Today, gas flaring emissions are considered by many to be one of the most challenging environmental problems facing the world. Gas flaring is the high-temperature combustion of unwanted gases and liquids produced during normal operations or unplanned over-pressure events in industrial processes. Due to technological limitations and other factors, gas flare combustion can be inefficient and release the greenhouse gases (GHGs) methane (CH4) and carbon dioxide (CO2), and the “indirect” GHGs carbon monoxide (CO) and nitrogen oxides (NOx). Of the two GHGs, CH4 is more harmful, having about 25 times greater global warming potential than CO2 on a mass basis. NOx gases react to form smog and acid rain and are central to the formation of fine particles called particulate matter (PM) and ground-level ozone, both of which are associated with adverse health effects. For traditional gas flares, reduced combustion efficiency is considered the norm.
It is generally recognized that approximately 95 percent of the waste gases flared consist of the hydrocarbons natural gas (90-percent CH4), propane, ethylene, propylene, butadiene and butane. During the flaring process, these hydrocarbons react with atmospheric oxygen to form CO2 and water. Upstream flaring (oil and gas production) accounts for approximately 90 percent of all flaring, while downstream flaring (refineries, transport facilities and chemical plants) accounts for 9-10 percent, and industrial flaring (water treatment plants, landfills and coal mines) accounts for less than 1 percent. Flaring from upstream sources releases CH4 and small quantities of other hydrocarbons and possibly inert gases such as nitrogen (N2), CO2 and CO. Flaring from downstream sources generally releases a mixture of hydrocarbons, and industrial sources release CH4 and CO2 along with small amounts of other inert gases.
Regulatory standards
One reason addressing the gas flaring environmental problem is so challenging is the complicated and evolving regulatory standards for gas flaring emissions. State and federal emissions standards vary by gas flaring application and change over time, generally becoming more stringent as the hazards posed to humans and the environment by the emissions’ constituents become better documented. California has been on the forefront of addressing environmental problems, such as the notoriously bad smog in Los Angeles, by legislating some of the most stringent pollution standards in the nation, many of which are lower than federal standards. Historically, many states have adopted California standards — and not just pollution standards. Rather than incur the costs of developing their own, they find it is more cost effective to follow the precedents set by a state whose standards are generally recognized as the most protective of public interests. An example of this is tailpipe emissions standards. As of June 2018, 14 states had adopted California’s tailpipe emissions standards, a trend that will most likely continue in the future. Why should this trend interest you? Because California’s standards may be coming to states where your company works.
Regulatory compliance: LEFs
To comply with these more stringent emissions standards, high-efficiency low emission flares (LEFs) have been developed. LEFs are capable of maintaining a 99-percent total hydrocarbon (THC) destruction efficiency and can reduce NOx emissions to less than 19 ppm and CO emissions to less than 78 ppm (corrected to 3-percent oxygen), independent of gas composition, gas-stream flow and weather conditions. Traditional gas flares can emit well over 54 ppm of NOx and 261 ppm of CO (corrected to 3-percent oxygen). Values can vary depending on other trace contaminants in the waste gas. The LEF reduces emissions by pre-mixing source gas and air in proportions that supply the quantities of excess air necessary to achieve high-efficiency burning. An optimal ratio of landfill gas to air is achieved using variable frequency drives on the air blowers to adjust the ratios based on gas heating value and stack temperature. This optimal mixture is then ignited. The use of multiple independently controlled burners allows the system to operate with extremely high flow turndown while maintaining the desired emission control efficiency.
LEFs are rapidly becoming the standard for mitigating landfill gases in non-attainment areas and are being used with increased frequency at water treatment plants and liquid natural gas (LNG) facilities and terminals due to their ability to reduce NOx, CO and nonmethane organic compounds (NMOCs) significantly more than traditional flares. Let’s look at a case where LEFs are meeting current emissions standards and will be able to meet new standards in the works.
LEFs in action
The City of Los Angeles Hyperion Treatment Plant (HTP) is one of the 10 largest wastewater treatment facilities in the world. The facility has a cogeneration power plant that uses biogas captured through HTP’s treatment process as a fuel source. Two LEFs are used to treat contaminants stripped from the biogas. These crucial components of the cogeneration process achieved extraordinary results. NOx emission levels are 0.007 pounds/mmBtu, 65-percent lower than the permit requirement of 0.02 pounds/ mmBtu, and CO emissions are 0.005 pounds/ mmBtu, 93-percent lower than the permit requirement. Currently, the South Coast Air Quality Management District (SCAQMD), which is the state agency responsible for regulating stationary sources of air pollution in Southern California’s South Coast Air Basin, is developing Proposed Rule 1118.1, Control of Emissions from Non-Refinery Flares, which applies to digester gas from wastewater treatment plants. Once approved, the emissions standards for associated gas flaring will be 0.025 pounds/mmBtu for NOx and 0.06 pounds/mmBtu for CO, standards the LEFs at HTP will easily meet.
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