Companies have been installing air-pollution control systems — or APCS, as they are sometimes called — to collect and destroy hazardous air pollutants for years. Until recently though, realizing a financial return-on-investment when installing a new system was never a consideration. For years, most companies thought of air-pollution control systems as necessary money pits: They allowed manufacturing operations to continue, but — in the mind of a buyer — there certainly was no justifiable return on investment. Fortunately, times have changed.
It all began with the 1970 passage of the U.S. Environmental Protection Agency’s Clean Air Act (CAA) — and subsequent amendments in both 1977 and 1990. Under the law, companies were required to control emissions from their process exhaust stacks to be in compliance. The CAA amendments called out 189 air pollutants of special concern, many of which are used in everyday production facilities and manufacturing atmospheres. Regulations state that at various allowable levels, stationary sources generating volatile organic compounds (VOCs) or hazardous air pollutants (HAPs) must control their facilities because these process emissions are precursors to the formation of ground-level ozone (or smog).
As these regulations went into effect, affected companies began installing various types of air-pollution control systems that were designed and produced by many different original equipment manufacturers (OEMs). The control devices included designs such as afterburners, scrubbers, bio-filtration, thermal oxidizers, catalytic oxidizers and solvent-recovery systems. Often, this capital expenditure was unplanned, so many companies made purchasing decisions based on the smallest capital investment without considering the long-term costs associated with life expectancy, operations and maintenance.
Fast forward to today: Many air-pollution control system projects being installed are replacing systems that were installed 10, 15, 20 or more years ago. Throughout the 1980s and 1990s, OEMs sold air-pollution control systems designed for a life expectancy of 10 to 15 years and yet much of this equipment is still in operation today. In most cases, this older equipment is ineffective, inefficient, has high annual maintenance costs and is wasting a tremendous amount of energy (typically natural gas and electricity).
Air-pollution control system technologies have evolved. Modern air-pollution control systems are simpler to install, easier to operate, require less maintenance (more reliable) and — when properly designed to meet a facility’s specific needs — energy efficient. Two energy-efficient air-pollution control system technologies offered are regenerative thermal oxidizers and rotary concentrators.
Regenerative Thermal Oxidizers
Designed to operate with setpoint temperatures of 1400 to 1600°F (760 to 871°C), a regenerative thermal oxidizer — also called an RTO — utilizes ceramic media packed in canisters as a high-efficiency heat exchanger. During operation, the process exhaust is routed through the ceramic media, mixed with oxygen and held at elevated temperatures in the combustion chamber while VOCs/HAPs are destroyed. To maximize heat exchange, switching valves alternate the airflow path between the canisters to continuously regenerate the heat stored within the ceramic media.
To reduce fuel consumption in regenerative thermal oxidizers, the oxidizers can be designed to provide up to 97 percent thermal energy recovery (TER). To reduce electrical use, low pressure-drop media is used to create the heat exchange, and NEMA premium-efficiency motors are used on all system fans and blowers (figure 1).
Rotary Concentrator Systems
A rotary concentrator system is a hybrid design that utilizes a rotating wheel impregnated with a hydrophobic zeolite in association with a thermal oxidizer that is sized one-tenth or one-fifteenth the size it would be without the rotary concentrator. In operation, VOC/HAP from the process airstream is adsorbed (concentrated) continuously onto the wheel as the air passes through the wheel. The VOC/HAP is thermally desorbed and then transferred to the oxidizer for destruction.
Often, the thermal oxidizer used in this hybrid system can provide the heated desorption air to the wheel so energy use is reduced further. A rotary concentrator is typically used when controlling large air volumes with low VOC/HAP concentrations (figure 2).
Secondary Energy Recovery
For a greater level of overall efficiency and maximum operational savings, secondary energy recovery also can be incorporated into new air-pollution control systems or retrofitted into existing systems. These secondary recovery systems employ heat exchangers or coils to capture the 250 to 1800°F (121 to 871°C) of heat energy, depending on the design and operating parameters of the air-pollution control systems in use. This energy, which normally would be vented from the air-pollution control system’s exhaust stack directly to atmosphere, can be retained and used elsewhere in the plant. The secondary energy-recovery system can be designed for minimal pressure drop so it will not affect the oxidizer operation, and electrical consumption is minimized.
Depending on site-specific variables and priorities, a custom-designed energy-recovery system can incorporate different equipment. For instance, if an air-to-air heat exchanger is used, filtered fresh air can be used to supplement building heat or provide process makeup air (ovens, dryers, kilns, curing zones, etc.). In some cases, the recovered energy could completely replace the need for natural-gas-fired burners within the manufacturing process itself. Alternatively, if air-to-liquid heat transfer coils are incorporated, hot water, low-pressure steam, a glycol solution or thermal oil can be used for facility heating or as a manufacturing process heat source.
Equipment Advancements and ROI Favor Replacement
A company may consider replacing an older air-pollution control system for many reasons. First and foremost, it is essential for a company’s air-pollution control system to meet regulatory compliance, and there are several reasons why an older system might no longer be in compliance. Over time, stainless steel heat exchangers deteriorate and cause internal leakage, or catalysts can become masked or poisoned by chemistry, so they can no longer convert VOC/HAP effectively. In either case, if such deterioration has occurred, it is likely that the air-pollution control system does not function within the company’s operating permit requirements.
Likewise, if production modifications and company growth over the years have required modifications to the facility’s ductwork collection system, and proper consideration was not given to the impact on the air-pollution control system, the air-pollution controls could now be severely undersized, reducing both energy and control efficiency.
In addition, due to the perceived cost of equipment replacement, and with the recent undesirable economic conditions, many companies have asked their maintenance departments to keep these older systems operational as long as possible. This often results in holding this type of equipment together with low-cost, short-term fixes that only last so long. In many cases, with proper research and a thorough design review, a new air-pollution control system could be installed with an annual operational savings realized instead of continual repair and maintenance costs.
In recent years, many companies have changed their older air-pollution control systems and have realized a return on investment. Depending upon the equipment size and operating circumstances, return on investments of less than 18 months have been achieved. Modern air-pollution control systems are designed to exceed all government regulations and to offer energy-efficient, adaptable secondary energy-recovery options. Often, a return on investment can be realized based solely on fuel savings. By making the change to a modern air-pollution control system, a company also can lower its electric bills, know that it will meet government compliance for many years to come, reduce equipment maintenance costs and, most often, add air-pollution control system capacity (volume) for future production growth. A qualified OEM is capable of designing a replacement system that potentially can be installed and fully operational in a long weekend, requiring only a few days of production downtime.