Even though the price of natural gas is currently low, the investment in high-efficiency gas burners makes sense economically. This article discusses different types of high-efficiency gas burners and radiant tubes now on the market. It will also explain the tradeoff between efficiency and NOx emissions and highlight a combustion technology which makes it possible to have the best of both worlds. Finally, the article will discuss a correction factor which can be used to adjust NOx emissions limitations based on combustion efficiency.
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The graph depicted in Figure 1 shows combustion efficiency (based on the lower heating value) as a function of exhaust gas temperature prior to the heat exchanger (if one exists for a particular burner type). In the case of a direct-fired burner, this is equal to the temperature of the furnace. In the case of a radiant tube burner, this temperature is higher than the furnace temperature by an amount which is related to the heat flux density across the radiant tube (cf. e.g. [1]).
The curve labeled ε = 0 represents a cold air burner (i.e. no combustion air preheat). At a temperature of 1,832 degrees F (1,000 degrees C), the best possible efficiency for this type of burner is approximately 50 percent. Many older cold air burners have an efficiency that is even lower than the theoretical maximum due to a variety of factors such as burner design, lack of maintenance, and improper tuning.
The curve labeled ε = 0.4 represents a burner equipped with either a plug-in recuperator or a central heat exchanger. In this case, the combustion air is pre-heated to approximately 40 percent of the exhaust gas inlet temperature. At the reference temperature of 1,832 degrees F (1,000 degrees C), this burner type has an efficiency in the range of 60 to 65 percent.
The curve labeled ε = 0.6 represents a self-recuperative burner. With this burner type, the heat exchanger is an integral part of the burner, and it sits directly inside the wall of the furnace. This arrangement helps to minimize heat losses to the ambient and thereby provides increased efficiency. In this case, the combustion air is preheated to approximately 60 to 65 percent of the exhaust gas inlet temperature. At the same reference temperature of 1,832 degrees F (1,000 degrees C), this burner type achieves an efficiency in the range of 70 to 75 percent. Self-recuperative burners are available with either a metallic or ceramic heat exchanger. The metallic type can operate at temperatures up to approximately 2,050 degrees F (1,120 degrees C), whereas the ceramic type can typically operate at temperatures up to approximately 2,372 degrees F (1,300 degrees C). Figure 2 shows a cross-section of a self-recuperative burner.
There is a new generation of self-recuperative burners which does not fall within the typical range depicted in Figure 1. This burner type is known as “gap flow.” With this design, the burner is equipped with many tiny tubes that serve as heat exchangers. This configuration effectively triples the heat transfer surface area, thereby increasing combustion air preheat to 75 to 80 percent of the exhaust gas inlet temperature. At the reference temperature of 1,832 degrees F (1,000 degrees C), this burner type has an efficiency in the range of 80 to 85 percent. It is available with either a metallic heat exchanger or a combination of a ceramic and a metallic heat exchanger. The metallic type can operate at temperatures up to approximately 1,832 degrees F (1,000 degrees C), whereas the ceramic/metallic type can operate at temperatures up to approximately 2,300 degrees F (1,260 degrees C). Figure 3 shows typical metallic and ceramic/metallic gap flow burners.
The curve labeled ε = 0.8 on the graph represents a regenerative burner. This burner type is equipped with heat storage media such as ceramic balls, discs, etc. Heat storage media is therefore in direct contact with either hot exhaust gas or cold combustion air depending on the point in the regeneration cycle. In the first half of the cycle, the hot exhaust gas heats the storage media to a very high temperature. Then switching valves are activated, and the flow path is reversed so that cold combustion air now flows over the heat storage media. With this arrangement, the combustion air is pre-heated to approximately 80 to 85 percent of the exhaust gas inlet temperature. At the reference temperature of 1,832 degrees F (1,000 degrees C), this burner type has an efficiency in the range of 85 to 90 percent. Traditional regenerative burners fire in pairs – one burner fires while the other exhausts and vice versa. A newer type known as the self-regenerative burner integrates all the regenerators and switching valves into one self-contained unit. Each burner contains six passageways, and each passageway contains a row of ceramic honeycomb discs that serves as the heat storage media. At any point in the cycle, three passageways are exhausting, and the other three passageways are admitting combustion air. After about 10 seconds, the switching valves cycle, and the flow path is reversed. Regenerative burners are also available in either metallic or ceramic varieties. The metallic type can operate at temperatures up to approximately 1,832 degrees F (1,000 degrees C), whereas the ceramic type can operate at temperatures up to approximately 2,372 degrees F (1,300 degrees C). Figure 4 shows a cross-section of a self-regenerative burner.
In many cases, gas burners fire directly into the furnace chamber. In a number of cases, however, the process requires that work load not be exposed to the products of combustion. For example, some processes require a protective atmosphere, such as nitrogen, to prevent oxidation on the surface of the parts. In these cases, indirect heating is used. One method of indirect heating is to fire the burners into radiant tubes which, in turn, transfer heat to the furnace and work load by virtue of a temperature differential.
Cold air burners, or those that use either plug-in recuperators or a central heat exchanger, are paired with traditional, non-recirculating type radiant tubes. With this tube type, the burner fires into one end of the tube and exhausts out the other. Examples of this tube type are the U-tube and the W-tube (see Figures 5 and 6).
Self-recuperative and self-regenerative burners are paired with recirculating type radiant tubes. These tubes each provide some sort of path for internal recirculation, and the burner fires into and exhausts out of the same end of the tube, leading to improved temperature uniformity over non-recirculating tubes.
In the case of a single-ended radiant tube (see Figure 7), exhaust gases flow through an inner tube and then back toward the exhaust through the annulus between the inner and outer tubes. There is a critical space between the tip of the burner and the beginning of the inner tube assembly. The exhaust gases reach this point, and the high velocity burner jet creates a Venturi effect to draw a portion of the exhaust gases back into the inner tube. In this way, the exhaust gases are recirculated several times before they finally pass over the heat exchanger and out of a port on the burner.
A P-tube (see Figure 8) is similar to a U-tube, except that it has a cross-connecting piece to promote internal recirculation. The burner fires into one leg of the tube, and exhaust gases flow back through the other leg and into the cross-connecting piece. The high velocity burner jet creates the same Venturi effect to draw a portion of the exhaust gases back into the firing leg. Likewise, the exhaust gases are recirculated several times before they finally pass over the heat exchanger and out of a port on the burner.
A Double P-tube (see Figure 9) is like a P-tube, but it has two return legs. In this arrangement, the burner fires into the center leg and the exhaust gases flow back through the side legs. This tube type provides a larger surface area and is therefore used with higher input burners.
Traditionally, there has been a tradeoff between combustion efficiency and NOx emissions. In order to achieve high efficiency, it is necessary to preheat the combustion air to high temperatures. These high combustion air preheat temperatures lead to high peak flame temperatures, which are the primary driver in NOx formation. NOx emissions are an exponential function of peak flame temperature, so they tend to increase rapidly with increasing furnace temperature and increasing combustion air preheat temperature. There are a number of techniques available to help combat this problem.
One such technique is known as air staging. With this technique, a portion of the combustion air is mixed with all of the fuel to generate a partial reaction and release some heat. Then, the rest of the combustion air is introduced a bit further downstream to complete the reaction and release some more heat. In this way, the reaction is spread out rather than concentrated at one point. This serves to reduce peak flame temperature and thereby decreases NOx emissions.
High velocity combustion also serves as an NOx reduction technique. Mixing exhaust gases thoroughly inside the furnace or radiant tube has a temperature averaging effect. Therefore, peak flame temperatures are reduced, and NOx emissions are decreased accordingly.
Likewise, flue gas recirculation can also serve as an NOx reduction technique. Exhaust gases are very hot, but not as hot as a flame. So pulling a portion of the inert exhaust gases back into the flame front actually produces a cooling effect. This effect serves to lower peak flame temperatures and hence NOx emissions.
All of these techniques are quite effective under normal conditions. However, when combustion air preheat temperatures reach very high levels, as in the case where self-recuperative or (self-) regenerative burners are used, the techniques are frequently not enough to reduce NOx emissions to acceptable levels. Fortunately, a revolutionary combustion technology has been developed to resolve this problem. This technology is known as FLOX combustion, or FLameless Oxidation (cf. e.g. [1] or [3] for more information). With this special technique, fuel and air are mixed with recirculated exhaust gases and a spontaneous combustion reaction that produces no visible flame takes place. By eliminating the flame from the combustion reaction, peak temperatures are reduced dramatically, and this suppresses NOx emissions to a fraction of the level achievable with traditional NOx reduction techniques. This process only occurs above the auto-ignition temperature, and some safety factor is required; so the FLOX transition temperature is typically set at 1,550 degrees F (850 degrees C). Below this temperature, the burner operates in a normal mode of combustion with a flame. Once the FLOX transition temperature is reached, the gas is injected in a fashion which produces a more favorable mixing/recirculation pattern and prevents flame formation and attachment. If the temperature drops below 1,550 degrees F (850 degrees C), the burner automatically reverts to “Flame” mode.
NOx emissions are limited by law or code in many locations throughout North America. While this is beneficial for the environment and society, the method used to define the limit can significantly alter the outcome if it does not account for the full extent of influencing factors. In particular, this is the case for combustion efficiency in industrial furnaces. A one-dimensional emissions limitation standard can lead to adverse effects, preventing a well-intended initiative from achieving its goals or even leading to the exact opposite. A very simple correction factor can be applied to properly reflect combustion efficiency and therefore achieve the intended goals.
In general, there are two possible methods to better define the emissions limitation in order to achieve the intended objective:
While Option 1 seems straightforward at first, it can be difficult to verify compliance in real-world applications, since there is typically no emissions monitoring device permanently installed to prove the true absolute amounts of a pollutant emitted per year. Option 2 could be used in a way very similar to today’s standard approach. Currently, the limitation of the concentration of a pollutant in the exhaust gases is spot-checked over the course of a typical operational cycle. In the case of NOx emissions, an exhaust gas analyzer probe is inserted into the exhaust system, and several NOx readings are taken over a certain period of time. These values are then averaged to compare them to the limit previously set for the audited furnace.At the same time, the exhaust gas analyzer typically also determines the efficiency of the combustion system based on the CO2 content and the temperature of the exhaust. These efficiency readings (or the published/guaranteed efficiency from the manufacturer) could then be used to adjust the emissions limitation.
Assume that the reference system (ref) is defined as the most efficient combustion technology that still achieves the emissions concentration limit (EB) as defined by the authorities.
If the reference technology is able to achieve lower specific emissions, EB should be set according to this lower value. Furthermore, assume that there is a more fuel-efficient technology (eff) available that does not meet the limitation for specific emissions.
The corrected emissions concentration limit (EN) shall serve as the new limit for the high-efficiency technology, because it represents the value at which the total absolute emissions of the more efficient system are equal to those of the less efficient system.
The formula is as follows:
As an example, assume that a forge furnace operates at 2,280 degrees F with cold air burners.
The emissions concentration of the cold air burners (EB) = 0.06 lb/MMBtu (50 ppm).
The efficiency of the cold air burners (ηref) = 37 percent.
The company is planning to replace the cold air burners with regenerative burners.
The efficiency of the regenerative burners (ηeff) = 78 percent.
For this example, the corrected specific emissions limit (EN) for NOx computes to:
Although the regenerative burner system emits 0.08 lb/MMBtu (70 ppm) @ 3 percent O2, it stays well below the corrected limit (EN) of approximately 0.13 lb/MMBtu (105 ppm) @ 3 percent O2, thus saving 33 percent NOx emissions per year compared to the cold air burners.
There are a variety of burner types available with varying levels of complexity and efficiency.
Most high-efficiency gas burners preheat the combustion air to increase the combustion efficiency. Traditionally, there is a tradeoff between efficiency and NOx emissions; however, FLOX combustion makes it possible to have the best of both worlds.
Finally, if combustion efficiency is not considered when establishing a limitation for NOx emissions, this can lead to the selection of equipment which actually produces higher absolute emissions.
However, a very simple correction factor can be applied to adjust NOx emissions limitations to properly reflect combustion efficiency.
About the authors
Steven R. Mickey is a graduate of Cornell University with a Bachelor of Science in Mechanical & Aerospace Engineering. He has been with WS Thermal Process Technology Inc. in Lorain, Ohio, for more than 17 years.
Martin G. Schönfelder is a co-owner of WS Thermal Process Technology Inc. He is based at the company’s headquarters in Renningen, Germany.
Joachim G. Wünning is a co-owner of WS Thermal Process Technology Inc. He is based at the company’s headquarters in Renningen, Germany.
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The Federal Energy Management Program (FEMP) provides acquisition guidance for large commercial boilers, a product category covered by FEMP-designated efficiency requirements.
FEMP's acquisition guidance and efficiency requirements apply to gas- or oil-fired, low-pressure hot water or steam boilers used in commercial space heating applications with a rated capacity above 2,500,000 and at or below 10,000,000 British thermal units per hour (Btu/h). High-pressure boilers (i.e., those used in industrial and cogeneration applications) are excluded, while residential boilers (i.e., those with a capacity less than 300,000 Btu/h) and small commercial boilers (above 300,000 Btu/h and at or below 2,500,000 Btu/h) are covered by the ENERGY STAR program.
This acquisition guidance was updated in June .
Federal purchases of commercial boilers must meet or exceed the minimum efficiency requirements and thermal efficiencies listed in Table 1. These efficiency levels can be voluntarily adopted by non-federal organizations, institutions, and purchasers.
Table 1. Efficiency Requirements for Large Commercial BoilersProduct ClassRated CapacityFuelHeating MediumEfficiency* (%)Large Gas-Fired Hot Water>2,500,000 Btu/h and ≤10,000,000 Btu/hGasHot WaterEc ≥ 96.0Large Gas-Fired Steam>2,500,000 Btu/h and ≤10,000,000 Btu/hGasSteamEt ≥ 83.7Large Oil-Fired Hot Water>2,500,000 Btu/h and ≤10,000,000 Btu/hOilHot WaterEc ≥ 89.0Large Oil-Fired Steam>2,500,000 Btu/h and ≤10,000,000 Btu/hOilSteamEt ≥ 85.8*Both thermal efficiency (Et) and combustion efficiency (Ec) are based on 10 CFR Part 431.86 - Uniform test method for the measurement of energy efficiency of commercial packaged boilers.With the Clean Energy Rule finalized in Spring , federal agencies must significantly reduce the use of on-site fossil fuels in new and majorly renovated facilities used for federal purposes. From FY to FY , federal entities must reduce the use of fossil fuels by 90% in facilities that are newly constructed or undergoing major renovation (exceeding a total cost threshold of $3.8 million in dollars for federally owned buildings). To comply, contracting officers should avoid purchases of commercial fossil fuel-fired boilers. Federal buyers are encouraged to consult the Clean Energy Rule webpage for further guidance. If no technically practicable alternative to a fossil-fueled product can be found that meets the mission requirements of the agency, an agency may obtain guidance, or request technical assistance, or petition for downward adjustment of the fossil fuel reduction target from FEMP by contacting the Clean Energy Rule team.
FEMP has calculated that a 3,000,000 Btu/h gas-fired hot water commercial boiler meeting the required combustion efficiency level of 96.0% Ec saves money if priced no more than $59,703 above the base model. The best available model saves the average user more: $66,839 in lifetime energy costs. Table 2 compares three types of product purchases and calculates the lifetime cost savings of purchasing efficient models. Federal purchasers can assume products that meet FEMP-designated efficiency requirements are life cycle cost-effective.
Table 2. Lifetime Savings for Efficient 3,000,000 Btu/h Gas-Fired, Hot Water BoilersPerformanceBest AvailableRequired ModelBase ModelCombustion Efficiency98.0%96.0%82.0%Annual Energy Use (therms/yr)35,,,000Annual Energy Cost ($/yr)$29,808$30,429$35,625Lifetime Energy Cost (25 year)$342,552$349,689$409,392Lifetime Energy Cost Savings$66,839$59,703======Annual Energy Use: 1,400 full-load hours per year, for 25 years.
Annual Energy Cost: Calculated based on an assumed natural gas price of 8.48¢ per therm, which is the average price at federal facilities in the United States (Site-Delivered Energy Use by End-Use Sector and Energy Type in Fiscal Year ).
Lifetime Energy Cost: Future electricity price trends and a 3% discount rate are from the Energy Price Indices and Discount Factors for Life-Cycle Cost Analysis – : Annual Supplement to NIST Handbook 135 and NBS Special Publication 709 (NISTIR 85--39).
Lifetime Energy Cost Savings: The difference between the lifetime energy cost of the less efficient model and the lifetime energy cost of the required model or best available model.
Calculated based on highest efficiency model identified in publicly provided manufacturer data as of June . Note that more efficient models may be introduced to the market after FEMP's acquisition guidance is posted.
Calculated based on FEMP-designated efficiency requirements. Federal agencies must purchase products that meet or exceed FEMP-designated efficiency levels.
Calculated based on the current federal minimum efficiency standard for this product type.
An efficient product is cost-effective when the lifetime energy savings (from avoided energy costs over the life of the product, discounted to present value) exceed the additional up-front cost (if any) compared to a less efficient option. FEMP considers up-front costs and lifetime energy savings when setting required efficiency levels. Federal purchasers can assume products that meet FEMP-designated efficiency requirements are life cycle cost-effective. In high-use applications or when energy rates are above the federal average, purchasers may save more if they specify products that exceed FEMP efficiency requirements (e.g., the best available model).
Federal laws and requirements mandate that agencies purchase ENERGY STAR-qualified products or FEMP-designated products in all product categories covered by these programs and in any acquisition actions that are not specifically exempted by law.
These mandatory requirements apply to all forms of procurement, including construction guide and project specifications; renovation, repair, energy service, and operation and maintenance (O&M) contracts; lease agreements; acquisitions made using purchase cards; and solicitations for offers.
Federal Acquisition Regulation (FAR) Part 23.206 requires agencies to insert the clause at FAR section 52.223-15 into contracts and solicitations that deliver, acquire, furnish, or specify energy-consuming products for use in federal government facilities.
To comply with FAR requirements, FEMP recommends that agencies incorporate efficiency requirements into technical specifications, the evaluation criteria of solicitations, and the evaluations of solicitation responses.
Agencies may claim an exception to the Clean Energy Rule if no alternative to a fossil-fuel powered product is found that meets the technical needs and mission requirements of the agency. If an agency wishes to obtain further guidance, request technical assistance or petition for downward adjustment on the fossil fuel reduction target from FEMP, they may do so by contacting the Clean Energy Rule team and following the petition process. Contracting officers should still aim to purchase products that meet the ENERGY STAR or FEMP-designated requirements and minimize emissions as much as possible.
Products meeting FEMP-designated efficiency requirements may not be life cycle cost-effective in certain low-use applications or in locations with very low rates for natural gas or fuel oil. However, for most applications, purchasers will find that energy-efficient products have the lowest life cycle cost.
Agencies may claim an exception to federal purchasing requirements through a written finding that no FEMP-designated or ENERGY STAR-qualified product is available to meet functional requirements, or that no such product is life cycle cost-effective for the specific application. Learn more about federal product purchasing requirements.
The federal supply sources for energy-efficient products are the General Services Administration (GSA) and the Defense Logistics Agency (DLA).
The U.S. Department of Agriculture (USDA) and U.S. Environmental Protection Agency (EPA) provide programs that help federal agencies buy products with positive environmental attributes.
Identification codes for product categories covered by sustainable acquisition requirements are provided by DLA and the United Nations Standard Products and Services Code (UNSPSC).
Under the Multiple Award Schedule program, GSA issues long-term governmentwide contracts that provide access to commercial products, services, and solutions at pre-negotiated pricing.
Federal buyers can use the GSA Multiple Award Schedules to find a vendor and pull up their latest price list. Alternatively, buyers can search for a specific product in GSA Advantage! or enter the product in GSA eBuy to get a quote from multiple vendors. Before purchasing a product through one of these channels or a preferred vendor, buyers should make sure the product meets the FEMP or ENERGY STAR efficiency requirements. For solicitations, buyers should include the relevant FAR clause and incorporate energy efficiency into the contract language and evaluation criteria to ensure compliance with the federal purchasing requirements.
DLA offers products through the Defense Supply Center Philadelphia and online through FedMall (formerly DOD EMALL).
Products sold through DLA are codified with a 13-digit National Stock Number (NSN) and, in some cases, a two-letter Environmental Attribute Code (ENAC). The ENAC identifies items that have positive environmental characteristics and meet standards set by an approved third party, such as FEMP and ENERGY STAR.
USDA's BioPreferred Program was created to increase the purchase and use of biobased products. Federal law, the FAR, and Presidential Executive Orders direct that all federal agencies and their contractors purchase biobased products in categories identified by USDA.
EPA offers several resources for choosing which products to buy. The Environmentally Preferable Purchasing Program helps federal government purchasers utilize private sector standards and ecolabels to identify and procure environmentally preferable products and services.
UNSPSC is a worldwide classification system for e-commerce. It contains more than 50,000 commodities, including many used in the federal sector, each with a unique eight-digit, four-level identification code. Manufacturers and vendors are beginning to adopt the UNSPSC classification convention and electronic procurement systems are beginning to include UNSPSC tracking in their software packages. UNSPSCs can help the federal acquisition community identify product categories covered by sustainable acquisition requirements, track purchases of products within those categories, and report on progress toward meeting sustainable acquisition goals.
GSA offers commercial boilers through Multiple Award Schedule Industrial Products HVAC and C schedules.
DLA's ENAC for commercial boilers is "HF."
The UNSPSCs for commercial boilers are , , , , and .
A boiler system should be capable of meeting the building's peak heating demand and also operate efficiently at part-load conditions. Selecting the right system and properly sizing a boiler requires knowledge of both the peak demand and load profile. If building loads are highly variable, as is common in commercial buildings, designers should consider installing multiple small (modular) boilers in addition to boilers that have modulating burners. In periods of low demand, some of the boilers can be isolated from the other boilers and not incur any standby losses or cycling losses. They can also be automatically staged such that each boiler is running at its most efficient operating point without incurring additional cycling.
For guidance on boiler rightsizing and quality installation, consult the American National Standards Institute/Air Conditioning Contractors of America Standard 5: HVAC Quality Installations Specification (ANSI/ACCA 5 QI ).
Federal procurement officers and buyers should consider specifying boilers with the following features:
Many new energy consuming commercial boilers come equipped with Internet of Things (IoT) sensing components, and network connectivity. Making a new purchase or replacement represents a prime opportunity to evaluate the vulnerabilities of your network. All IoT-enabled devices introduce novel exposures to potential data breaches. Building controls and heating, ventilating, and air conditioning systems are no exception. Security can almost never be networked in after the fact, and so it is important to ensure that your networked devices are secure. Also, regularly testing for network vulnerabilities is key. For more information on how to build cybersecure networks of building technologies, consult FEMP’s Energy and Cybersecurity Integration resources and Cyber-Securing Facility Related Control Systems fact sheet.
Several diagnostic and maintenance procedures are important to maintain efficient boiler operation. Flue gas temperature monitoring is useful in detecting efficiency and operating problems. Maintaining steady excess air levels (with an oxygen trim sensor) ensures that burners will mix air and fuel properly. Low water levels can damage boilers, so water levels should be checked frequently as part of a regular maintenance program. Water treatment can prolong boiler life as well as increase efficiency. Waterside and fireside surfaces should be cleaned annually.
The Boiler Efficiency Institute provides maintenance and operation manuals for boilers and boiler control systems. To encourage quality operations and maintenance, building engineers can also refer to ASHRAE/ACCA Standard 180: Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems. In addition, the FEMP O&M Best Practices Guide, Release 3.0, Chapter 9 provides valuable information on operation and maintenance of boiler systems.
Lawrence Berkeley National Laboratory provided supporting analysis for this acquisition guidance.
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