Many companies feel pressured to spend the least amount possible to meet environmental regulations, primarily because compliance is not a profit-generating endeavor. What some environmental and compliance managers do not realize is that emission control equipment can quickly become a profit-decreasing endeavor with this “penny wise, pound foolish” approach. With mandatory greenhouse gas (GHG) reporting on the horizon, manufacturers could soon be paying for the carbon emissions generated by some of these pollution control systems, adding to the capital and operating costs associated with regulatory compliance.
Thermal and catalytic oxidizers are used in many industries for the destruction of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). These manufacturing-related emissions are destroyed through the process of high temperature combustion, or oxidation, and can be explained using the following equation:
CnH2m + (n + m/2) O2 ⇒ n CO2 + m H2O + Heat
During this process, heat breaks apart the contaminated compounds, which then reform naturally, bonding into carbon dioxide and water. Fossil fuels are required to bring the oxidizer up to temperature and, in some cases, maintain destruction.
Several oxidizer options should be considered when buying a new system to prevent GHG emissions. For plants with an existing oxidizer not yet ready for replacement, there are add-on features and operating practices that can help plants reduce their carbon footprint.
Some experts argue that while oxidizer systems prevent hazardous chemicals from being released into the atmosphere, they also emit significant amounts of carbon dioxide (CO2) and nitrous oxides (NOX). Contrary to popular belief, CO2 and NOX are not automatically and necessarily byproducts of air pollution control devices, especially the newer technologies. However, there are certain features that an oxidizer should have in order to achieve self-sustaining, fuel-free destruction.
For instance, the regenerative thermal oxidizer (RTO) emits a fraction of the GHGs that its predecessors do. It is generally considered to be the most energy-efficient control technology when compared to the recuperative, catalytic and direct-fired systems because of its ability to capture heat from the purified exhaust air and preheat incoming airflow. Under standard process conditions, the regenerative thermal oxidizer is the only self-sustaining, fuel-free control technology (figure 1). If process conditions allow, upgrading from an older oxidizer technology to a regenerative thermal oxidizer should be considered before implementing any other modifications suggested.
Make Process Modifications and Efficiency UpgradesQuantifying GHG emissions specifically from an oxidizer in the stack is not an easy task. Often times, CO2 is generated by the process dryers, ovens or burners and not the oxidizer. Making modifications or efficiency improvements to your process equipment such as recirculation or heat recovery can be an excellent way to reduce operating costs as well as your carbon footprint.
The other source in the stack that is often overlooked is the actual pollution generating solvents and carcinogens. It is a function of oxygen and process temperature -- with processes operating in the 500 to 1,600°F (260 to 871°C) range -- that causes the molecules to break apart, oxidizing the carbon in organic solvents (VOCs) to CO2. Figure 1 also demonstrates just how significant the carbon emissions can be from the solvent itself.
Among the process modifications and efficiency upgrades that can be implemented, consider whether some of the following can benefit your process.
Turn Off the Oxidizer Burner. When properly designed and applied, supplemental fuel injection (SFI) is an excellent means of reducing system operating cost and providing a cleaner “burn.” Natural gas is injected directly into the emission-laden airstream, typically at or near the inlet of the oxidizer, through a quill in the ductwork transition. As a general rule of thumb, NOX is created when temperatures reach 1,500°F (815°C) or higher, but the combustion burner is not in operation when SFI is used, thereby eliminating the NOX production and reducing combustion air.
Concentrate High-Volume, Low-VOC Airstreams Prior to the Oxidizer. If a significant portion of the air entering your oxidizer is at or near ambient temperature with low levels of VOC loading, a concentrator may be applicable for reducing the heat input required by your oxidizer system. A concentrator can take exhaust streams at or near ambient temperatures and concentrate them so that the volume of airflow actually sent to the oxidizer is reduced by a factor of eight to 15. This greatly reduced stream is typically rich in VOCs and much less of an operating cost burden on the oxidizer system. In fact, it generally allows for self-sustaining, fuel-free destruction.
Improve Primary Heat Recovery. Typically, oxidizers are designed with some form of internal or primary heat recovery. The hot purified gases leaving the combustion chamber usually are used to preheat the incoming, solvent-laden airstream. Projects that improve the primary heat recovery often offer the quickest payback because they provide additional heat recovery whenever the oxidizer is in service.
For recuperative thermal and catalytic units, this typically consists of adding additional passes to the internal air-to-air heat exchanger.
For regenerative thermal oxidizers and regenerative catalytic oxidizers (RCOs), this would be handled by increasing or changing the type of ceramic heat recovery media or altering the control scheme that dictates how often beds are switched from inlet to outlet. For example, if an average sized (25,000 scfm) regenerative thermal oxidizer, originally designed for 95 percent thermal energy recovery (TER), slips to 93 percent TER for a full year, this could cost as much as $65,000 in additional operating costs. New media types can offer 97 percent or higher TER with lower pressure drop for reduced electrical consumption.
Properly Maintain Existing SystemsNo matter how well an overall system is designed, it cannot continue to operate at a high efficiency level without proper maintenance. A handful of small inefficiencies in system operation can lead to large operating costs over the course of a year. For instance, making sure burners are tuned properly and not firing on excess combustion air can drastically reduce fuel consumption and GHG emissions. At today’s energy prices, regular calibration of feedback instruments and control loops can pay for themselves many times over. All too often, the decision-makers at production facilities take the “no news is good news” approach to the air pollution control equipment when the personnel really should be chasing the benefits of “company stays green -- saves green” instead.
In the oxidizer industry even just a dozen years ago, oxidizer manufacturers asserted that the equipment released “harmless” CO2 and water vapor. Things are much different today than they were even five years ago, as individuals and businesses alike are trying to reduce their environmental impact. This article only focuses on the primary means of GHG, fuel and operating cost reductions to maximize return on investment. Plant personnel should consult with emission control professionals as each application is unique.
Achieving the upcoming standards for GHG emissions won’t come without challenges, but putting “energy” into your emission control device will help your facility meet these goals while reducing operating costs.