to NOx may be 50-60%. The formation rate of thermal NOx is dependent on the reaction temperature, the local stoichiometry, and the residence time. The fuel-NOx formation mechanism is more complex, depending upon fuel pyrolysis and subsequent reaction between many intermediate nitrogenous species and the oxidant species. The rates for formation of both thermal NOx and fuel NOx are kinetically or aerodynamically limited, with the amount of NOx formed being much less than the equilibrium value. The rate of formation of NOx is dominated by combustion conditions and can be suppressed by modifying the combustion process. Both thermal and fuel NOx are promoted by rapid mixing of oxygen with the fuel. Thermal NOx is greatly increased

Trimming NOx from Furnaces

FIGURE 2. Actual measurements (in ppm) of nitrogen oxides in fluegases can be converted to the more common way of representing emissions, in lb NOx (as NO2)/ million Btu of gross heat released

by long residence time at high temperature. Emission limits are usually specified in terms of pounds of NOx per million Btu of gross heat released, or pounds per hour. NOx concentrations, however, are measured in terms of ppm (volume) basis. Since operating conditions vary among various furnaces, the NOx measurements are converted to standard conditions at 3% oxygen. By calculating the dry combustion production per million Btu and the heat release rate, R, it is possible to convert from ppm to lb/million Btu or lb/h. NOx emission calculations are made on the basis of NO2 (molecular weight of 46), although NO2 is only 10-15% of the total NOx. ppm vol. (at 3% O2) = ppm vol. measured x (21-3)/21 - % O2) where, % O2 = vol. % O2, dry basis NOx, in lb/million Btu = (ppm NOx) x (DSCF/ million Btu) x 46/ (1,000,000 x 379.3)

Modifying combustion conditions to inhibit the mechanisms for formation of NOx Lowering NO x generated during combustion by either catalytic or noncatalytic reduction. The NOx-control processes discussed below utilize one or a combination of the above techniques. Flue gas recirculation (FGR) extracts a portion of the flue gas from the stack and returns it to the furnace along with combustion air (Figure 3). This lowers the peak flame temperature, and cuts thermal-NOx formation. The addition of fluegas also reduces the oxygen available to react with the nitrogen. A comparison of the two heat duties for a furnace with and without – flue gas recirculation is shown in the table. Increasing the recirculation rate generally corresponds to a decrease in thermal NOx, but flame instability and a decrease in the net thermal output limits the recirculation rate. Recirculation rates for gas-fired units are limited to about 15% to 20%, resulting in maximum thermal-NOx reductions on the order of 50%. It is useful where low nitrogen fuels, such as natural gas, are used. Recirculating flue gas temperature should For quick NOx level estimations, the not be more than 600oF. following DSCF – dry volume of flue-gas Flue gas recirculation has been in standard cubic feet/ million Btu at 3% mostly applied to forced-draft burners. O2 concentration – values are Installation requires additional duct work, recommended: a flue gas recirculation fan, a flow control damper, special burners and combustion Natural Gas 10,127 control instrumentation (such as Propane 10,127 continuous oxygen and carbon monoxide Butane 10,127 analyzers in the stack). If the heat content Fuel Oil 10,684 of the fuel is highly variable, a flame safeguard system is required to monitor the flames continuously. The technique is Post Combustion NOx Treatment The concentration of NOx in combustion suitable for heaters with a few burners, flue gases can be cut by: such as vertical, cylindrical heaters. Flue gas recirculation does not affect the overall efficiency of fired heaters if the temperature of fluegas leaving the convection zone is the same as that of the flue gas being recirculated. However, the split of radiant heat and convection heat duty will change, since the TABLE. Fluegas recirculation changes the split recirculating flue gas acts as a diluent, reducing the of convection -and radiant - heat duties.


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Trimming NOx from Furnaces uptake of heat in the radiant section and deactivation. However, this can be reduces NOx to N2 and H2O. Ammonia is increasing it in the convection section. avoided if care is taken during the design injected into the upper part of combustion stage. For example, if SO2 is present in chamber or into a thermally favorable Selective catalytic reduction (SCR) the flue gas, then a minimum temperature location downstream. The various involves injecting ammonia into the flue of 608oF is recommended for SCR reactions are: gas upstream of a catalyst bed. The operation. A catalyst’s life depends on its chemical reaction involved is: type, the application and other factors, 6NO + 4NH3 5N2 + 6H2O with numbers of three to six years being O2 + 4NO + 4NH3 4N2 + 6H2O reported in oil and gas applications. SCR systems have the highest 6NO2 + 8NH3 7N2 + 12H2O NOx and NH3 combine on the catalyst’s installation costs and requires the greatest surface, forming an ammonium salt amount of space of all NOx-control intermediate that subsequently methods. They can be easily retrofitted in Recently, a urea-based regent is decomposes to produce elemental fired heaters with air-preheating systems, increasingly being used in place of NH3 nitrogen and water. The catalyst lowers since all this involves is re-rating of the because it is safer and easier to handle. the activation energy of the NOx fan and re-routing the duct to the air Urea decomposes into NH3 and carbon decomposition reaction, thereby enabling preheater via an SCR unit. The left dioxide inside the firebox. use of this technology at lower flue gas portion of Figure 3 shows a typical SCR The flue gas temperature is critical to temperatures. The optimum temperature unit retrofitted in a furnace with an air the successful reduction of NOx. For range for SCR is 600oF to 700oF. preheating system. convectional combustion, the optimal SCR removes 70 to 90% of the NOx, range for NH3 injection is 1600o to using between 0.9 to 1.0 mole NH3 for Selective noncatalytic reduction (SNCR) 1,750oF; for urea, 1,000o to 1,900oF. As every mole of NOx; this leaves behind 5 is a post combustion-control method that the temperature increases, the NH3 reacts to 10 ppm of unreacted NH3. The major Figure 3: Popular post-combustion methods of removing NOx components of an SCR system are a catalyst bed reactor, an ammonia injection include selective catalytic reduction (left part of figure) and grid, and an ammonia storage unit. fluegas recirculation. Ammonia can be injected in anhydrous form or as an aqueous solution. Typically a residence time of 0.5 to 1.0s allows for adequate mixing of the ammonia and NOx before the catalyst bed. Several factors in addition to operating temperature influence SCR performance. These include the catalyst composition and configuration, sulfur and metals content of the fuel, and the design of the ammonia-injection system. Catalysts are commercially available in a wide variety of materials. These include such metals (such as titanium, vanadium and platinum), zeolites and ceramics. Catalyst shapes include honeycomb plates, parallel-ridged plates, rings and pellets. Each combination presents advantages and disadvantages in terms of allowable operating temperatures, catalyst fouling and pressure drop. Typical gas velocities over the catalyst are around 50ft/s, and the pressure drop is 3-4 in. (water column.) The early applications of SCR had been prone to a number of problems. These include: catalyst plugging by fineparticle dust; catalyst poisoning by SO2; conversion of SO2 to SO3; formation of ammonium bisulfate; and the deposition of ammonium bisulfate on the catalyst at temperatures below 518oF. All these factors lead to catalyst


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Trimming NOx from Furnaces While SCR and SNCR maintain control over NOx after is has been formed in the combustion reaction, modifications of the combustion equipment or the burners can also significantly reduce NOx formation. There are a number of advantages in using such modified burners, the major ones being simplicity and low cost. At the same time, since burners form the heart of a furnace. The process of implementing new ones should always be tried cautiously. Staged air burner systems divide the incoming combustion air into primary and secondary paths: All of the fuel is injected into the throat of the burner and is combined with the primary air, which floss through the venturi and burns (Figure 4). In this fuel-rich zone, the fuel partially burns and the nitrogen is Figure 4: Two ways of modifying the fuel air stoichiometry during comconverted into reducing agents. These nitrogenous compounds are bustion is to use a staged fuel burner (left) or a staged air burner. subsequently oxidized to elemental more with oxygen than with NO, forming to be used along with a second NOx- nitrogen, thereby minimizing the more NOx. At flue gas temperatures reduction technique. generation of fuel NOx. below the optimal range, the rate of Also, the peak flame temperature is reaction declines, resulting in reduced Redesigning The Equipment lowered in the fuel-rich primary NOx control and greater amounts of combustion zone, since the generated heat unreacted NH3 slipping into the dissipates rapidly. Recirculation of effluent. combustion products within the burner The NOx reduction achieved is of further cuts the flame temperature and the order of 50-60%, with the NH3 slip oxygen concentration, reducing NOx (in in the 20 to 30 ppm range. The this case the thermal NOx even more. In technique is effective in the presence of the secondary-combustion zone, carbon monoxide, and with oxygen additional air is injected through contents of up to 1% (calling for very refractory ports to complete combustion close control of excess air). and optimize the flame profiles. The disadvantages of SNCR are Staged air burners are simple and similar to those of SCR: Ammonium inexpensive, and NOx reductions as high salts, namely ammonium sulfate and as 20 to 35% have been demonstrated. bisulfate, may form if excess NH3 The main disadvantage of the burners is reacts with sulfuric acid, form a prior the long flames, which need to be reaction between SO3 and water. controlled. Further, staged-air burners Ammonium bisulfate can contribute to have proven to be quite successful in fouling and corrosion in low formed-draft applications, and have even temperature heat-recovery equipment. been used with flue gas-recirculation Ammonium chloride can also be systems. formed, which is undesirable since it causes visible plumes. High levels of Staged fuel burners inject a portion of the NH3 slip, up to 50 to 100 ppm, can fuel gas into the combustion air, and the occur if the NH3-to-NOx ratio is not resulting combustion is very lean (i.e. air optimized. Overall, the method has not rich). This lean combustion reduces become widely popular with process thermal NOx. The remainder of the fuel fired heaters since it cannot meet gas is injected into a secondary Figure 5. NOx levels can be cut very low NOx-reduction requirements by combustion zone through secondary levels by combining staged fuel burners with itself, and needs nozzles (Figure 4). internal fluegas recirculation


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Trimming NOx from Furnaces used to reduce NOx to very low levels. In this design, the fuel gas’s pressure or external agents, such as medium-pressure steam or compressed air, are used to induce flue gas recirculation within the burner. Low excess-air burners works on the principle that low levels of excess air suppress NOx formation. Typically, excess air levels are maintained at 5%. The burners are often of a forced-draft design, and employ a self-recirculating technique to produce a multi-stage combustion effect. A NOx level of 0.06 to 0.08 lb/ million Btu is typically encountered. Generally, it has been found that reducing excess air from 30% to 10% cuts NOx emissions by 30%. References Air Pollution Engineering Manual, AP42, U.S. Environmental Protection Agency. NOx Control In fired heaters, Martin, R.R. and W.M. John Zink Co. Cleaning Up NOx Emissions, McInnes, R., and M.B. Van Wormer, Chem. Eng., Sept 1990 Reduce Heater NOx In the Burners, Seabold, J.G., Hydrocarbon Process, Nov. 1982. The author

The combustion products and inert gas from the primary zone reduce the peak temperatures and oxygen concentration in the secondary zone, further the inhibiting NOx formation. Some of the NOx formed in the first stage combustion zone is reduced by the hydrogen and carbon monoxide that is formed in staged combustion. Staged fuel burners can reduce NOx emissions by as much as 50-60%. This type of burner can operate with a small


flame length, and at lower excess-air levels than can staged air burners. The flames in staged fuel burners are about one and a half times longer than those in standard burners. Staged fuel burners have been found more effective in reducing NOx in gas-fired heaters and, so, the majority of the applications are gas fired.

Asutosh Garg is Manager of Thermal Engineering at Kinetics Technology International Corp. He has more than 18 years of experience in process design, sales and troubleshooting of all combustion systems. He graduated in Ultra Low NOx Burners, a combination of chemical engineering in 1974 from Indian staged fuel burning and internal flue gas Institute of Technology, Kapur. He is circulation (Figure 5), have recently been registered professional engineer in

6 *Reproduced with the permission of Chemical engineering.

Trimming NOx From Furnaces

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