Case History: Ethanol Facility Retrofit with a Lower Emissions, Improved Fuel Efficiency Thermal Oxidizer and Dryer Burners Control # 1074 Prepared by L. Crynes, P. Melton, H. Hohl, C. Baukal, M. Lorra, and M. Fleifil John Zink Company, LLC Tulsa, OK ABSTRACT An extensive engineering study was conducted on an existing thermal oxidizer system in a corn ethanol plant. The system included a horizontal thermal oxidizer, a recuperative preheater, HRSG boiler, and feedwater economizer. Several problem areas were studied including excessive vibration, refractory failures, and preheater tube damage. System efficiency and emissions were also studied. The study concluded that the thermal oxidizer geometery, burner design, and waste injection technique were the causes of the problems. This equipment was subsequently replaced and the problems were solved. INTRODUCTION Ethanol Production Ethanol is made by fermenting sugars produced from corn starch. Ethanol is the most widely used oxygenate for complying with current oxygenated fuel requirements. Ethanol s high oxygen content allows automobile engines to combust fuel better, resulting in reduced tail pipe emissions. 1 It has been added to gasoline since the late 1970s. 2 Since that time, U.S. fuel grade ethanol production capacity has grown to billion of gallons per year (see Figure 1). Ethanol plants are scattered around the Midwestern U.S. as shown in Figure 2. Ethanol is a clean-burning renewable fuel that helps reduce emissions of carbon monoxide (CO) and smog-forming volatile organic compounds (VOC). 3 1
Figure 1. U.S. ethanol production (Source: ). Figure 2. U.S. ethanol plants (Source: ). The ethanol industry manufactures ethanol for blending with automobile fuel, principally from industrial corn. During the ethanol manufacturing process, dry mills (see Figure 3) burn off gases which emit volatile organic compounds and carbon monoxide into the air. Environmental regulations continue to reduce the allowable NOx emissions from thermal oxidizers. 4 This has led to the development of low NOx thermal oxidation systems 5 that include low NOx burners 6 and post-treatment cleanup equipment. Figure 4 shows a schematic of a typical thermal oxidizer system with a steam generating boiler. 2
Figure 3. Schematic of the dry mill ethanol production process (Source: ). Flue Gas Waste Gas Steam Air Fuel Thermal Oxidizer Boiler Figure 4. Thermal oxidizer system generating steam. Ethanol production from corn has increased rapidly in the last few years (see Figure 1). A number of plants to process the corn into ethanol have been built during this time. The product remaining after fermentation of the starch in the corn to the primary product, ethanol, is called distillers grains. It contains the non-fermentable components of the corn and is rich in cereal and residual yeast proteins, fat, minerals, and vitamins. It is an excellent digestible protein and 3
energy source for all ruminants and can make up 20 to 30 percent of a ration s dry matter. It is a profitable co-product of corn ethanol production. Case Study Ethanol Plant The distillers grains product from the plant under consideration here is produced by condensing and drying the distillers grains solubles (DGS) that remain after fermentation. The DGS must be dried to a uniform moisture content depending on the final product composition desired. Dried distillers grains with solubles (DDGS) contains about 90% dry matter while the DGS contains only 35 to 50% dry matter. To achieve the proper moisture content, rotary dryers utilize hot air generated by air heaters burning natural gas (methane) to remove the excess moisture. Although the process of producing ethanol and DDGS ends with the dryers, the dryer exhaust contains small amounts of odorous hydrocarbon from the corn products and products of incomplete combustion (PICs) from the air heater burners that must be eliminated. To accomplish this, a thermal oxidizer (TO) is added to the end of the overall process. The dryer exhaust, which may contain 10 to 15% oxygen, enters the TO where it is heated by firing additional natural gas and additional fresh air to burn out the hydrocarbon and PICs. Heat from the TO flue gas is recovered by several methods. A gas-to-gas heat exchanger is used to preheat some of the dryer exhaust coming into the TO. A waste heat boiler downstream of the heat exchanger generates steam for use in the plant. An economizer is located downstream of the boiler to preheat boiler feed water. Finally, cook water heaters are located downstream of the economizer to heat water used in the ethanol production process. Since both the air heater and TO require large amounts of natural gas, they are responsible for a significant amount of the operating cost for a plant. It is important that the natural gas use is efficient and effective. Efficient burning of natural gas will result in minimal amounts of unburned natural gas (methane, i.e. CH 4 ) and partially burned CH 4 in the form of carbon monoxide (CO). Also, nitrogen oxides (NOx) must be minimized. Inefficient or improperly utilized dryer burners or TO burners will result in excessive amounts of these components in the exhaust gas from the dryers and/or the TOs. PLANT PROBLEMS An onsite evaluation was conducted of the thermal oxidizer systems at the plant. This evaluation was followed by an engineering study to address the following issues: 1. Significant vibration in the existing thermal oxidizer had resulted in numerous shutdowns and damage over the first year of the plant s existence. 2. Fiber blanket modules had repeatedly failed and been replaced. 3. The recuperative preheater had experienced tube damage. 4
4. The plant was planning to increase capacity such that the thermal oxidizer load and the HRSG steam production would increase about 20%. 5. The system would need to meet the plant s existing emissions requirements while operating at this expanded capacity. 6. An additional stream from the CO 2 scrubber exhaust would be added to the thermal oxidizer for reduction of VOC emissions from the plant. Some of these issues are considered next. Excessive Vibration The thermal oxidizers vibrated at a low frequency. The magnitude of the vibration appeared to be causing damage to other areas of the thermal oxidizer system. For example, ceramic fiber modules in the TO had come loose, perhaps because of excessive flexure and vibration of the shell, and some heat damage had occurred. The preheater for the dryer exhaust had experienced tube failures. The thought was that the vibration may have caused or contributed to those problems. Also, fabric expansion joints had failed because of excessive cycles of pulsation. Changes had been made in the process (most recently the combustion air flow had been increased), but the changes had not corrected the vibration and the extra air simply increased the fuel usage. Emissions One of the problems identified was the need to reduce emissions of CO, VOC, NOx, etc. so there is more of a cushion between permitted levels and actual levels. Efficiency Another problem was the need to increase steam production, and maintain natural gas usage at less than 32,000 Btu per gallon of ethanol produced. System Reliability Although fixing the vibration problem and repairing the refractory were the immediate concerns, plant personnel were also interested in improving other process and mechanical operational portions of the plant as well. A general concern was to increase the long-term run time with no shutdowns, by increasing overall system reliability. ANALYSIS The existing thermal oxidizer was a horizontal unit with a U-shaped layout. A recuperative preheater, boiler (HRSG) and feedwater economizer were attached to the exit of the oxidizer. The L:D (length:diameter) of the unit appeared to differ from the standard recommended value of 3:1. The initial section of the unit was fairly short, after which the flue gases entered a 90 5
turn and the unit transitioned from round to rectangular walls. The flue gases then immediately went through another 90 turn before exiting the thermal oxidizer. Pulsation/Vibration An initial evaluation indicated that the thermal oxidizer geometry was a primary cause of the vibration problems. In addition, the burner design and waste injection technique appeared to result in poor mixing and delayed combustion. Recycled flue gas was being introduced with the combustion air. The flue gas recirculation (FGR) rate and details of injection were not known by plant personnel. A spin-type burner was being used in the thermal oxidizer. Waste (dryer exhaust) was injected into the front of the oxidizer through numerous ports surrounding the outside of the burner. A stainless cylinder had originally extended from the burner tile in order to delay mixing of the dryer exhaust and burner combustion products. This cylinder was now warped and the top 180 had been removed. Visual inspection of the flame seemed to indicate poor mixing and delayed combustion. Extreme vibration was noted during the site walk and loss of numerous fiber modules was evident. The pulsation and magnitude of pulsation were the product of two conditions. Large (high heat release) burners generate more combustion noise than small burners. However, although the volume (sound power level) of the noise is greater, the frequency of stable combustion noise is usually relatively high (200-500 Hz) and can be attenuated. The problem arises when the combustion is unstable, which generates low frequency pulsation in the 10-15 Hz range, and the combustion chamber is large but short (L/D < 3) and it has a flat end with a side outlet. The amplification occurs because the combustion chamber geometry has a harmonic frequency that is the same as one of the low frequencies produced by the unstable combustion. The resulting strong, low frequency pulsation/vibration can easily damage the combustion device itself and travel both upstream and downstream to damage other equipment in the system. The existing slow-spin burner was apparently meant to run fuel-rich in the center to delay combustion, resulting in lower flame temperature and, therefore, lower NO X production. There was also some cooled flue gas being recycled into the combustion air, reducing the actual O 2 content and further delaying and cooling the combustion process. Unfortunately, the delayed combustion is not stable and the first part of the problem condition is established. Therefore, to minimize one of the two conditions causing the damaging vibration, it was recommended to change the burner to a more stable design configuration. There are three types of burners that could be utilized for this. One is a relatively simple, stable staged fuel low NO X burner that introduces all of the air and less than half of the fuel gas at one point and the rest of the fuel gas at some point downstream. The drawback is that air has to be supplied for all the fuel combustion. Another type is a more complicated, proprietary combination staged fuel and staged air ultra low NO X burner that also uses some internal flue gas recirculation to produce even less NO X that the previously mentioned burner. Again, the drawback is that all the air for the fuel has to pass through the burner. The third option is also a proprietary technology that utilizes staged combustion and oxygen in the dryer exhaust in such a way that combustion air and therefore fuel usage (as well as emissions) are minimized. Some of the fuel gas and enough combustion air for that fuel are introduced at the front of the burner and the rest of the fuel is injected at locations downstream along with dryer exhaust. The oxygen in 6
the dryer exhaust is utilized to oxidize the remaining portion of the fuel. Since less combustion air is required, the amount of fuel needed is reduced. The components of this option are larger in size with more complicated fuel piping and, therefore, more costly. However, the low emissions and fuel savings should compensate for the added expense in a reasonable amount of time. The first two options would be better if excess firing to generate more steam is desired because the fuel required for those cases is up to 30% greater than normal operation with the third option. Regardless of the burner changes made, the thermal oxidizer geometry should be modified to attempt to change the harmonic frequency of the chamber. Changing the 90 turns to 45 turns by adding a corner wall may help. Perhaps other disruptions can be achieved even though the primary shape is not changed. (Fleifil to provide vibration analysis discussion and data) Emissions The combined stack emissions will be the sum of the emissions from the dryers and TOs. The NO X generated by the dryer burners is not destroyed when it passes through the TOs. At a TO operating temperature of 1500 to 1550 F, CO formed by the dryer burners will only be partially oxidized in the TOs. VOCs coming from the dryers will be reduced significantly but the amount from the dryers is not great anyway. SO X cannot be reduced or increased by the currently used equipment. Basically, the sulfur that enters the combustion equipment will exit the combustion equipment as SO X. SO X can be removed from flue gas but only by adding more equipment to react the SO X with caustic or some other alkali material. The emission levels can be minimized with the right technology. However, even the best TO burner configuration, producing very low levels of NO X, CO, and VOC, may not be able to compensate for the NO X and CO from the dryer burners. In that case, the dryer burners would have to be replaced with lower emission burners. For example, from the NO X standpoint only, if the dryer burners are rated at 75 MMBtu/hr each and the TO burners are rated at 125 MMBtu/hr each, the total rated heat release would be 550 MMBtu/hr (4 x 75 + 2 x 125). If the maximum NO X is 91.98 TPY, for an 8,424 hour-year (50 weeks operation - 2 weeks downtime), the average NO X produced would be about 0.04 lb NO X /MMBtu of rated fuel input. Because the dryer burners contribute more than half (54.5%) of the total heat release, by ratio, they generate more than half of the NO X if both produced 0.04 lb/mmbtu. If they generate more than 0.04 lb/mmbtu, the TO burners would have to be designed for much less than 0.04 to compensate. Minimize Natural Gas Usage/Increase Steam Generation Based on a number of heat and material balances completed, for the given amount (~182,000 lb/hr) of dryer exhaust (DE) entering the TO, the option 3 burner configuration would require about 106 MMBtu/hr of fuel gas and about 6,000 SCFM of outside air to operate at 1550 F, with reasonable heat loss, if some of the oxygen in the DE is utilized and the excess O 2 level in the stack is maintained at 3% on a wet basis by volume. Cooling the flue gas from 1550 F to 400 F 7
(i.e., no dryer exhaust preheat) will only produce about 83,000 lb/hr of steam per TO or an overall amount from both boilers of about 166,000 lb/hr of steam. To increase steam production to 235,000 lb/hr, about 237 MMBtu/hr of heat would have to be recovered. Assuming a 5% loss in blowdown, etc., the total heat available would have to be about 249 MMBtu/hr or 125 MMBtu/hr for each TO. Again, assuming the flue gas is cooled from 1550 F to 400 F only to generate steam, each TO would have to fire about 152 MMBtu/hr with 30,600 SCFM of outside air in addition to the 182,000 lb/hr of dryer exhaust to produce enough flue gas. In other words, to produce 235,000 lb/hr of steam, 304 MMBtu/hr of natural gas would have to be burned. To maximize steam production in all conditions, it would be necessary to remove the dryer exhaust preheaters. A refractory lined spool installed between the TO and the boiler could replace the preheater. However, to minimize fuel gas usage, it is necessary to utilize the DE preheater to its maximum capability. For this reason, while we can try to achieve the lowest possible amount of natural gas use per gallon of ethanol produced under normal operating conditions, we cannot generate 235,000 lb/hr of steam unless the firing rate is increased proportionally. Another important consideration is that if the permitted NO X, CO, and VOC emission levels were not increased with increased fuel firing to generate more steam, the average allowable emission levels per MM Btu heat release would be reduced. That reduction would either have to be supported by reduced-emission TO burners (because that is where the fuel increase would occur) or by replacing dryer and TO burners as mentioned in the emissions section. A CFD analysis was recommended to evaluate the existing oxidizer and develop solutions to reduce vibration. (Lorra to provide typical CFD results) 8
Table 1. Plant operating conditions. Dryer Exhaust Temperature, F 140 to 200 Pressure, in. w.c. (approx.) -0.5 to -0.1 Molecular Weight 24.4 Flow, lb/hr 181,700 Component Mole % H 2 O <48 CO 2 <7 O 2 8 10 N 2 CO VOCs (as C) NOx Balance <5630 ppmv (dry) 400 1700ppm (wet) <13 ppmv (dry) SOLUTION The proposed system consisted of a forced draft burner, combustion chamber, horizontal thermal oxidizer, and miscellaneous instrumentation. This system utilizes the existing combustion air blower, induced draft (ID) fan, control system, and downstream heat recovery equipment (boiler, economizer, etc.). Combustion air and cooling drum air are fed to the burner and thermal oxidizer via the existing combustion air blower. Natural gas is fired in the burner to attain the minimum destruction temperature and as needed to meet the required steam production. Dryer exhaust is fed into the combustion chamber in a manner that provides efficient mixing and stable combustion such that VOC, CO, and NOx emissions are minimal. The overall system is designed to minimize vibration and prevent the problems often associated with vibration. The horizontal thermal oxidizer furnace provides sufficient residence time for destruction of VOCs, along with any CO that may be generated in the upstream dryer burners. Flue gases exit this furnace into the existing boiler where energy is recovered to produce steam. The flue gas then passes through the existing downstream economizers where further energy is recovered. The existing ID fan is used to draw the flue gases through the oxidizer and the heat recovery equipment. The thermal oxidizer s minimum operating temperature of 1,550 F was maintained by burning natural gas. Dryer exhaust is introduced after the oxidizer reaches 1,550 F in order to guarantee 9
destruction of unwanted waste components. The unit was designed to fire enough natural gas to provide the 135,000 lb/hr gross steam production requested for the plant s process and the deaerator. When less steam is required, the thermal oxidizer can be turned down to conserve fuel while still meeting the full requirements for destruction of dryer exhaust compounds. A proprietary staged combustion system was recommended to meet the low NOx emissions required by the plant s permit. A linear, horizontal layout was used to minimize the risk of vibration. The recuperative preheater was removed at the customer s request due to previous problems with control and overheating. The new unit utilizes the oxygen contained in the dryer exhaust stream as a source of oxygen for combustion. Because of the low temperature and low oxygen concentration of the dryer exhaust, this can result in flame instabilities, flame-out or vibration. However, the injection technology allows this while still maintaining stable combustion. By utilizing this oxygen, the new system is able to operate with less fresh combustion air than many other designs, thus saving fuel and providing additional turndown capabilities when the plant has a lower steam demand. In addition, the existing air blower, ID fan and controls were reused. However, the controls were significantly upgraded to provide improved control for the new design as well as offer redundancy for improved uptime and upgrade the safety level of the control system. Operational Reliability and Process Control Because the market for ethanol is strong and because the co-product of ethanol production is also a valuable product, there is a large advantage of minimizing down time. During the review of the existing system, some areas were found, noted below, that could be improved. Some items were simple and others would be more difficult to modify. Also, after becoming more familiar with all of the plant combustion/heat transfer related equipment, other areas may be identified that can be enhanced to improve control and/or reliability. Scanners After the vibration problem, one of the first things we noticed was that the TO burners used only one scanner. (Interestingly, it was wrapped in a plastic bag to avoid moisture apparently dripping from some ductwork.) To minimize the total package costs for multiples of the same model commercial burners, often only one scanner is used. However, when that scanner needs maintenance or if it fails, the burner cannot be in service. Since there are only six burners (four air heaters and two TOs), the cost of adding a scanner to each burner is insignificant compared to loss of production. It was recommended that a second scanner be added to each burner, isolation valves added between each scanner and the burner front plate, and the control logic revised to have the flame proved signals in parallel. The isolation valves will allow each scanner to be taken out of service and removed from the burner front plate for repair or replacement while the remaining scanner safe burner operation. 10
Expansion Joints As previously noted, at least one fabric-type expansion joint had failed and been replaced with an equivalent fabric joint. The replacement joint was observed flexing in and out because of the pulsation. Unfortunately, fabric joints are not designed for continuously flexing conditions. Their primary purpose is to handle gradual differential growth between adjacent pieces of equipment rather than pressure cycles that continuously flex the fabric. It was recommended that when currently installed fabric joints fail, they be replaced by metallic expansion joints. Even with some pulsation, the metal joint will not flex and break because of excessive flexure cycles. Belt Drive Blowers Based on previous experience with blowers having motors larger than 100 HP, it is recommended that direct drive (through a properly designed coupling) and utilization of proper inlet flow control will require far less maintenance and repair than belt drive blowers. Belt drives place a lateral thrust on the motor and blower shaft bearings usually decreasing bearing life. Belts must be adjusted regularly as they stretch and finally replaced, sometimes on a yearly basis. Belt drives are often used when there is some anticipation that the blower speed will have to be changed to increase or decrease flow rate and/or supply pressure because of unknown or unplanned process variations. It is relatively easy to change pulley sizes and belt lengths but with direct drive units, the wheel or the whole blower has to be changed to correct an undersize condition. Belt drives can also save some plot space. If shaft bearing and belt failures continue with the dual motor blower, we suggest going to direct drive blowers. Another alternative may be to attempt to balance the lateral (downward) thrust on the shaft bearing with an adjustable pulley mounted above the blower shaft. CONCLUSIONS REFERENCES 1. www.epa.gov/compliance/civil.programs/caa/ethanol/index.html 2. Renewable Fuels Association, Fuel Ethanol: Industry Guidelines, Specifications, and Procedures, Renewable Fuels Association publication #960501, Washington, DC, 2003. 3. National Corn Growers Association web site: www.ncga.com/ethanol 4. Baukal, C.E., Industrial Combustion Pollution and Control, Marcel Dekker, New York, 2004. 5. Melton, P. and Graham, K., Chapter 21: Thermal Oxidizers, in Baukal, C.E. (ed.), The John Zink Combustion Handbook, CRC Press, Boca Raton, FL, 2001. 6. Baukal, C.E. (ed.), Industrial Burners Handbook, CRC Press, Boca Raton, FL, 2004. 11