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Reducing emission treatment costs for gas processors

Regenerative thermal oxidising abatement technology is effective in amine tail gas treatment operations due to energy-efficient operation and reliability

Kevin Summ
Anguil Environmental Systems
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Article Summary
An April 2011 study by the US Energy Information Administration (EIA) estimates that 48 shale gas basins in 32 countries hold 5760 trillion cubic feet (tcf) of natural gas. As countries scramble to develop the infrastructure necessary to extract, process and distribute gas from these basins, there is a lot of concern over the environmental impact of these efforts. Much of the recent attention has been given to the implications of hydraulic fracturing (or “fracking”) used to release tightly bound natural gas reserves. Many milestones have been achieved in this regard. However, air pollution from drilling and processing of natural gas has gone somewhat unnoticed. In many countries, there are impending new air pollution regulations, which are heightening the industry’s need for viable control solutions.

Similar to other industries, oil and gas producers are often required by government regulatory agencies to prevent untreated air pollutants from entering the atmosphere. In the US, this is enforced by the Environmental Protection Agency (EPA) under the Clean Air Act amendment. In addition to their harmful effects on plants and trees, these pollutants, known as volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), cause respiratory ailments, heart conditions, birth defects, nervous system damage and cancer in humans.

For natural gas processors, meeting environmental regulations is not a profit-generating endeavour. Simply put, the time and money spent on protecting air, water and land does not produce more natural gas. Where environmental and compliance managers succeed is when they reduce the cost of air pollution control initiatives by saving money on capital equipment expenditures and operating costs. For instance, many of the pollution control devices used to abate emissions consume large amounts of fossil fuels to achieve VOC and HAP destruction. Operators can significantly reduce operating costs by utilising the most energy-
efficient technologies. Older, less efficient technologies will also emit considerably larger amounts of carbon dioxide (CO2) and nitrous oxides (NOx). With impending mandatory greenhouse gas (GHG) reporting, processors 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.

Emission abatement history
Certain production techniques and processes used by natural gas companies are, or soon will be, regulated as emission sources. From stationary combustion engines to amine systems, the industry is facing some fairly strict legislation. One area of great concern in the midstream market is amine tail gas treatment. Amine systems are a very common and critical component used by natural gas processing facilities to remove acid gases such as hydrogen sulphide (H2S) and CO2. This is accomplished by running the gas through a column with amine liquid flowing in the opposite direction, stripping acids from natural gas and absorbing them into the liquid. The natural gas is then sent for further processing, while the amine is sent to be regenerated. The amine regeneration process removes the acid gases from the amine solution, allowing it to be reused again. The byproduct of the amine regeneration process, known as tail gas, contains VOC and HAP emissions, which are then delivered to a control device.

Thermal and catalytic oxidisers are technologies commonly used on a wide variety of applications where VOC, HAP and odour abatement are required. They destroy harmful emissions through the process of high-temperature oxidation or combustion:

CnH2m + (n + m/2) O2 → n CO2 + mH2O + heat

Midstream companies have historically used flares, vapour combustors, direct-fired thermal oxidisers (TOs) or recuperative systems for emission destruction. Applications where these devices are applied range from amine tail gas treatment, nitrogen rejection units and liquefied natural gas (LNG) processes. The temperature in these systems is maintained somewhere between 1400°F and 1800°F so that hydrocarbons are converted to CO2 and water vapour, while the H2S is converted to sulphur oxides (for example, SO2 and SO3).

When designed properly, these older technologies are fairly dependable, but their effectiveness and efficiency can result in more emissions and higher operating costs. In the case of flares, water is often injected into the device to reduce visible black smoke. This drastically reduces destruction efficiency and the regulatory community is taking note.

While TOs and vapour combustors can achieve destruction efficiencies of around 99%, they share a common negative aspect with flares: high auxiliary fuel consumption. These technologies generally use large amounts of fossil fuels required for combustion to bring the emissions up to proper destruction temperature. Rather than use the heat generated from combustion to preheat incoming pollutants, the energy is simply released into the atmosphere, along with CO2 and NOX, which are classified as GHGs. Figure 1 demonstrates just how significant the carbon emissions can be from the various technologies.

Fuel-efficient abatement technology
Over the years, gas production companies have focused mainly on new process technologies to streamline production and improve profits. It was not until recently that the search for newer and more efficient technologies broke away from the process equipment, amine systems, dehydrators and other areas of the plant. Through energy optimisation, auxiliary fuel consumption can be reduced, keeping the businesses greener and more profitable.

All too often, production facilities take the “no news is good news” approach to air pollution control equipment when they really should be chasing the benefits of the “company stays green and saves green” approach. The proprietary, more fuel-efficient Regenerative Thermal Oxidizer (RTO) abatement technology is now being applied to tail gas treatment where it was once thought impossible (see Figure 2).

New to the midstream market, RTOs have been successfully applied over the past three decades on applications such as printing presses with solvent-based inks, composite applications for styrene control and numerous others. What differentiates it from other technologies is its ability to use the proper mix of temperature, residence time (or dwell time), turbulence and oxygen to convert pollutants into CO2 and water vapour. The technology reuses thermal energy from the exothermic oxidation reaction to reduce operating costs. In most cases, emission destruction can occur without any additional 
natural gas or other auxiliary fuel once the RTO is brought up to temperature.
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