- FINAL REPORT -
ATMOSPHERE RECOVERY AND REGENERATION
IN HEAT TREATING OPERATIONS
September 30, 1997
1.0 Summary of Original Projects Goals
Heat treating and similar high temperature furnace operations typically maintain a reducing gas “atmosphere” over parts being processed. A major furnace operation is “carburizing” - heat treating of steel parts in the presence of gaseous carbon monoxide to produce hard, wear resistant surfaces. An estimated 4,500 or more U.S. manufacturing plants use carburizing furnaces, most of which have atmospheres produced by partial combustion of natural gas and air in a catalytic chamber. In current industry practice, the waste furnace atmosphere gases are discharged directly through one or more gas-fired flares into ambient air with no attempt at recycling, reuse or energy recovery and without additional pollution control. This practice introduces high concentrations and large amounts of carbon monoxide into ambient air and wastes significant energy. Federal and state air permitting, monitoring and emission fees and increasing natural gas prices have reduced U.S. competitiveness for industries using this process. Anticipated Federal air emission “maximum achievable control technology” will require additional expensive and energy consuming pollution control devices and result in a further competitive disadvantage.
Dana Corporation, a Fortune 100 heat treater proposed to install a new prototype technology developed by Atmosphere Recovery, Inc. (ARI) to reduce at the source 90% or more of its furnace atmosphere gas discharge. Support (cash, equipment, and in-kind) was to be provided by the Dana Plant, the NICE3 Grant, and additional contributions from Dow Chemical (Generon), Air Liquide America, BOC Gases, ARI, and the Minnesota State Department of Public Service.
2.0 Variance from Project Goals
The principal deviation in project goals related to project completion scheduling. The prototype component selection process and testing process, particularly for the gas-tight dry compressor, the ultra-low pressure transducers, gas sensing hardware interfaces, and furnace safety systems took longer than originally anticipated. The NICE3 funds were received later than originally planned and Dana’s heat treat production needs slowed furnace modification and safety testing. Two extensions were requested and granted and the additional funds required provided by Dana.
Another variance from the original proposal was total (100%) recovery of atmosphere from one test furnace instead of lesser percent recovery from three. This change was made because the pressure regulation of multiple furnaces with a single system was more complex and less safe than desired, and with better than anticipated separation and control performance, the preferred design became zero-discharge atmosphere recovery and reuse, using one unit per furnace.
The final significant variation was a modification of project participation. After a staffing change early in the project, Air Liquide America no longer participated (although they recently expressed renewed interest). BOC Gases contributed information early on, but did not contribute further until this month (this month’s estimated contribution is included in the final project budget). Dow Chemical’s former Generon group was purchased by M.G. Industries, but otherwise provided support and technology to the project. Additional support (in the form of equipment trials, discounts and technical support) were provided by more than 10 additional vendors, discussed more fully in Section 8.0.
3.0 Future Technology Requirements
No major technology barrier exists for near-term commercialization of the technology. Some prototype components need improvement, longer term testing, and possible industrial certification. One component (the primary gas sensor) needs further development or an alternate approach must be selected before widespread commercial installation. The higher pressure gas compressor used in the prototype can be replaced by modified lower pressure, commercial systems, however this type of compressor has not been tested in this application.
During prototype construction, ARI used commercially available technology whenever possible. However, some commercial technologies (such as the ultra-low pressure transducers, the computer controller software and high speed communication boards) needed modification or remain too “bleeding edge” to be considered proven. Vendors of these systems are assisting ARI in refining their products for commercial use.
Some components (such as the primary gas sensing system and the membrane control valve) are not currently commercially acceptable because they were custom built or require excessive service and calibration to be used by the heat treat industry. Alternatives to these components are now being developed by ARI, under Dana funding and with some assistance from BOC Gases.
Three items need improvement - the intake air filter (excessive pressure drop under suction), moisture sensing (the dew point detectors foul with oil mist after a short period of time), and the furnace vestibule low pressure safety relief valve (leakage during operation). These problems should be resolvable in production versions using alternative furnace connection and compressor locations, use of the improved primary gas sensor, and a custom built valve (respectively).
4.0 Technology Application and Result
The system was installed on a large (700 cubic feet per hour - CFH) Surface Combustion “Allcase” brand batch furnace at Dana Corporation’s axle manufacturing facility in Plymouth, Minnesota. The project consisted of prototype component and system development, construction, operation and testing using a membrane-based pollution-preventing and energy conserving system to recover and reuse discharged furnace atmosphere from heat treat carburizing furnaces. Tests on full loads of pinion gears were used to develop standardized performance data, and the results compared with conventional furnace operation on similar production parts treated by conventional methods. Appendix C presents the atmosphere data collected during a typical trial run (conducted 9/25/97).
ARI’s atmosphere recovery and regeneration process (the ARI process) currently uses commercially available nitrogen gas separation membrane technology and a proprietary gas monitoring, control and constituent gas correction procedure. In the prototype installation, the furnace atmosphere discharge vent is sealed gas-tight, and the formerly discharged gas cooled and piped to the ARI regeneration unit for compression and removal of impurities. The processed gas is then piped back to the furnace for reuse. The prototype unit removes the excess hydrogen, oxygen, carbon dioxide and water vapor found in the discharged gas and establishes appropriate nitrogen, CO, and hydrogen ratios. 96% of the recovered atmosphere is currently reused during operation, exceeding the initial goal of 90%. The primary separation process functions by preferentially retaining the nitrogen and carbon monoxide in the waste atmosphere gas components on the “upstream” (pressurized) side of membrane surface while allowing the impurities to pass through the membrane at relatively higher rates. Small amounts of endo gas is introduced and controlled to replace furnace leakage losses (not required if furnace doors are modified). The prototype system uses non-dispersive infra-red and gas chromatograph gas quality sensors and multiple programmable controllers and man-machine interfaces to optimize operation. The recovered gas is returned to the furnace as replacement atmosphere through the existing endo supply lines.The removed gas products (4% of the total recovered) is currently flared away from the furnace, but has hydrogen content sufficient so that production units can use the gas for processing or burner fuel.
To date the project has been an unqualified success, and has the apparent potential to revolutionize atmosphere furnace operation. The heat treated test parts were carburized within proper or greater depth specification on standard control cycles indicating the potential for greater furnace throughput. Emissions from discharge pipe flaring were positively (through use of a gas tight ball valve) reduced to zero, and modifications to apply the technology on existing furnaces proved to be minimal. Combined with improved furnace sealing, total process carbon monoxide use and emissions has the potential to be reduced to less than 10% of current levels, and a minimum of 25% of heat treat process energy can be saved. Economics for developed commercial systems are likely to be better than originally assumed due to additional labor savings through automation, predictive failure maintenance, lower part rejects, and the increased through put potential.
5.0 Operational Findings
5.1 Typical Industrial Unit Definition
High temperature furnace processes used in manufacturing (including heat treating, brazing, sintering and related operations) typically maintain reducing or inert gas “atmospheres” over parts being processed. A major heat treating operation of this type is “carburizing” primarily used to produce “case hardened” (hard and wear resistant) surfaces on steel parts. Many U.S. industrial sectors manufacture products containing case hardened parts, examples of which are gears, bearings, driveshafts, piston rings, universal joints, hand and machine tools, gas turbine blades, and steel fasteners. The automobile, farm and construction equipment, aerospace and defense industries, and tool and die manufacturing industries are significant producers of such parts. An estimated 4,500 to 5,700 U.S. facilities have carburizing operations, using 15,000 to 20,000 furnaces.
Typically, parts to be carburized are placed in batch or continuous (conveyor or index driven) furnaces heated by natural gas (resistance electricity is sometimes also used) to temperatures of 1600 to 1750 degrees Fahrenheit (oF) in the presence of gaseous carbon monoxide (CO). The most common “atmosphere” used in carburizing is principally a mixture of 20% CO, 40% hydrogen and 40% nitrogen (called “endothermic” or “endo” gas). As the endo gas contacts the hot part surfaces, some of the carbon in the CO portion dissolves and diffuses into the steel and enhances the carbon content at the part surface and for a distance inside. The released oxygen reacts with the hydrogen to form water vapor, and the nitrogen acts primarily as an inert gas.
Most often the endo (and other similar blend) atmospheres are formed away from the furnace(s) by reacting natural gas with air in heated catalytic chamber “generators”. Additional natural gas (or electricity) is used as fuel to continually heat the catalyst. After generation, the hot endo gas is cooled to maintain gas component ratios and is then piped to the carburizing furnace(s) at a fixed flow-rate sufficient to always maintain a “reducing” (non-oxidizing) gas blanket over the parts, provide sufficient CO to react with the part surface, react with the released CO oxygen, and react with air leaking into the furnace (from loading doors and furnace holes). Carburizing is less often conducted with fixed-flow blends of nitrogen and methanol dissociated directly in the furnace, producing endo-like gas without the need for endo generators.
To adjust carbon and hydrogen levels, natural gas is usually added to the endo mixture in the furnace during the carburizing process. The parts remain in the furnace and carburizing continues until an appropriate “case depth” (depth from the surface of the carbon reaction) forms. This carburizing process typically takes 6 to 12 hours per part tray, after which the parts are removed from the furnace and immediately “quenched” (rapidly cooled) to “lock in” the proper carbon content and prevent oxidation of the steel surface.
Furnace capacities are usually expressed by weight capacity, but from a atmosphere gas energy usage and environmental impact evaluation standpoint, endothermic gas flow rates to the furnaces are more appropriate. Typical industrial-scale carburizing furnaces use endo (or nitrogen-methanol) flows in the 300 to 3000 cubic feet per hour (CFH) range, the majority ranging from 600 to 1,500 CFH, with 1,100 CFH an estimated per furnace median. Using the average of the U.S. furnace and plant numbers indicated above, a typical heat treating facility would need 3,600 CFH of endo gas at capacity. For “reliability” of endo supply, endo generation capacity typically exceeds use needs by 50% to 150% (assumed to be 100% on average). Therefore a “typical” U.S. heat treater would have about 7,200 CFH generation capacity (which even when “idling” consumes energy and emits air pollutants).
Based on these estimates, in 1994 the Dana plant was almost precisely a “typical” heat treat industry plant, having a 3,600 CFH endo usage capacity (2,100 CFH in three batch furnaces and 1,500 in a single continuous furnace) and 7,200 CFH generation capacity (in two 3,600 CFH endo generators) that year (capacity has since been expanded). The plant’s 1994 heat treat energy and emission data is thus considered the reference and definition of a “typical unit” in this report.
5.2 Energy Savings
All endo generation and use energy is typically wasted because the discharged atmosphere gas must be replaced by newly generated endo gas. Given the toxicity of the discharged gas, heat recovery is seldom used. Independent calculations from the major carburizing furnace vendor (Surface Combustion, Maumee, OH) and from Dana Corporation facility data indicate that a minimum of 25% of the natural gas consumed by the overall carburizing process is a result of endo generation, use and discharge (about 220,000 therms out of the 880,000 therms used at the Dana plant in 1994 for heat treating operations). This figure does not include heating, ventilating and air conditioning expenses related to indoor air quality for CO control in a typical facility.
Combined with improved furnace sealing and gas routing, installation of four (4) ARI production units could eliminate the typical plant thermal catalytic generation requirements and eliminate all waste atmosphere gas discharge venting and flaring from each furnace. Based on energy estimates from the prototype unit (that has operated from 400 to 1600 CFH during different trials), each of the four typical production units would require an average of 2.0 kilowatts (kW) of electricity (8.0 kW total) to operate (this energy includes associated gas compressors, cooling fans and all electronic controls) for a total of about 70,000 kilowatt-hours (kWh) at 8760 hours per year. This electric consumption is about 35% of that initially estimated due to greater than anticipated membrane performance and lower pressure than anticipated operation. ARI unit replacement gas constituents for a typical plant are still estimated at 10% of typical unit amounts of natural gas - 20,000 therms per year (2,000 Million British Thermal Units - BTUs). However, this figure could be cut to nearly zero with additional furnace modifications (primarily improved door seals). The “Actual Energy Savings” table presented in Appendix A summarizes the per unit (typical plant) energy information assuming four units were installed.
Using 20,000 Million BTUs per year as typical unit average gas savings, fully commercialized installation of the proposed system at the lower estimate of 4,500 captive and job-shop heat-treating operations (possible by 2010) would result in an annual energy savings of about 90 trillion BTUs (.09 Quads) of natural gas and/or methanol-nitrogen equivalent. This total does not reflect the additional savings that may be achieved through additional furnace modifications, reduced furnace heating and scrap loss from improved process control, nor the avoided additional energy consumption required for avoided air pollution control systems. Total energy savings may more than double if these factors are included (to about 0.2 Quads per year). Substantial additional energy savings from reduced plant ventilation requirements are also anticipated which are not reflected in any of the tables.
5.3 Environmental Benefits
Prior to flaring, waste carburizing gas contains almost 200,000 parts-per-million (ppm) CO and elevated nitrogen and sulfur oxides. Following flaring, this waste carburizing gas typically contains from 10,000 ppm to 40,000 ppm unburned CO, and the furnace discharge from a typical unit facility averages 80-200 tons per year (TPY) of actual CO emissions. An additional 20-80 TPY of CO is typically emitted from bypassing spare endo generator capacity, since minimum flows must be maintained in the generators to produce an acceptable endo quality. Assuming the “typical” heat treating plant has average leakage, flaring, and generator bypassing emissions, overall actual process CO emissions from a typically sized current technology endo gas carburizing unit (facility) totals about 200 TPY. This amount is two-thirds of the 300 TPY value assumed in the NICE3 proposal and is based on more extensive sampling since the proposal was written. The methanol-nitrogen endo equivalent atmosphere systems eliminates the idling and spare generator waste (actual CO furnace emissions of about 100-150 TPY in a typical facility) but introduces hazardous volatile organic emissions from methanol use and storage and an unknown amount of additional CO from methanol and nitrogen manufacture. Because furnace door seals have not been improved during prototype operation thus far, the CO and other estimated emissions presented in the “Actual Waste Savings” table in Appendix A still assumes 90% overall recovery by the ARI process (nearly 100% is possible after the ARI process and door technology is developed).