|Title||Regenerative Heat Recovery/Burner System as a replacement for Conventional Recuperators|
|Abstract||Cycling regenerator/burner systems are well proven as Best Practice for energy efficiency and CO2 reduction in a number of high temperature industrial heating applications. The cost-effectiveness of these systems becomes more difficult to justify in processes with waste gas temperatures below about 1000oC. A simplified regenerative heat recovery system has been developed which can provide the extremely high heat recovery of the cycling regenerators while reducing the overall system cost. Burners are de-coupled from the cycling regenerators via a hot-side valving system suitable for the process conditions. Essentially the regenerator system can take the place of a conventional recuperator. The compact and cost-effective system design make it ideal for retrofit/upgrade projects for energy efficiency, productivity and/or CO2 emission control in a variety of industrial heating applications. A case study of an operational LTRTM ‘Low Temperature Regenerative System' will be presented including operational results and a discussion of the process impacts.|
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1 Regenerative Heat Recovery/Burner System as a replacement for Conventional Recuperators Author: David Schalles, Bloom Engineering Co., Inc. Cycling regenerator/burner systems are well proven as Best Practice for energy efficiency and CO2 reduction in a number of high temperature industrial heating applications. The cost-effectiveness of these systems becomes more difficult to justify in processes with waste gas temperatures below about 1000oC. A simplified regenerative heat recovery system has been developed which can provide the extremely high heat recovery of the cycling regenerators while reducing the overall system cost. Burners are de-coupled from the cycling regenerators via a hot-side valving system suitable for the process conditions. Essentially the regenerator system can take the place of a conventional recuperator. The compact and cost-effective system design make it ideal for retrofit/upgrade projects for energy efficiency, productivity and/or CO2 emission control in a variety of industrial heating applications. A case study of an operational LTRTM ‘Low Temperature Regenerative System' will be presented including operational results and a discussion of the process impacts. Keywords: Regenerative, Fuel efficiency, CO2 reduction, GHG reduction, waste heat recovery, BACT 2 INTRODUCTION Capture of flue gas waste heat for preheating of combustion air is often chosen as a convenient and cost-effective method for boosting the thermal efficiency of industrial combustion processes. Bloom Engineering has developed and successfully implemented a system for regenerative heat recovery which can provide improved economic payback over a range of common industrial heating processes. The essential combustion system components are illustrated in Figure 1: Combustion Air Fan Exhaust Fan Fuel Supply FLUE Bloom Engineering - Low Temperature Regenerative (LTRTM) Combustion System Cycle Valves Cycle Valves NOTE: Diagram shows 1 of 2 regenerative cycles Cycle Valves Cycle Valves Regenerator B Regenerator A Preheated Combustion Air Burner Burner Burner Burner The Bloom Low Temperature Regenerative (LTRTM) system has been developed to improve the fuel efficiency of furnaces and heaters for processes with flue gas temperatures approximately in the range of 1000F up to 1800F (approx. 540C to 980C). In this window the LTR system can often be justified when compared with cold air or recuperative combustion systems. At greater temperatures, the system design considerations will typically favor the coupled burner+regenerator hardware design1. An alternative to periodic regeneration is the rotary-type (Lungstrӧm) regenerator which theoretically could 1 ‘Regenerative Burners-Are They Worth It?' David G. Schalles, AFRC Fall 2004 Symposium 3 provide similar thermal performance but suffers from an ‘achilles heel' of being difficult to seal against the required combustion air pressure at these temperatures, among other drawbacks. Since the large majority of industrial furnaces are fired with hydrocarbon fossil/nonrenewable fuels, improvements in fuel efficiency correspond directly to reductions in CO2 emissions. DESIGN ISSUES The basic idea of periodic (time-cycling) regenerators has been in use for over 150 years. Initially the primary purpose was to achieve elevated flame temperatures sufficient to achieve the desired process temperature, such as glass melting, iron blast furnaces and steel open hearth production. In modern times, the coupled burner+regenerator has been extensively applied to many types of metals industry furnaces for fuel savings and furnace throughput increases. The concept of the regenerator system de-coupled from the burners using intermediate valving is described by Schmidt and Wilmott2. The Bloom LTRTM system has been developed to improve the cost-effectiveness of thermal regenerative heat recovery to lower temperature processes. The size (and hence cost) of the heat storage beds can be minimized through the maximization of the heat storage effectiveness (e) =Qactual/Qmax theoretical and the minimization of the time period (P). Bloom has developed a proprietary calculation code for packed-bed heat exchange based on the following models: Description of the temperature fields in the media and chamber walls according to Fourier-Kirchoff law Heat transfer between ball surface and gas flowing through fill according to Kunii and Levenspiel theory Pressure loss in the fill according to Krischer, Kessler and Kast A simulation code using finite element methods was developed to solve these models to a pseudo-steady state. Essentially the model is run for a single bed design alternating between cold and hot inlet fluids (in counter flow) and solved for the condition at the end of the selected period. Time-average fluid exit temperatures can be extracted to allow averaged thermal performance. This has enabled us to optimize the heat storage system design and study various alternatives as they become available. The design options for the heat storage media can be classified as structured (such as honeycomb, open cell foam, checker brick) or loose fill (spheres, random ‘pebbles', rings, etc.). Material choices could include ceramic, metal alloys, and even raw mineral shapes. Bloom has conducted thorough evaluations of these options. The key attributes for the ideal heat storage media design would include: High thermal conductivity High strength including thermal shock resistance and resistance to breakage during filling/removal 2 ‘Thermal Energy Storage and Regeneration', F.W. Schmidt and A.J. Wilmott, McGraw-Hill, NY (1981). 4 High surface-to- volume ratio Resistance to chemical/corrosion attack Repeatable/consistent pressure drop characteristic Low cost and readily available in desired configuration For most metals industry applications we have concluded that a simple packed bed of high-alumina spheres provides the best balance of the above selection criteria. If weight is a key concern due to a particular structural situation, then a structured media with thin sections such as ceramic foam may be chosen. Our investigation of structured metal alloy media which is produced for rotary regenerators used on power plant ‘heat wheel' devices was found to be significantly higher cost than ceramic materials for a given performance specification. The upper process temperature limit of the LTR system is dictated by the hot-side valve and piping material and design limitations. The use of high alloys, outboard shaft bearings and high temperature seals are used to achieve high cycle duty switching valves at temperatures approaching 1000 oC. One valve supplier that we found has claimed success with around 1200C using ceramic components, however the costs at this time do not seem to justify such as system as compared with coupled regenerative burners. Dilution air is of course an option for system design, but generally the efficiency penalty of reduced flue gas temperature into the regenerator will negatively affect the fuel savings cost justification for the system. Another consideration is cost and heat loss through the hot air ducting to the burners. The design selection for a given application requires a cost/benefit evaluation regarding piping and insulation options. It can be stated as obvious that the piping costs will be more favorable in the case of shorter piping distances and larger flow rates. Burner designs for the high air temperatures of the LTR system require consideration for internal materials, fuel gas temperatures (hydrocarbon decomposition leading to sooting) and NOx emissions. Also the burner ignition, stability and turndown characteristics may be important to handle the period of time during bed switchover. Conventional recuperative hot air burners will in most instances require design modifications to address these requirements. On the other hand, ‘high-temperature' regenerative burners as used in the coupled-regenerator systems may prove to be over-designed for this application. The optimum design for a given project will best be determined via an engineered approach which takes the above criteria into account. System control can be very similar to a conventional recuperative air combustion system. The additional requirements include switching valve and burner cycling logic, to handle the bed reversal periods. In the simplest two-bed system, there will be a brief period (generally 5% of total operating time) when the air flow experiences an upset interruption. Options to address this include burner cycling, cold air bleed into the air header or fuel interruption. The choice of approach is simplified for systems operating above the auto-ignition temperature. The Bloom LTR system includes proprietary sequencing logic which has been developed through our R&D program and is now proven in service. 5 ADVANTAGES An analysis of the LTR system thermal efficiency as compared to alternative combustion systems has shown that the fuel efficiency, CO2 emissions and utility-based operating costs all favor the LTR system, as shown in Figure 2: Operating Cost Comparison Per Hour For 10 MM BTU/hr. (2.52 x 106 Kcal/hr.) Net Heat Input to Furnace Cold Air Recuperative 400°C Preheat Regenerative 720oC Air Preheat Oxy- Fuel Equivalent Burner Input (106Kcal/Hr.) 4.46 3.6 3.07 3.12 Natural Gas (NM3) 501.1 404.5 344.9 350.5 Fuel Cost ($) $106.24 $85.75 $73.13 $74.32 Oxygen (NM3) 724.4 Oxygen Cost ($) $67.57 Electrical Cost for Blowers ($) $1.09 $1.82 $2.37 Total Cost/Hr. ($) $107.33 $87.57 $75.50 $141.89 CO2 emissions ton/hr. 1.04 0.84 0.72 0.73* *Does not account for significant off-site CO2 produced due to electrical power required for Oxygen production (air separation). Accounting for off-site CO2 could raise oxy-fuel CO2-equivalent emissions by 50% or more. Basis: • Efficiencies calculated on furnace exhaust gas temperature of 800°C • Fuel Cost $6.0/MM BTU • Oxygen Cost $0.25/ccf (liquid oxygen) • Electricity Cost $0.075/kWh A full economic analysis of a project would of course include equipment, installation and maintenance cost factors, future utility price expectations and environmental-impact compliance costs. In general, we believe that regenerative heat-recovery technology represents the highest available fuel efficiency and lowest operating cost option for furnace applications with POC exhaust temperatures in the range of roughly 600-1000C. 6 Once the decision is made to explore regenerative heat recovery technology, the next step is to evaluate the various options with regard to a given application. As mentioned previously, additional styles of regenerative systems include coupled burner+regenerator systems, as well as rotary ‘heat wheel' designs. The following chart illustrates the general design differences and offers our opinion of the positive and negative aspects of each: COMPARISON OF REGENERATIVE SYSTEMS PROS CONS Rotary Continuous/no cycling hardware needed Leakage at moving bed seals Fewer burners than coupled Loss of POC gases according to wheel design/number of segments Bed rotation mechanism maintenance Temperature limited by hot-side valving Coupled No hot-side valves. Temp limit at least 1500°C Double number of burners required. Cost and space required increase Special design/housing options can help to offset this No hot-side ducting Cycling hardware required LTRTM Fewer burners than coupled Retrofit may be easier Cycling hardware required Minimized amount of cycling equipment Temperature limited by hot-side valving APPLICABILITY The LTR system is intended as a high-efficiency option suitable for almost any direct-fired industrial furnace application where metallic recuperators are currently employed. Our current market targets include metals-industry processes such as heat treatment, annealing and reheating of various metals and alloys. Some high-temperature fluid-heating applications might benefit from this technology; however fluid processes typically can utilize convective heat exchanger systems to transfer POC heat directly to the product, resulting in final POC temperatures low enough that further heat recovery is not practical. Cost justification by fuel savings alone is often difficult, especially for retrofit situations. In many cases, the additional benefits of the LTRTM System can help to offset the upfront costs. These factors may include: 7 1.) Reduced maintenance costs as compared with metallic recuperators. 2.) Flexible, compact geometry 3.) Small, low temperature fan-driven exhaust system eliminates the need for large refractory lined flue and stack system. 4.) Minimized CO2 emissions due to increase fuel efficiency. CASE STUDY To prevent steam/H2 explosion potential, prior to charging aluminum solid metal ‘sows' into a re-melting furnace, they must be dried to insure there is no remaining moisture. The heat imparted to the sows then also reduces the additional heat required for melting. Often this is performed in a gas-fired furnace operating just below the melting point of the metal, for fastest processing. An existing furnace with a damaged metallic recuperator was converted to the Bloom LTRTM regenerative system including one pair of regenerators and 6 new burners, plus associated valving and a new exhaust blower. The project was justified on the basis of achieving 12% fuel savings compared to the existing system, which was essentially running with cold combustion air due to the deteriorated recuperator. Unreliable burner operation was contributing to extended cycle times and excessive fuel consumption. The installation of the LTRTM System (commissioned in Fall 2010) achieves documented fuel savings of greater than 25% compared with the data prior to conversion. Air preheats as high as 525C are achieved with furnace temperatures of 600C. In fact, some modifications to the air piping materials are to be carried out, as the plant personnel had hoped that the existing steel piping would be adequate, as they apparently did not really expect to achieve such a temperature in practice. 8 Future Outlook Further planned developments for extending the applicability of this technology include the following: 1.) Investigation of extended temperature range valving, such as ceramic or specialty alloys flappers and special bearing designs. 2.) Optimization of low NOx burner designs to suit a general air preheat range of 600-900C. 3.) Further study of heat storage media selections optimized for various temperature regimes. 4.) Development of a spreadsheet-based cost analysis model which will include cost inputs for fuel, other utilities and equipment. This could also include air emissions estimates and comparisons to other modes of heat recovery. Such a model could be made available for client use in evaluating BACT emission reduction options for NOx and CO2.