Environmental Aspects of Gas Processing & Natural Gas Emissions

The processing and utilization of natural gas, despite its relative cleanliness compared to other fossil fuels, are associated with significant environmental aspects that demand careful consideration. These aspects span the entire production cycle: from primary gas processing to its final combustion. The primary environmental impacts include atmospheric emissions of pollutants such as methane and carbon dioxide, which contribute to the greenhouse effect and climate change.

Furthermore, technological processes at natural gas processing plants can lead to water and soil pollution due to the discharge of wastewater containing chemical reagents and hydrocarbons. Given the global role of natural gas in the energy mix, understanding and mitigating these impacts through appropriate protocols and environmental management systems become critically important tasks for the sustainable development of the industry.

Introduction

Currently, natural gas represents approximately 24 % of the energy consumed in the United States with increases in use projected for the next decade. These increases are expected because emissions of greenhouse gases are much lower with the consumption of natural gas relative to other fossil fuel consumption. For example, natural gas, when burned, emits lower quantities of greenhouse gases and criteria pollutants per unit of energy produced than other fossil fuels. This occurs in part because natural gas is fully combusted more easily and in part because natural gas contains fewer impurities than any other fossil fuel.

However, the major constituent of natural gas, methane, also contributes directly to the greenhouse effect through venting or leaking of natural gas into the atmosphere.

Natural gas is seen by many as an important fuel in initiatives to address environmental concerns. Although natural gas is the most benign of the fossil fuels in terms of air pollution, it is less so than nonfossil-based energy sources such as renewable sources or nuclear power. However, because of its lower costs, greater resources, and existing infrastructure, natural gas is projected to increase its share of energy consumption relative to all other fuels, fossil and nonfossil, under current laws and regulations. Indeed, natural gas consumption and emissions are projected to increase more rapidly than other fossil fuels, at average annual rates of 1,7 % through 2020. However, this represents reductions in totalcarbon emissions derived from the environmental advantages of natural gas.

Concern about global warming and further deterioration of the environment caused by escalating industrial expansion and other development is being addressed by worldwide initiatives that seek a decrease in emissions of greenhouse gases and other pollutants. Natural gas is expected to play a key role in strategies to lower carbon emissions because it allows fuel users to consume the same Btu level of energy while less carbon is emitted. If carbon-reduction measures are implemented, natural gas demand will increase. However, emissions from natural gas consumption would also rise, but the natural gas share of total emissions would increase only slightly.

This article describes efforts on the part of the Everything about Natural Gas in Modern Industrynatural gas industry to lessen the environmental impact of natural gas processing and discusses the environmental effects of the use of natural gas, including comparing the emissions from natural gas to other fossil fuels.

Environmental Impacts of Natural Gas Processing

The Natural Gas Processing and Liquids Recoveryprocessing of natural gas poses low environmental risk, primarily because natural gas has a simple and comparatively pure composition. However, two important sources of emissions in the gas processing plants are discussed.

Air Pollutant Emissions

The atmosphere is a mixture primarily of the gases nitrogen and oxygen, totaling 99 % with almost 1 % water and very small amounts of other gases and substances, some of which are chemically reactive. With the exception of oxygen, nitrogen, water, and the inert gases, all constituents of air may be a source of concern due either to their potential health effects on humans, animals, and plants or to their influence on the climate.

In the United States the Clean Air Act (CAA), which was last amended in 1990, mandates that the Environmental Protection Agency regulates criteria pollutants that are considered harmful to the environment and public health.

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There are 188 substances that are identified as air toxics or hazardous air pollutants, with lead being the only one that is currently classified as a criteria pollutant and thus regulated. Air toxic pollutants are more acute biological hazards than most particulate or criteria pollutants but are much smaller in volume. Procedures are now underway to regulate other air toxics under the CAA.

In the natural gas and refining industries, as in other industries, air emissions include point and nonpoint sources. Point sources are emissions that exit stacks and flares and, thus, can be monitored and treated.

Nonpoint sources are fugitive emissions that are difficult to locate and capture. Fugitive emissions occur throughout refineries and arise from, for example, the thousands of valves, Pipelines in Marine Terminals: Key Considerations for Handling Liquefied Gaspipe connections, seals in pumps and compressors, storage tanks, LNG Carrier Pressure Relief Systemspressure relief valves, and flanged joints.

While individual leaks are typically small, the sum of all fugitive leaks at a refinery can be one of its largest emission sources. These leaks can release methane and volatile organic compounds (VOCs) into the air. Companies can minimize “fugitive emissions” by designing facilities with the fewest possible components and connections and avoiding components known to cause significant fugitive emissions. When companies quantify fugitive emissions, this provides them with important information they can then use to design the most effective leak repair program for their company.

Directed inspection and maintenance programs are designed to identify the source of these leaks and to prioritize and plan their repair in a timely fashion. A reliable and effective directed inspection and maintenance plan for an individual facility will be composed of a number of components, including a methods of leak detection, a definition of what constitutes a leak, set schedules and targeted devices for leak surveys, and allowable repair time. A directed inspection and maintenance program begins with a baseline survey to identify and quantify leaks. Quantification of the leaks is critical because this information is used to determine which leaks are serious enough to justify their repair costs. Repairs are then made only to the leaking components that are cost effective to fix. Subsequent surveys are then scheduled and designed based on information collected from previous surveys, permitting operators to concentrate on the components that are more likely to leak. Some natural gas companies have demonstrated that directed inspection and maintenance programs can profitably eliminate as much as 95 % of gas losses from equipment leaks.

Most refinery process units and equipment are manifolded into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquid and gases that are either automatically vented from the process units through pressure relief valves or that are manually drawn from units. Recirculated process streams and cooling water streams are often purged manually to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shutdown before normal or emergency shutdowns. Blowdown systems utilize a series of flash drums and condensers to separate the blowdown into its vapor and liquid components. The liquid is typically composed of mixtures of water and hydrocarbons containing sulfides, ammonia, and other contaminants, which are sent to the wastewater treatment plant.

The gaseous component typically contains hydrocarbons, hydrogen sulfide, ammonia, mercaptans, solvents, and other constituents and is either discharged directly to the atmosphere or is combusted in a flare. The major air emissions from blowdown systems are hydrocarbons in the case of direct discharge to the atmosphere and sulfur oxides when flared.

Other potential hazardous air pollutant emission points are the tail gas streams from amine-treating processes and sulfur recovery units.

Emissions from the sulfur recovery unit typically contain some hydrogen sulfide (H₂S), sulfur oxides, and nitrogen oxides. To address the risk associated with sour gas exposure and risks associated with explosions, sour facilities are generally required to have lower rates of fugitive emissions than How and For What Liquefied Petroleum Gas Reliquefaction Plants Workgas plants that do not process sour gas (sweet gas facilities) and to have in place emergency response plans that address the hazard of hydrogen sulfide exposure.

Other emissions sources from refinery processes arise from the periodic regeneration of catalysts. These processes generate streams that may contain relatively high levels of carbon monoxide, particulates, and VOCs that are usually referred to as Gas Conditioning and Natural Gas Liquids Recovery Technologiesnatural gas liquids or natural gasoline. Before being discharged to the atmosphere, such off-gas streams may be treated first through a carbon monoxide boiler to burn carbon monoxide and any volatile organic compounds and then through an electrostatic precipitaor or cyclone separator to remove particulates.

Facilities that dispose of acid gas by deep well injection generally have far lower emissions of sulfur dioxide than facilities that recover sulfur or that flare acid gas. Acid gas injection provides the additional benefit of “sequestering” the carbon dioxide part of the acid gas stream. Acid gas disposal is a proven technology that a growing number of new and existing oil and Project Management of the Large-Scale Liquefied Natural Gas Facilitiesgas facilities are using. Companies must dispose of acid gas in an underground formation where there is no chance that the gas will escape and contaminate other formations. Workers must specifically design the acid gas injection well bore to handle highly corrosive wet acid gas.

Glycol dehydrators can also be significant sources of hydrocarbon emissions. Most Implementing Advanced Gas Processing Plant Controls for Optimizationgas processing plants use glycol dehydration to remove water from gas as it enters a pipeline to prevent hydrate formation and corrosion. In addition to extracting water from natural gas, glycol will extract some benzene, toluene, ethyl benzene, and xylene (collectively referred to as BTEX) molecules. When heat is used to regenerate the glycol, both water and BTEX molecules are driven off.

Operators commonly vent these emissions to the atmosphere. There are a number of ways operators can reduce or eliminate emissions from glycol dehydrators. On existing dehydration units, operators can reduce emissions by optimizing the dehydration unit, for example, reducing the glycol circulation rate to the minimum required to ensure adequate freeze protection, or optimizing the temperature of the unit. Operators can also use a separator to remove water from gas before it enters the dehydration unit; this also reduces the amount of glycol they need to use in the dehydrator and reduces the quantity of emissions. If plant operators collect vapors from the regeneration column, the glycol pump, and any gas-operated instrumentation and then flare or incinerator these vapors, they can achieve near-zero emissions from glycol dehydrators.

Other alter-natives, which can be an improvement over glycol, include:

1. line heater to heat the gas at the well site and raise the gas temperature above the freezing point
2. and molecular sieve dehydrator, which is typically a closed system that removes water by heating the crystals to above the boiling point of water.

This releases the water and regenerates the crystals so that they can be reused. This process almost eliminates vapor and BTEX emissions. Because of its closed system process, the molecular sieve is suitable for dehydration of sour gas where the release of H₂S could be lethal.

The burning of large quantities of natural gas to drive engines, motors, and heaters is also a key source of air emissions (primarily carbon dioxide and nitrogen oxides) from gas processing facilities. If facilities use electrically powered units, this will avoid the creation of local combustion emissions. However, if using electricity requires power generation facilities to be expanded or new sources of power to be developed, this may contribute to the creation of emissions and impacts wherever the power is generated.

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Because medium to large gas processing facilities contain many sources of air pollution, they can emit a large volume of pollutants into the atmosphere. For this reason, when regulatory authorities issue operating permits or approvals to companies running these facilities, they specify which processing units are allowed to emit pollutants to the air, and they sometimes set limits on how much of a pollutant the facility can release.

Companies should place air quality monitoring equipment at their property line or close to the plant so that they can monitor air contaminants of concern. Companies usually place this equipment in the area where they expect there to be the highest concentrations of an air pollutant at ground level (also called the “maximum point of impingement“). Regulators can use the monitoring devices to make sure the facility is operating in compliance with the approval it was given.

Gas Flaring Emissions

Flaring is used to consume waste gases (including hydrogen sulfiderich gases and gases burned during emergencies) in a safe and reliable manner through combustion in an open flame. It is used routinely to dispose of flammable gases that are either unusable or uneconomical to recover. Sometimes gas plant workers must do emergency flaring for safety purposes when they depressure equipment for maintenance.

In theory, the complete combustion of pure hydrocarbons produces only water and carbon dioxide. Low-efficiency flares do not completely combust all of the fuel gas, and unburned hydrocarbons and carbon monoxide are emitted from the flare with the carbon dioxide. If the waste fuel entering the flare contains impurities and/or liquid droplets, many other by-products can also be emitted from the flare stack. These products include particulate matter, VOCs such as benzene, toluene, and xylene, polycyclic aromatic hydrocarbons (PAH), and small quantities of sulfur compounds such as carbon disulfide (CS₂) and carbonyl sulfide (COS).

Flaring is both a concern to the public and a government priority because of the potential health risks and environmental concerns associated with the activity and also because it wastes a valuable nonrenewable resource. Furthermore, the noise, odor, and smoke produced from flaring activities can interfere with nearby residents and their enjoyment of the outdoors.

Flaring is an environmental concern regarding to global warming and acid deposition. Emissions of carbon dioxide and unburned natural gas from flares contribute to the greenhouse gas effect and global warming.

Flaring at sweet and sour gas plants and acid gas injection facilities can generate a large volume of air emissions that could negatively affect local air quality. Sometimes it will be less expensive for plant operators to flare gas for a long time while they try to fix a problem instead of shutting down the plant completely. Operators can reduce the impacts of flaring at gas processing/disposal facilities by minimizing the total volume of gas they flare at the plant and by reducing the frequency and duration of upset flaring events. Operators can prevent air impacts from ongoing flaring by shutting down all or part of a facility in phases, according to preset time increments. This will still give them some time to resolve any problems while the plant is running.

The efficiency of a flare is a measure of how effective that flare is in converting all of the carbon in the fuel to CO₂. Previous studies have indicated that flares have highly variable efficiencies, on the order of 62-99 %.

More recent research concludes that the yearly averaged combustion efficiency of gas flares could exceed 95 % if certain design and operating conditions are met. Gas flares are operated in uncontrolled conditions.

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The tips of the flare can be exposed to wind, humidity, and temperature variations that reduce efficiency and increase variability. Other factors that can adversely affect the efficiency of gas flares are the composition of the waste gas stream entering the flare, which varies from site to site, and improper flaring practices that cause unsteady combustion conditions.

Poor flare performance is often associated with incomplete combustion, leading to smoke and unburned hydrocarbon emissions. Under such conditions a review of the flare system will often bring benefit in operational cost, and ensuring minimum purge gas is demanded by the requirements of safe flare operation. An operation flare review will give operators the ability to discuss specific operational issues with a flare consultant and to examine the flexibility in the operation to enhance the flare system performance to achieve improved performance in terms of environmental, economic, and safety goals. Miles examined how a flare system can be designed safely to meet the challenge of reducing emissions to the environment as well as reducing the cost of operating the overall flare system.

Incineration can potentially be a more efficient method to dispose of waste gases, although a more costly and sophisticated one. If operated properly, incinerators generally have more efficient combustion than flares because combustion occurs in an enclosed chamber, away from the effects of wind and weather, and the air-to-fuel ratio required for complete combustion can be controlled precisely. Although they can be highly efficient, incinerators are mainly used at sour gas processing plants and not for routine waste gas flaring. The reasons for this are that incinerators are more costly to install, require more frequent maintenance and monitoring, and are difficult to install and operate in remote locations. Other alternatives to flaring include:

1. conserving the waste gas for processing at natural gas facilities;
2. reinjecting the waste gas underground to maintain reservoir pressure during production;
3. using the gas to power microturbine generators for electricity production;
4. and ensuring that flare systems are properly designed, constructed, and maintained through guidelines, codes of practice, or regulation. There are several proven economic and environmental benefit solutions now in service.

Flare gas recovery (the zero flare option) is a major step forward in securing a brighter and cleaner future for the industry. Flare gas recovery, however, may not be possible in all cases. There is a basic precondition that the gas recovered has to be useful. Fuel gas-deficient facilities are ideal; however, future trends toward centralized power generation may make a significant future impact. Where flare gas recovery is simply not feasible, then the best flare solutions should be sort.

Staged flares with the benefit of variable orifice solutions probably offer the best all round solution. The combination of these flare systems and flare gas recovery will give economic and environmental benefit.

Methane Emissions

Methane is both the primary constituent of natural gas and a potent greenhouse gas when released to the atmosphere. Reducing methane emissions can yield substantial economic and environmental benefits. The implementation of available cost-effective methane emission reduction opportunities in the gas processing industry can lead to reduced product losses, lower methane emissions, and increased revenues.

Methane losses from natural gas processing plants account for 15 % of total worldwide methane emissions. Emissions result primarily from normal operations, routine maintenance, and system disruptions. Emissions vary greatly from facility to facility and are largely a function of operation and maintenance procedures and equipment conditions.

Some of the more significant sources of methane emissions and selected technologies and practices applicable to the gas processing section are presented.

Pneumatic Devices

At processing facilities without electrical power, workers can use pneumatic devices, which can run on natural gas from oil and gas formations, to drive pumps as well as power instrumentation and control equipment.

High-bleed pneumatic devices can be major sources of methane emissions. Alternative technology is available that, while still using natural gas to drive pumps and instruments, does not vent to the atmosphere. Many companies in all natural gas sectors have achieved significant savings and methane emissions reduction by replacing, retrofitting, or improving maintenance of the high-bleed pneumatic devices. Field experience shows that up to 80 % of all high-bleed devices can be replaced with low-bleed equipment or retrofitted. Although low-bleed devices cost more, most operators that install them (either initially or as a retrofit) end up making their money back on the investment. Another option available at facilities with available electric power is to replace their natural gas-powered pneumatic control systems with compressed instrument air systems, eliminating 100 % of emissions from pneumatics. However, instrument air systems require electrical power to be available on site. In some cases, bottled nitrogen instead of instrument air or natural gas can be used for smaller services.

Dehydrator Systems

Many natural gas dehydrator systems in the processing sector use triethylene glycol to remove water from the natural gas stream in order to meet pipeline specifications. Often the glycol circulation rate is set much higher than it needs to be in order to achieve this objective. Overcirculation leads to increased methane emissions.

Operators can adjust this circulation rate at no additional cost and decrease methane emissions from the dehydrator system. Another way to decrease methane emissions from triethylene glycol systems is to install flash tank separators. Flash tank separators capture approximately 90 % of the methane entrained in the triethylene glycol systems, preventing the methane from being boiled off into the atmosphere when the triethylene glycol systems passes through the regenerator.

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Desiccant dehydrators can be good alternatives to triethylene glycol systems under certain circumstances. Glycol dehydrators can be replaced altogether with desiccant dehydrators at a higher cost, nearly eliminating gas emissions and also saving both fuel gas used for the glycol reboiler (sometimes a gas heater) and pneumatic gas used for glycol unit controllers. With no regenerator, desiccant dehydrators produce methane emissions only when they are being refilled with desiccant, and even then the volumes are far below those from triethylene glycol systems.

Vapor Recovery Units

During storage of condensates, methane and other gases vaporize and collect in the space between the liquid and the fixed roof of the tank.

As the liquid level in the tank fluctuates, these vapors are often vented to the atmosphere. One way production sector companies can prevent these emissions is to install vapor recovery units on condensate Accidents Involving LNG and LPG Storage Tanksstorage tanks. A vapor recovery unit draws over 95 % of the hydrocarbon vapors out of a storage tank or set of tanks under low pressure. The vapors are then routed to a scrubber and then used as an on-site fuel supply or sold.

Compressors

Reciprocating compressors are the dominant type of compressors used in the gas processing industry. A Gas Research Institute (GRI) report estimates that reciprocating compressors formed 85 % of the total compressor population in the gas processing industry. Components associated with reciprocating compressors are subject to high thermal and vibrational stresses that make them prone to leaks and are therefore among the largest source of emissions in the gas plant. Clearstone Engineering Inc. (Clearstone) data estimate much higher emissions from compressor seals than the GRI study. Clearstone has also identified valves and connectors as big sources of emissions that are not explicitly listed in the GRI study.

Thus, according to Clearstone, compressor seals, valves, and connectors are the top three emission sources in that order and together constitute 90 % of the emissions from reciprocating compressors. This rate would justify the practice of cost-effective rod packing replacement and could potentially save another billion cubic feet of methane emissions annually.

Replacement of wet seals with dry seals also leads to substantially reduced operating and maintenance expenses, improved reliability, and reduced contamination of the gas.

Reciprocating and Centrifugal Compressor Comparison for Natural Gas CompressionCentrifugal compressors are represented as a smaller source of emissions compared to reciprocating compressors. GRI shows an individual centrifugal compressor emitting more than an individual reciprocating compressor. However, due to the low numbers of centrifugal compressors used in the gas processing sector, they do not contribute as much as reciprocal compressors to the total emission volume.

Cryogenic Equipment

This category only contains turboexpanders and the equipment associated with them. This category of equipment was not identified in the GRI study and so is a “new” source of methane emissions. The total emissions from turboexpanders are small and contribute less than 3 % of total methane emissions in a gas plant. The individual components contributing to emissions are valves, connectors, pressure-relief valves, compressor seals, and open-ended lines.

Flares

This category includes process flares and flare system piping. The GRI study does not identify flares as a source of methane emissions. Clearstone data estimate flares to be the largest single source of methane emissions, contributing more than 40 % of the total methane emissions at a gas plant.

It is unclear from Clearstone data whether the flares were lit or unlit when evaluated or what the source of gas in the flare stack was. However, Clearstone provides a total hydrocarbon emissions rate that was determined by engineering calculations rather than direct measurement. The methane content of this gas was assumed to be the same as the fugitive emissions from all other components.

Methane Emissions Reduction

In natural gas plants, one can reduce methane emissions by upgrading technologies or equipment and by improving management practices and operational procedures. Opportunities to reduce methane emissions generally fall into one of three categories.

• Technologies or equipment upgrades, such as low-emission regulator valves, that reduce or eliminate equipment venting or fugitive emissions.
• Improvements in management practices and operational procedures to reduce venting.
• Enhanced management practices, such as leak detection and measurement programs, that take advantage of improved measurement or emission reduction technology.

There are numerous ways natural gas companies can reduce their methane emissions, and many of these technologies and practices cost less to implement than the value of the gas they save. Cost-effective opportunities for reducing methane emissions in the gas processing plants vary greatly from country to country based on the levels of physical and institutional infrastructure. Many of the available cost-ffective abatement options and technologies, however, can be applied universally throughout the gas processing industry.

Water Pollution

Refineries are also potential contributors to surface water and groundwater contamination. Wastewater in refineries may be highly contaminated and may arise from various processes (such as water from cooling, distillation, product fractionator reflux drum drains, and boiler blow-down). This water is recycled through many stages during the refining process and goes through several treatment processes, including a waste-water treatment plant, before being released into surface waters. The wastes discharged into surface waters are subject to state discharge regulations and are regulated under the Clean Water Act (CWA). These discharge guidelines limit the amounts of sulfides, ammonia, suspended solids, and other compounds that may be present in the wastewater. Although these guidelines are in place, contamination from past discharges may remain in surface-water bodies.

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When managing surface water, companies must prevent contact between rainwater and contaminants at the processing facility. They must also design the sites properly to ensure that rainwater that lands on a gas plant is collected in a central location where it can be stored, treated, and reused or released into the environment in a controlled manner.

Companies that use effective liners and secondary containment systems, and continue to monitor groundwater throughout the life of the project, can ensure that leaks and spills are prevented and any contamination that does happen can be detected and cleaned up quickly.

Soil Pollution

Contamination of soils from the refining processes is generally a less significant problem than that of contamination of air and water. Soil contamination, including some hazardous wastes, spent process catalysts, tank bottoms (this can be minimized by using recirculating pumps or mixers inside tanks to keep heavier parts suspended throughout the condensate rather than allowing them to collect on the bottom), sludge from the treatment processes, filter clay, and incinerator ash can occur from leaks as well as accidents or spills on- or off-site during the transport process. Treatment of these wastes includes incineration, land treating off-site, land filling on-site, land filling off-site, chemical fixation, neutralization, and other methods.

Pollution Prevention

Pollution prevention is the reduction or elimination of pollutants to the environment. Pollutants such as hazardous and nonhazrdous wastes and regulated and unregulated chemicals from all sources may be discharged as air emissions, wastewater, or solid waste. All of these wastes are treated. However, air emissions are more difficult to capture than waste-water or solid waste. Thus, air emissions are the largest source of untreated wastes released to the environment.

The limits of pollutants emitted to the atmosphere, land, and water are defined by various pieces of legislation that have been put into place over the past four decades. Following passage of the Pollution Prevention Act (PPA) of 1990, the US EPA developed a formal definition of pollution prevention and a strategy for making it a central guiding mission. Under Section 6602(b) of the PPA, Congress established a national policy that:

• pollution should be prevented or reduced at the source whenever feasible;
• pollution that cannot be prevented should be recycled in an environmentally safe manner whenever feasible;
• pollution that cannot be prevented or recycled should be treated in an environmentally safe manner whenever feasible;
• and disposal or other release into the environment should be employed only as a last resort and should be conducted in an environmentally safe manner.

The particular suite of practices and measures adopted for a specific gas processing project should reflect local circumstances. Not all of the best practices or measures just listed are appropriate in all cases. However, this hierarchy of preferred options for dealing with environmental pollution officially places prevention at the top of the list.

Pollution prevention can be accomplished by reducing the generation of wastes at their source or by using, reusing, or reclaiming wastes once they are generated. However, as stated before, environmental analysis plays a major role in determining if emissions-effluents (air, liquid, or solid) fall within the parameters of the relevant legislation.

Pollution prevention is often touted as an economically advantageous, strategically wise way for companies to protect the environment while protecting themselves (from liability, legal infractions, and unforeseen or unnecessary costs). Setting up a pollution prevention program does not require exotic or expensive technologies. Some of the most effective techniques are simple and inexpensive. Others require significant capital expenditures, but many provide a return on that investment. Speight presents a description of various methods by which effluents and emissions are treated.

Emissions from Natural Gas Use

Combustion Emissions

Composed primarily of methane, the main products of the combustion of natural gas are carbon dioxide and water vapour. The combustion of natural gas also produces significantly lower quantities of other undesirable compounds, particularly toxics, than those produced from the combustion of petroleum products or coal. The toxic compound benzene can be a component of natural gas, but the levels in natural gas are considered insignificant and are not generally monitored by gas processing plants and most pipeline companies. However, because the benzene content of the natural gas varies by source and can range from less than 0,4 to 100 ppm, depending on the efficiency of the combustion, some of the benzene will be oxidized to carbon dioxide and water, some will pass through unburned, and some will be converted to other toxic compounds.

Natural gas is the cleanest of all the fossil fuels. Coal and oil are composed of much more complex molecules, with a higher carbon ratio and higher nitrogen and sulfur contents. This means that when combusted, coal and oil release higher levels of harmful emissions, including a higher ratio of carbon emissions, nitrogen oxides (NO_x), and sulfur dioxide (SO₂).

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Coal and fuel oil also release ash particles into the environment, substances that do not burn but instead are carried into the atmosphere and contribute to pollution. The combustion of natural gas, however, releases very small amounts of sulfur dioxide and nitrogen oxides, virtually no ash or particulate matter, and lower levels of carbon dioxide, carbon monoxide, and other reactive hydrocarbons. Coal and oil plants beget masses of solid waste-up to 590 tons per day-while gas plants create none. Natural gas plants also release less waste heat due to their higher efficiency.

The most significant conventional pollutants released by gas combustion are oxides of nitrogen (NO_x) formed by heating air around the point of combustion. Harmful to human health itself, NO_x combines with airborne hydrocarbons to form ozone, a pervasive urban scourge. NO_x emissions are a precursor of airborne particulate pollution, which causes over 50 000 deaths per year in the United States. Because of their low NO_x emissions, some renewable energy technologies can make a greater immediate impact on environmental problems than natural gas plants. Advanced gas combustion technologies also reduce NO_x emissions significantly, although the majority of plants in service now use older technologies.

The environmental case for using natural gas as a bridge to a renewable energy future can be summarized as follows. First, natural gas pollutes more than renewable energy but less than oil or coal. Second, the supply of gas has limits, but for at least the next few decades, gas can generate far more electricity than all the renewable energy technologies combined. Third, due to its immediate availability, natural gas can displace many more tons of coal now and in the near future than renewable energy can.

Acid Rain Formation

Acid rain is another environmental problem that affects much of the eastern United States, damaging crops, forests, and wildlife populations and causing respiratory and other illnesses in humans. Acid rain is formed when sulfur dioxide (SO₂) and nitrogen oxides (NO_x) react with water vapor and oxidants in the presence of sunlight to produce various acidic compounds, such as sulfuric acid and nitric acid. Precipitation in the form of rain, snow, ice, and fog causes about half of these atmospheric acids to fall to the ground as acid rain, while about half fall as dry particles and gases. Winds can blow the particles and compounds hundreds of miles from their source before they are deposited, and they and their sulfate and nitrate derivatives contribute to atmospheric haze prior to eventual deposition as acid rain. The dry particles that land on surfaces are also washed off by rain, increasing the acidity of runoff.

The principal source of acid rain causing pollutants, sulfur dioxide and nitrogen oxides, are coal-fired power plants. Because natural gas emits virtually no sulfur dioxide and up to 80 % less nitrogen oxides than the combustion of coal, increased use of natural gas could provide for fewer acid rain causing emissions.

Smog Formation

Smog is a pressing environmental problem, particularly for large metropolitan cities. The primary constituent of smog is ground-level ozone created by photochemical reactions in the near-surface atmosphere involving a combination of pollutants from many sources, including motor vehicle exhausts, volatile organic compounds such as paints and solvents, and smokestack emissions. Because the reaction to create smog requires heat, smog problems are the worst in the summertime.

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Natural gas use is not much of a factor in smog formation. As opposed to petroleum products and coal, the combustion of natural gas results in relatively small production of smog-forming pollutants as it emits low levels of nitrogen oxides and virtually no particulate matter. For this reason, it can be used to help combat smog formation in those areas where ground-level air quality is poor. The smog-forming pollutants literally cook in the air as they mix together and are acted on by heat and sunlight. The wind can blow smog-forming pollutants away from their sources while the reaction takes place, explaining why smog can be more severe miles away from the source of pollutants than at the source itself.

Greenhouse Gas Emissions

The Earth’s surface temperature is maintained at a habitable level through the action of certain atmospheric gases known as greenhouse gases that help trap the sun’s heat close to the Earth’s surface. The main greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and several man-made chemicals, such as chlorofluorocarbons. Most greenhouse gases occur naturally, but concentrations of carbon dioxide and other greenhouse gases in the Earth’s atmosphere have been increasing since the industrial revolution of the 18th century with the increased combustion of fossil fuels and increased agricultural operations. Of late, there has been concern that if this increase continues unabated, the ultimate result could be that more heat would be trapped, adversely affecting Earth’s climate. Consequently, governments world-wide are attempting to find some mechanisms for reducing emissions or increasing absorption of greenhouse gases.

One of the principal greenhouse gases is carbon dioxide. Although carbon dioxide does not trap heat as effectively as other greenhouse gases (making it a less potent greenhouse gas), the sheer volume of carbon dioxide emissions into the atmosphere is very high, particularly from the burning of fossil fuels. On a carbon-equivalent basis, 99 % of anthro-pogenically sourced carbon dioxide emissions in the United States are due to the burning of fossil fuels, with 22 % of this attributed to natural gas.

One issue that has arisen with respect to natural gas and the greenhouse effect is the fact that methane, the principal component of natural gas, is itself a very potent greenhouse gas. Methane contributes directly to the greenhouse effect because of its ability to trap heat in the atmosphere, which is estimated to be 21 times greater than the ability of carbon dioxide to trap heat. According to the Energy Information Administration (EIA), although methane emissions account for only 1,1 % of total US greenhouse gas emissions, they account for 8,5 % of the greenhouse gas emissions based on the global warming potential.

Water vapor is the most common greenhouse gas, at about 1 % of the atmosphere by weight, followed by carbon dioxide at 0,04 % and then methane, nitrous oxide, and man-made compounds, such as chlorofluoro-carbons. Each gas has a different residence time in the atmosphere, from about a decade for carbon dioxide to 120 years for nitrous oxide and up to 50 000 years for some of the chlorofluorocarbons. Water vapor is omnipresent and cycles into and out of the atmosphere continually.

In estimating the effect of these greenhouse gases on climate, both the global warming potential (heat-trapping effectiveness relative to carbon dioxide) and the quantity of gas must be considered for each of the greenhouse gases.

Oil and gas companies should develop and carry out greenhouse gas management plans to minimize the cost of complying with the Kyoto protocol, ratified by Canada in December 2002, and with subsequent emission reduction requirements. Such plans should include ways to reduce emissions through internal energy efficiency, investments in offsets and “green power“, and a commitment to limiting absolute volumes of emissions.

Industrial and Electric Generation Emissions

The use of natural gas to power both industrial boilers and processes and the generation of electricity can significantly improve the emissions profiles for these two sectors. Natural gas is becoming an increasingly important fuel in the generation of electricity. As well as providing an efficient, competitively priced fuel for the generation of electricity, the increased use of natural gas allows for improvement in the emissions profile of the electric generation industry. According to the National Environmental Trust (NET) in their 2002 publication entitled “Cleaning up Air Pollution from America’s Power Plants”, power plants in the United States account for 67 % of sulfur dioxide emissions, 40 % of carbon dioxide emissions, 25 % of nitrogen oxide emissions, and 34 % of mercury emissions.

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Coal-fired power plants are the greatest contributors to these types of emissions. In fact, only 3 % of sulfur dioxide emissions, 5 % of carbon dioxide emissions, 2 % of nitrogen oxide emissions, and 1 % of mercury emissions come from noncoal-fired power plants.

Natural gas-fired electric generation and natural gas-powered industrial applications offer a variety of environmental benefits and environmentally friendly uses, including fewer emissions, reduced sludge, reburning, cogeneration, combined cycle generation, and fuel cells. Essentially, electric generation and industrial applications that require energy, particularly for heating, use the combustion of fossil fuels for that energy. Because of its clean-burning nature, the use of natural gas wherever possible, either in conjunction with other fossil fuels or instead of them, can help reduce the emission of harmful pollutants.

Protocol ang Environmental Programs

Natural gas is a collection of pollutant chemicals. Such gaseous emissions are often characterized by chemical species identification, e. g., inorganic gases such as sulfur dioxide (SO₂), nitrogen oxides (NO_x), and carbon monoxide (CO) or organic gases such as the hydrocarbon constituents of natural gas. The rate of release or concentrating in the exhaust air stream (in parts per million or comparable units) along with the type of gaseous emission greatly predetermines the applicable control technology.

Recommended protocols must occur as a preclude to the cleanup of emissions and mitigating future releases. Necessary actions for the cleanup of natural gas emissions are:

1. identification of the emissions;
2. identification of the emission sources;
3. estimation of emission rates;
4. atmospheric dispersion, transformation, and depletion mechanisms;
5. emission control methods;
6. air-quality evaluation methods;
7. effects on stratospheric ozone;
8. and regulations.

Members of the natural gas industry have a commitment to ensuring that their operations are environmentally sound and that every effort is being made to ensure that the environmental impacts of activities related to the processing of natural gas are as minimal as possible. Part of this commitment includes participating in voluntary industry programs aimed at maintaining the best possible environmental record for the natural gas industry. These programs include the following.

• The EPA natural gas STAR program. This program, sponsored by the EPA, is intended to reduce methane emissions – a potent greenhouse gas – from the oil and gas industry by implementing improved technologies, better work practices, and improved maintenance and inspection of distribution networks in order to minimize leaks and emissions.
• The API STEP program. The American Petroleum Institute runs a program called STEP (Strategies for Today’s Environmental Partner-ships). This program serves to encourage petroleum industry members to commit to environmental stewardship in their policies and principles and to develop programs to ensure safe, environmentally sound operating practices.

In addition to the aforementioned programs, the US natural gas industry is actively involved in international programs that serve to share “best practices” with respect to environmental preservation. Usually, this is accomplished through affiliation with an association dealing with significant environmental matters.

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In addition to industry participation in maintaining a healthy environment, the federal government has enacted a number of pieces of legislation to ensure that the natural environment is preserved and maintained into the future. The EPA is the primary federal government agency charged with protecting human health and safeguarding the natural environment. The EPA develops and enforces environmental regulations that exist under environmental laws in the United States, as well as leading a number of voluntary and educational programs intended to reduce pollution and protect the environment.

Environmental Management System

As stated earlier, organizations are increasingly concerned to achieve and demonstrate sound environmental performance by controlling the impact of their activities, products, or services on the environment, taking into account their environmental policy and objectives. For this purpose, the ISO 14001 standard has been developed to provide organizations with the elements of an effective environmental management system, which can be integrated with other management requirements, to assist organizations to achieve environmental and economic goals. This standard enables an organization to establish, and assess the effectiveness of, procedures to set an environmental policy and objectives, achieve conformance with them, and demonstrate such conformance to others. The overall aim is to support environmental protection and prevention of pollution in balance with socioeconomic needs.

The organization should review and continually improve its environment management system to achieve overall improvement in environmental performance. At regular intervals management should carry out a review of the environmental management system to ensure its continuing suitability and effectiveness. The scope of the review should be comprehensive, although not all elements of an environmental management system need to be reviewed at once and the review process may take place over a period of time.

Author

Olga Nesvetailova

Freelancer

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