Put energy into emission control projects

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Courtesy of Anguil Environmental Systems, Inc.

Many companies feel pressured to spend the least amount possible to meet environmental regulations, primarily because compliance isn’t a profit generating endeavor. What some environmental and compliance managers fail to 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, it’s about to get a whole lot worse for those who went down this road. 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 a wide variety of industries for the destruction of Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs). These process emissions are destroyed through the process of high temperature combustion – CnH2m + (n + m/2) O2 Þ n CO2 + mH2O + heat – which requires some type of fossil fuel to bring the oxidizer up to temperature. This article focuses on the oxidizer options that someone should consider when buying a new system to prevent GHG emissions. It will also explore some of the add-on features and practices that plants should implement on an existing oxidizer to 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 necessarily a by-product of these 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. The energy efficient options that “weren’t in the budget” when the system was purchased could now hit the bottom line.

Before we begin examining some of the options, it is important to note that the Regenerative Thermal Oxidizer (RTO) emits a fraction of the GHGs than 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 pre-heat incoming airflow. If process conditions allow, upgrading from an older oxidizer technology to the RTO should be considered before implementing any of the modifications referenced below. As you can see in Chart A, under standard process conditions the RTO is the only self-sustaining, fuel-free control technology.

Make Process Modifications and Efficiency Upgrades
Quantifying GHG emissions specifically from an oxidizer in the stack is not an easy task, often times CO2 is generated by the process driers, ovens or burners and not the oxidizer. Making modifications or efficiency improvements to your process equipment such as recirculation or heat recovery is an excellent way to not only reduce operating costs but also your carbon footprint.

The other source in the stack that is often over looked is the actual pollution generating solvents and carcinogens. It’s a function of oxygen and temperature (500-1600°F) that causes the molecules to break apart, oxidizing the carbon in organic solvents (VOCs) to CO2. Chart A demonstrates just how significant the carbon emissions can be from the solvent itself. Limiting the amount of solvents used or changing to a more environmentally safe chemical can reduce the GHGs that ultimately exit the stack to atmosphere. However, doing this also takes fuel from the oxidizer and may negatively impact your system performance so an expert should be consulted before changing solvents.

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 1500°F or higher but the combustion burner is not in operation when SFI is used thereby eliminating the NOX production and reducing combustion air. SFI also gives the oxidizer a more uniform temperature profile that improves energy recovery and overall efficiency.

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.

As a result of recent regulations, many facilities around the United States have been forced to improve localized VOC capture as well as prove high destruction efficiency in their system. In many cases this has lead to the installation of additional capture hoods or enclosures and increased the amount of air to be treated by a particular oxidizer system. A concentrator can take exhaust streams at or near ambient temperatures and concentrate it so that the airflow actually sent to the oxidizer is reduced by a factor of 8 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
Oxidizers are typically designed with some form of internal heat recovery. Usually the hot purified gases leaving the combustion chamber are used to pre-heat the incoming solvent laden airstream. This is referred to as the ‘primary heat recovery’ of an oxidizer system. Projects that improve the primary heat recovery often offer the quickest payback because they provide additional heat recovery at all times 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 RTOs and RCOs (Regenerative Catalytic Oxidizers) 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 RTO (25,000 SCFM) originally designed for 95% TER (Thermal Energy Recovery) slips to 93% TER for a full year this could cost upwards of $65,000 in additional operating costs. New media types can offer 97% or higher TER with lower pressure drop for reduced electrical consumption.

Consider Secondary Heat Recovery
If improving primary heat recovery is not cost effective or operating conditions do not allow it then secondary heat recovery may be the best option for reclaiming heat from an oxidizer system. When added to the stack, heat exchangers capture the excess heat so that it can be reused to generate hot air, water, steam or even electricity. There are a wide variety of low back-pressure designs that can be added to an oxidizer’s stack without requiring a replacement of the system fan.

Payback for these projects is greatly improved if the captured heat can be used back in the exhaust generating process itself, because again – it is assumed that the process is operating at all times the oxidizer is running. For example, fresh air is passed through a secondary heat exchanger in an oxidizer exhaust stack and supplied back as heated supply air for the oven zones that the oxidizer is treating. Every time the oxidizer is on the oven zones require heat, so this heat recovery project pays back all year long. If the same fresh air was supplied back to the plant as tempered makeup air, this may only provide payback during the heating season.

Following this logic, comfort heat applications may have been ignored in the past. But considering today’s unstable and rising fuel costs, coupled with the energy recovery grants available to facilities, these projects deserve attention; as do emerging technologies like heat-to-power. Sometimes referred to as cogeneration, heat-to-power is a technology capable of sending electrical power directly back into a facility. The concept has been implemented on different applications throughout the world but is only now being integrated with combustion devices such as oxidizers. As electricity costs increase and greater efficiencies are achieved with the technology it will be a very attractive option in the near future. Today, heat-to-power is not necessarily a cost reduction strategy but rather a green initiative that could be used to promote a company as a leader in energy conservation.

Properly Maintain Existing Systems
Finally, no 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 production facilities take the “No News is Good News” approach to their air pollution control equipment when they really should be chasing the benefits of “Company Stays Green and Saves Green” headline instead.

When I first started in the oxidizer industry over 13 years ago, we used to claim that our oxidizers released “harmless” CO2 and water vapor. Back then I probably would have laughed uncontrollably if someone had told me that Al Gore, former Vice President of the United States and once Presidential hopeful, would star in a 2006 movie that brought climate change to the forefront by drawing millions of viewers from all over the world. Indeed, 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 covers the primary means of GHG, fuel and operating cost reduction; to maximize return on investment plants should consult with a professional 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.

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