issue: June 2007 APPLIANCE Magazine
The Cornerstones of Boiler Technology
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by Steve Connor, Cleaver-Brooks
The following paper demonstrates how boiler system manufacturers are reevaluating their product offerings and redesigning them to save energy and the environment, while positively influencing reliability and safe operation.
Figure 1. A burner must be able to ignite and generate a stable flame that fits the furnace geometry.
Those in the appliance industry, and any other industry for that matter, are faced with ever-rising and erratic supply issues with fuel. Adding to that challenge is the fact that 80% of U.S. boilers are nearly 30 years old. Aging boilers can be plagued with inefficiencies that waste fuel at alarming rates. Attention to the efficiency of a boiler system can mean thousands, if not millions, of dollars in savings every year, and it can’t be ignored.
Beyond energy conservation is the environmental impact of burning carbon-based fuels. Any time oil and gas are burned, they give off CO2, a greenhouse gas that contributes to global warming. By having a more efficient boiler system, users are not only saving money and energy resources, but are emitting less of these harmful gases into the environment.
During the combustion process, boiler systems also create forms of nitrogen oxides, or NOx, which, in the presence of sunlight, turn to ozone or smog. The Cleaver-Brooks burner technology discussed in this article—flue gas recirculation (FGR)—reduces this effluent dramatically, lowering the ozone, or smog, emitted into the environment.
Besides being eco-friendly, there are many regulations on these emissions that boiler operators must be aware of and comply with, most notably the Clean Air Act Amendments and soon the Industrial Boiler maximum achievable control technology (MACT) regulations. Each U.S. state and local government can also enact its own, stricter versions of these regulations as they feel necessary.
This technical paper is written to educate readers on boiler technology and the steps that boiler system manufacturers are taking to reevaluate their product offerings, redesign them to meet the demands of the retrofit aftermarket, and to design completely new packages to deliver the desired results for new installations, replacements, and expansions. It delves into the key issues and what can be done from an overall design standpoint to save energy and the environment while positively influencing reliability and safe operation.
Engineers, boiler owners, and operators have long observed that the combustion chamber or furnace geometry and construction greatly affect burner performance. It has been proven many times that the same burner installed in different boilers will not produce the same results. The concept of a universal burner that fits all is a technical utopia that has long plagued the boiler industry.
Facing the challenge of the 21st century to generate steam and hot water efficiently, reliably, and safely with ultralow emissions requires using engineering practices that incorporate a more advanced technical toolbox than employed in the past. Burner design can no longer be performed independently from the furnace in which it is to be installed. To the contrary; it must integrate the furnace effects and provide a means of adapting combustion aerodynamics to match the furnace. This trend will lead to a total integration of the boiler, burner, and controls, resulting in high efficiency and low-contaminantemitting steam- and hot-water-generating systems. It is very likely that the boiler industry will be following a technological path similar to that of the gas turbine industry.
In gas turbine engines, the constraints of volumetric heat generation are significantly higher than in current boilers. Yet, the high level of temperature uniformity obtained by fast and efficient mixing allows for extremely low NOx emissions, even without FGR. The achievements of the aircraft and gas turbine industries are largely due to the use of advanced test facilities and computer simulation involving computational fluid dynamics (CFD), which essentially correlates fluid mass flows and velocities with temperature profiles and gradients, accurately predicting and modeling thermodynamic outcomes.
It is not the intent of this paper to elaborate on computer simulation, but rather to try to demystify the fundamental physics of burner and combustion chamber interaction. In addition, this paper will explain a technically innovative strategy on how to achieve sub-9-ppm NOx emissions when burning natural gas with less than 25 ppm CO in various furnaces, including smaller or more-confined ones. The paper will also address the continual need for reliability in the total boiler package and the need to keep safety at the forefront of any development work.
Efficiency Begins with the Burner
Whether it is a premix or a nozzle-mix or diffusion-type flame, the very first function of a burner is to ensure that the fuel is evenly mixed with air so that it burns completely within a confined combustion chamber volume. The less the excess air required for complete combustion in a given volume, the more efficient the burner. The efficiency of a burner can in fact be defined as an inverse function of the amount of excess air required to achieve complete combustion in a given flame volume.
For a nozzle-mix burner, the mixing power required to obtain the desired mixing rate is the sum of the combustion airflow pressure drop and the fuel flow pressure drop. On the air side, the burner dissipates some fan horsepower to affect mixing. On the fuel side, gas pressure multiplied by volumetric flow through the gas injectors provides another source of mixing power. These air and fuel pressure drops at a given volumetric flow rate provide the potential mixing power for a burner to accomplish its task of mixing the two streams.
The volumetric heat generation of the flame of any burner is a function of this quantity: The larger it is, the smaller the resulting flame. An example of the potential mixing power can be explained by referring to a 350-MMBtu/hr (8365-hp) natural gas burner running at full load with 12 psig of gas pressure and 8 in. of air pressure drop.
Although the gas flow is a considerable order of magnitude less than the airflow when comparing volumes in cubic feet per minute, the natural gas mixing power still represents more than 75% of the total mixing power. Additionally, the air mixing power requires spending fan electrical horsepower while the gas mixing power source is essentially free. Designing for low gas pressure at the burner is a waste of that potential mixing source and resultant burning efficiency.
Stabilizing and Shaping the Flame to Match the Furnace
As a second function, a burner must be able to ignite and generate a stable flame that, most importantly, fits the furnace geometry (see Figure 1). Flame stabilization is generally performed without much difficulty by a bluff effect and/or a critical swirl effect occurring at the mixing point of fuel and air. Flame shape can be described by its length-to-maximum diameter ratio and is determined by the aerodynamics of the air and fuel injection pattern. The swirl, defined as the ratio of tangential momentum to axial momentum fluxes, added to the other injected fluid streams, is the key controlling parameter for flame shaping.
To achieve good practical results, it is imperative that the burnergenerated flame be compatible with furnace geometrical constraints. This is most critical in water-cooled furnaces such as boilers and especially in packaged boilers. If flame impingement occurs on cold surfaces, carbon monoxide and other by-products of incomplete combustion will be produced at the point of impingement and, in large proportions, produce severe furnace vibrations upon violent reignition of these by-products.
Besides this condition being an energy waste and a pollution contributor, safety is also compromised by this lack of proper matching. In other words, the situation should be avoided at all cost.
Figure 2. Engineers are turning to a PLC-based platform for burner/boiler management and control.
The Dynamics of NOx Generation in Boiler Combustion
Considering that NOx is massively formed in areas where flame temperatures exceed 2800°F (1538°C) and that boiler furnaces are designed to operate at average outlet temperatures ranging from 1800° to 2400°F (982° to 1316°C), NOx formation is simply the result of poor temperature uniformity within the furnace and, more particularly, within the flame zone. The primary reason for this is that many boiler manufacturers are operating separately from the burner manufacturer of choice. This separation, if not bridged by collaborative design engineering and testing between the two parties, will lead to inadequate design of the burner and its required compatibility with a given furnace’s aerodynamics. This condition then leads to inadequate mixing rates not only between the air and the fuel, but also with the products of combustion inside the furnace.
If, on the other hand, compatibility is assured, temperature peaks are reduced and cold spots are minimized, thus reducing both NOx and CO emissions. Furthermore, the improved mixing uniformity results in a significant reduction of the flame size and of the peak heat fluxes on the boilers’ heat-transfer surfaces. Consequently, ultra-low-emission performances can be achieved in much smaller furnaces with less thermal stress for better longevity and reliability of the boiler proper. This integral design philosophy has already proven successful in other combustion applications such as gas turbines and car engines with their multiple-valve designs.
Reducing NOx and Other Pollutants
Since the mixing rate and its specific distribution determine the temperature and chemical species concentration within a given furnace, the design and configuration of the burners in relation to the geometry and flow pattern in the combustion chamber have a significant influence on NOx, CO, and particulate formation.
The emission of pollutants is intimately linked with furnace confi guration and distribution of heat dissipation. It is obvious that reducing pollutants successfully in a given furnace requires that the burner configuration be sufficiently adaptable to properly match the furnace flow and temperature characteristics.
To achieve ultralow NOx emissions (<9 ppm), the burner mixing power must not only provide enough mixing between air and fuel to obtain complete combustion, it must also provide enough mixing power per unit furnace volume to evenly mix the inert combustion products within that radiant chamber.
A perfectly mixed combustion zone results in negligible NOx formation that could theoretically be close to zero even without the use of FGR; however, such perfect mixing would require a level of mixing power per unit furnace volume that would be cost prohibitive for many boiler applications. Additionally, even a somewhat imperfectly mixed furnace with temperatures within 1300° to 2900°F (704° to 1593°C) would still result in very low emissions for both CO and NOx.
Figure 3. The Model CB firetube boiler with Hawk ICS PLC controller, which enables instant communication with building automation systems and the boiler operator, improved uptime, and overall boiler operation cost reduction.
Finding the Optimum Solution for Emission Control
Balancing the benefits of high mixing technology and uniform temperatures with the realities of cost containment and practicality has led to the use of FGR as an additional inert mass released into the combustion zone, reducing the peak temperature of the flame and lowering NOx production. The major drawback of FGR is that it can also reduce the temperature of colder zones in the furnace, increasing CO emissions. Another factor of FGR use is its dilution effect on the fuel-oxidant, increasing flame size. At a certain FGR rate, this can cause flame impingement and induce furnace vibrations and rumbling.
To reach ultralow NOx levels (<9 ppm) with FGR alone, the amount of FGR typically used by low-mixing-power burners is in the 30–40% FGR range. Under these conditions, the bulk of the flame zone is approaching the lower flammability limit of methane, the major component of natural gas.
Under extremely lean conditions, combustion becomes radically unstable, especially with rapidly varying flame modulation rates working in conjunction with standard commercial control systems. In these situations, only extremely slow firing-rate changes can be achieved without adversely affecting reliability and safety.
Another important factor to consider with FGR is that it has led to many burner manufacturers requesting that the boiler manufacturers increase the size of their furnaces to fit the new ultra-low- NOx burner. This recent trend in boiler design is detrimental not only for the obvious elevated material cost and footprint, but also for the boilers’ convection bank pressure drop. Since furnace size is often increased, it compresses the boiler’s convection section. The result is a boiler with higher draft losses, increasing fan horsepower requirements.
Another possible downside to FGR is that due to the larger furnace volume, more burner mixing power will be required to obtain the same temperature uniformity, the driving factor being the mixing horsepower per unit of furnace volume. Higher FGR rates will affect the required fan horsepower, driving up electrical cost.
Lastly, FGR in the 30–40% range may result in considerably higher burner windbox temperatures, which can adversely affect mechanical devices located in the vicinity of the hot recirculation gases and—if superheaters are a part of the package—impact their output as well.
Integrated Design and Technologies Provide the Key
With advanced design tools such as CFD, an integrated boiler and burner manufacturer can build a combination of burner and combustion chamber that displays an unrivaled temperature homogeneity compared with what was accomplished in the past.
As explained above, by narrowing the temperature extremes within the furnace, very little FGR becomes necessary to reduce the peak temperature(s) to achieve less than 9 ppm NOx. Typically with the right burner design and matching, levels as low as 20–25% FGR have proven to be sufficient to reach the desired emission level on Cleaver- Brooks integral packages. This is because the lowest temperatures are reduced as a consequence of better homogeneity, with virtually no unburned fuel and very little of its precursor (CO). Carbon monoxide emissions between 0 and 10 ppm across the range were easily demonstrated with this design strategy.
Another point worth noting is that flame volume becomes much smaller when homogeneous mixing is achieved. Thus, the combustion chamber can be made much shorter than before, and because of this, the next generation of low-emission boilers will exhibit a much smaller footprint with fire-tube boiler efficiencies approaching or exceeding 90%.
Besides proper design and mating of the burner, another key to achieving high efficiency and low emissions is the overall design of the heat exchanger proper. This begins with maximizing the effective emissivity ratio in the furnace—attaining the highest achievable Reynolds numbers (ratio of flue-gas inertia forces to viscous forces) within the (fireside) convective section of the vessel. Concurrently, it is essential that the external circulation on the fluid side of the shell be designed to enhance proper hydraulics, optimizing pressure and density differentials to achieve the highest degree of flow and heat exchange possible.
Secondly, the total package (burner and boiler) must include the proper devices or system to automatically and safely control, monitor and communicate the operation of the boiler package, from prepurge to postpurge and shutdown.
Figure 4. Boiler operator remotely tracks boiler system operation and efficiency for Model CB firetube boilers.
Using PLC-Based Controls
In this day of advanced electronics, engineers are turning to a programmable logic controller (PLC)–based platform (see Figure 2) for burner and boiler management and control, since it provides the greatest flexibility and almost limitless opportunities for expansion, upgrading, and communications. The technology achieves all of this, while also playing a collateral role in emission reduction and a direct role in improving efficiency, reliability, and safety within the boiler operation.
Energy and Emission Reduction. Achieving proper burner and boiler design and integration, though extremely important in achieving an efficient, low-emitting boiler, is compromised if the optimum control scheme is omitted from the conceptualized package. This is because the advanced controls available today use electronics (as opposed to electromechanical sensing and actuation) and are infinitely more sensitive, responding to load changes far more rapidly and limiting over- or undershooting while maintaining set point. This saves energy and, as such, reduces emissions.
Using PLC platforms, the control scheme can also integrate variable- speed drives (VSD), oxygen trim systems, and actuators for controlling fuel and air (parallel positioning). That reduces electrical demand while adjusting for atmospheric changes that adversely affect fuel:air ratios, and eliminating linkages which, if not maintained and adjusted, can also lead to improper fuel:air ratios and wasted fuel.
Reliability. The essence of a PLC, and having it part of the overall boiler control scheme, is achieving the added benefit of optimum information gathering and communication flexibility. With ever-rising fuel costs, pressures to reduce noxious emissions, and the extremely high cost of unexpected outages, it becomes imperative for the boiler owner to have the ability to closely monitor and track the boiler’s operation, communicating conditions on a moment’s notice. PLC-based packages afford this capability, amassing volumes of data that can be trended on spreadsheets for developing predictive maintenance procedures or highlighted for important cost impact areas involving energy use and emission variances.
Further enhancing these awareness benefits, PLC-based technology allows the instant communication of this information via pager, phone, local terminal, or over the World Wide Web. This helps improve equipment uptime while improving the overall cost of operation for the entire plant or facility.
Safety. For more than 100 years, boiler manufacturers have recognized the importance of producing a safe unit, manufacturing their products under rigid ASME codes, and controlling them in accordance with third-party standards such as those of UL, cUL, NFPA, CSD-1, and the various insurance companies that assume property risk on behalf of the client. Though these standards have enhanced the safe operation of thousands of units throughout the years, accidents still occur despite the tireless efforts on the part of manufacturers to produce and provide the most safe and reliable boiler possible. This is where PLC-based controls can further assist in the effort.
Using the PLC-based platform, the overall boiler management system can integrate the best in-burner management control, assuring the user maximum safety from preignition interlock, checking through prepurge, trial for pilot, proof of main flame and postpurge following load satisfaction; all this while continually checking itself hundreds of times a minute and looking for any faults triggered by the boilers’ safety circuit.
Additionally, the PLC-based platform allows incorporation of other safety devices in addition to the burner management control. An example of this is the C-B Level Master, a low-water cutoff and pump control that uses magnetostrictive technology in conjunction with solid-state and microprocessor integration to sense and control level. With most boiler occurrences happening because of unexpected low-water conditions, the new technology provides continuous information on precise water level during standby and operation, alarming when it is too high or alarming when it is too low before turning the burner off. It will also remind the operator to blow the column down, advising if it was done properly, and logging results and any faults to an alarm history with date and time stamping.
Total Commitment Leads to Ultimate Success
There are many things that responsible and dedicated boiler manufacturers can do to assist users of steam and hot-water generators to achieve their desire for equipment that will conserve energy and the environment, operating reliably and safely for years to come. Several ways have been presented in this paper.
Another way, not discussed but which needs to be mentioned, is supplying the various users of steam- and hot-water-generating equipment with the training programs and the tools they need to safely and reliably operate the equipment with optimum longterm results. These programs should be dynamic and flexible in nature, fitting an array of needs, and should be readily accessible to meet expanding demand.
In the final analysis, all the solutions start with the manufacturer’s commitment and steadfast dedication to the business. Research and development dollars and other resources should be applied toward the creation of new products and programs, and the envelope should be stretched to accomplish the objectives with a constant eye on the customers and their ever-changing needs.
About the Author
Steve Connor is a 40-year boiler industry veteran. His experience in the steam-generating field spans a broad scope of disciplines, including engineering, service training, and field application sales. He is director of marketing and communications at Cleaver-Brooks. If you wish to contact Connor, e-mail email@example.com.