Fossil fuels play a critical role in today’s economy and are critical for future global economic development and prosperity. The rise of unconventional resources in recent years has contributed to the growth of petroleum and natural gas production (Figure 1), but at the same time it has also increased their energy intensity. From extraction to refining, the oil and gas industry requires significant energy expenses — approximately a quarter of the energy contained in a barrel of oil is used to produce, refine and transport it. Adding to the mix high price volatility and a constant increase of environmental regulations, demand for technologies that can improve economic performance while reducing emissions is at an all time high. The oil and gas industry is now facing significant pressure to increase energy security by implementing energy efficiency and conservation measures while reducing their overall environmental impact.
The combustion of natural gas and other fossil fuels in oil and gas applications results in the emission of smog-producing chemical compounds including nitrogen oxides (NOx) — one of the criteria pollutants designated by the EPA. In recent years, several highly populated and industrial regions and countries with severe air pollution issues have begun to introduce strict emissions regulations. For example, California’s South Coast Air Quality Management District (SCAQMD) and the San Joaquin Valley Air Pollution Control District (SJVAPCD) have introduced some of the most stringent NOx emissions limits, mandating reductions to below 9 parts per million (ppm) in 2015, with plans for further reductions in the coming years.
Both upstream and downstream, the oil and gas industry deploys a range of boilers and furnaces, which in the United States are primarily fueled with gaseous fuels such as refinery or natural gas. Typically, natural gas is used to produce steam, which is in turn a widely used in a range of applications. For example, operators worldwide deploy enhanced oil recovery (EOR) techniques to aid the extraction of heavy oil. The most common, thermal EOR, requires the burning of large amounts of natural gas to generate steam, which is then injected into the reservoir to reduce the viscosity of crude and facilitate its extraction. Furthermore, oil refineries are highly energy intensive and require significant amounts process heat for the production of gasoline, diesel fuel and other chemicals — mostly supplied by burning refinery gas and natural gas.
Traditionally, operators have been able to meet environmental regulations with the use of ultra-low NOx burners. However, as regulations become more strick, two kinds of technologies to curb NOx emissions beyond the capabilities of burners that treat post-combustion gases have been deployed: selective catalytic reduction and flue gas recirculation systems. To better understand the differences and similarities between these options, let’s take a look at how they work.
Selective Catalytic Reduction (SCR)
SCR systems act on the exhaust gases produced during the combustion process to reduce the amount of NOx released into the environment. Flue gases pass through a catalyst bed, where a liquid reactant — generally ammonia or urea — is injected. SCR systems set off chemical reactions that convert the NOx contained in the flue gases back into molecular nitrogen, and molecular oxygen.
Flue Gas Recirculation (FGR)
FGR is another commonly used NOx emissions reduction technology, particularly for industrial boilers and refinery applications. FGR captures part of the flue gases produced during the combustion process and re-injects them in the burner with the addition of fresh air. The cooled flue gases are able to absorb heat from the flame, thereby lowering peak flame temperatures and inhibiting the formation of thermal NOx.
Challenges Associated with Post-Combustion Technologies
SCR and FGR technologies can be effective, but they also create a number of significant issues for operators of combustion equipment in the oil and gas industry. First and foremost, these solutions require expensive and complex designs, which can have significant implications on the performance and cost of combustion systems. For example, because SCR systems perform best within narrow temperature bands, they require the installation of control units to ensure that the exhaust gas temperatures are within range for the reaction to occur. SCR systems also require additional space and infrastructure (mechanical, electrical, structural), which may not be available at the site or the plant. Similarly, FGR technologies require additional ductwork, blowers and fans. This technique can be significantly energy intensive, resulting in increased energy consumption and reduced efficiency.
The use of harsh chemicals such as ammonia can actually pose new environmental threats — an over-injection can cause an ammonia slip and the release of hazardous compounds into the atmosphere, which can be harmful in the event of direct exposure.
The underlying issue with SCR and FGR technologies is that they operate in the post-combustion process, meaning they are attempting to mitigate the problem of emissions after they’ve been created rather than solving the issue at the source. Recently, a new generation of technologies has entered the market with a new take on air pollution control. Instead of functioning as filters or scrubbers, these technologies operate at the flame level, eliminating varying levels of pollutants directly at the source.
Low-NOx (LNB) and Ultra-Low NOx Burners (ULNB)
LNBs and ULNBs for boilers and process heaters typically use some variant of staged combustion and delayed mixing. During this process, fuel-rich and fuel-lean zones are deliberately formed in diffusion flames to bring down average temperatures and inhibit NOx formation. Unfortunately, these solutions can also have a major impact on operating costs. LNBs and ULNBs create elongated flames, which are inherently less stable and can cause poor flame patterns in the heater and lead to potentially severe impingement of flames on process tubes. Furthermore, LNBs and ULNBs have more limited operating ranges, compromising turndown in order to maintain stability. Aside from of these trade-offs, these burners alone have not shown they are capable of meeting the levels of NOx emission required by upcoming regulations, and may require the use of external flue gas recirculation or SCR to achieve these objectives.
Problems Associated with Elongated Flames
Elongated flames create a range of issues in industrial burners and process heaters in the refining and petrochemical industries. While fuel and air momentum are strong near the burner, the momentum falls sharply away from the fuel nozzles, often resulting in lazy and buoyant flames, prone to entrainment by furnace currents — often leading to flame impingement on process tubes. This undesirable phenomenon causes a host of problems, the most severe being tube overheating, which in turn leads to coking inside the process tubes. As a thermal insulator, coke interferes with the normal heat transfer and causes overheating on the outside tube wall — an issue commonly referred to as hot spots, which can severely weaken the tubes and accelerate tube failure. Also, intermittent flame impingement can gradually corrode the oxide layer inside the tubes, thinning the tube walls and leading to catastrophic tube ruptures.
Advanced Technologies
Advanced systems, which can be easily applied to traditional burners, boilers and process heaters, provide a radically different approach to combustion, enabling significant NOx emissions reductions at the flame level. This can be achieved without the limitations experienced by LNBs and ULNBs, while increasing burner efficiency and actually improving operations and management costs. Not only are these technologies more cost-effective to operate compared to traditional emissions control solutions, but they also require less upfront investment and can be easily retrofitted into existing systems.
The daily fluctuations in oil prices directly affect operations and future planning for oil and gas companies, which tend to be risk-averse and careful when it comes to adopting new technologies. Particularly when oil prices are low, oil companies tend to reduce investments in new technologies and instead focus on incremental improvements. As a relatively low-cost investment, advanced emissions control technologies offer an additional economic incentive to operators, independently from gas price variations.
By lowering NOx emissions to record-breaking lows — at or below 5ppm — these systems provide oil and gas operators with an economical compliance pathway to meet environmental regulation. A recently developed technology in particular enables such results by making use of a porous ceramic matrix through which the combustion is uniformly sustained, while simultaneously reducing flame length by up to 80 percent and better controlling flame patterns and heat distribution, eliminating the risk of flame impingement.
This ducted ceramic tile increases thermal efficiency, optimizing heat transfer in the system’s radiant section and avoiding excessive loading of downstream convection heat transfer surfaces. Additionally, this technology achieves better fuel-air mixing, which allows for reduced excess air and resulting in incremental fuel efficiency.
Without the flame impingement, a frequent problem with LNBs and ULNBs, process tubes will require less frequent de-coking and both process and steam tubes will enjoy lower failure rates and longer lifetimes, thus enabling operators to avoid downtime and the expense of unscheduled maintenance. Figure 2 shows how a simple combustion system, when modified to operate with an advanced post-combustion treatment technology, can significantly abate NOx emissions, lowering concentrations from more than 100 ppm to 5ppm or below.
As increasingly stricter environmental regulations are imposed in major markets around the world, oil and gas operators are searching for a solution that can reduce emissions without sacrificing system performance. Today, operators have the opportunity to choose from a variety of technologies that can help them abate NOx emissions. As previously discussed, the traditional methods provide limited benefits at high capital and operational costs. Advanced systems reduce pollutants at the source, eliminating the need for post-combustion treatment technology and significantly lowering both capital and operating costs while yielding a positive return on investment for system operators.
Roberto Ruiz, PhD, is senior vice president of product development at ClearSign Combustion Corporation. He is an oil and gas industry veteran with decades of leadership experience. Before joining ClearSign, he served as the president and chief operating officer of OnQuest, a provider of EPC services for fired heaters, waste heat recovery units and LNG, hydrogen, ammonia and bio-fuels plants.
