Both natural- and forced-draft burners are utilized in millions of process heating applications throughout the world. They are applied across many industries to heat, process, refine, store and transport the myriad of products that we rely upon each day.
Producers in the upstream, midstream and downstream sectors of the global oil-and-gas industry rely on heating applications, commonly referred to as heaters, to accomplish objectives that support the production and processing of hydrocarbons. These objectives include separation, hydrogen sulfide (H₂S) removal, transportation, dehydration, compression and waste destruction. Each of these heating technologies requires and employs a burner.
Due to the generally low cost and minimal maintenance requirements, natural-draft burners tend to be the predominant choice of burner type, specifically in the upstream and midstream oil-and-gas industry. This article will briefly discuss why that is the case.
Natural- vs. Forced-Draft Burners
It is important to highlight the main differences between natural- and forced-draft burners to understand the reasoning behind the adoption of natural-draft burners in the upstream and midstream oil-and-gas industry.
- Are typically easier to install on upstream and midstream heater applications.
- Require no power, fan or blower.
- Have few or no moving parts.
- Typically are more cost effective.
- Are higher efficiency.
- Require less fuel pressure.
- Do not rely on atmospheric conditions to create draft for combustion.
In many industries and applications, the higher efficiency provided by the forced-draft burners justifies the additional cost and complexity. That is not always the case, however. In some instances, natural-draft burners provide more benefits. For example, in remote locations, where regular service is not feasible, the lack of moving parts in a natural-draft burner becomes a significant benefit. In other areas, the power required to operate the blower of a forced-draft burner is unavailable. Natural-draft burners do not require such a blower.
Also, it is worth noting that high efficiency forced-draft systems require the water created by the combustion process to be condensed. In a commercial or residential setting, the condensed water is disposed of easily. In a remote field site, this water would need to be hauled out by a truck, negating the efficiency benefits gained by condensing the water.
Proper equipment, installation and maintenance are key components of the efficient operation of natural-draft burners.
How Does a Natural-Draft Burner Operate?
A natural-draft burner is a mechanical device that utilizes the principles of combustion, draft and pressure to produce the airflow necessary to support the intended performance of the heater application. Although there are many varieties and designs of burners, typically they share similar characteristics: a primary mixer, venturi, orifice and nozzle or tip. Natural-draft burners rely on primary air being drawn into a burner mixing chamber via kinetic energy that is created by the gas stream entering the burner orifice. Prior to gas being introduced to the burner, a pilot is lit with the same principles described to provide an ignition source for the main burner. Once the main burner flame is established, the heater stack or chimney is heated by the combustion process, creating a vacuum known as the chimney effect. As the hot air rises, the fresh air required for combustion is continually drawn into the combustion chamber and burner.
Natural-draft burners rely on primary air being drawn into a burner mixing chamber via kinetic energy that is created by the gas stream entering the burner orifice.
Natural-Draft Burner Considerations
As mentioned, there are many varieties and designs. For the most part, however, natural-draft burners can be consolidated into three main categories associated with overall performance. A high-end, high performance burner normally will be comprised of a mixer, venturi sleeve and a high quality burner nozzle. Other features may include a quick-change orifice, flue gas recirculation, secondary air control, lightweight construction and suitability for a larger range of heater outputs (BTU/hr). High performance burners generally will have a high turndown range capability, the ability to achieve higher efficiency ranges and, potentially, the capability to provide lower NOₓ. Mid- and low-range burners are less efficient, offer less flexibility, have reduced turndown capability and may be heavier and more difficult to work with and service. Each performance category contains pros, cons and trades-offs, including factors such as efficiency expectations and provisions, emission requirements, noise abatement, turndown capability, flame stability and characteristics, installation, serviceability, maintenance and budget.
Natural-draft burners are used in many simple upstream and midstream oil-and-gas processing applications such as line heaters. Efficient and safe operation of these types of heaters starts with a proper burner setup.
The key to reducing emissions is improving efficiency. Basically, the higher the efficiency, the more heat is utilized for the appliance’s intended function. In natural-draft applications, the optimization zone lies from about 75 to 82 percent. To achieve these levels, a high performance burner with integrated secondary air control is required. Combustion efficiency is based on two factors: excess oxygen and stack-exit temperature. Optimizing the excess oxygen that exits the stack will result in higher efficiencies and less wasted heat. Efficiency capabilities differ vastly between the low- and high-end burner spectrum.
Emissions. A key issue facing humanity and the oil-and-gas industry is to reduce emissions. In terms of natural-draft burners, low range burners offer few options for emissions control. A high performance burner may offer heat scavenging or flue gas recirculation features to support lower NOₓ emissions and higher efficiency.
Noise Abatement. Many heated appliances in the oil-and-gas industry are found in locations where noise mitigation is desired or required. This can be in relation to surroundings, workplace safety and other environmental concerns. High-end burners have the capability to operate significantly quieter. As a point of comparison, 50 dB is one-fourth as loud as 70 dB.
Turndown. Often, heated appliances do not need to operate at full capacity due to changing conditions or loads on the appliance. Turndown ratio provides users with flexibility, allowing the burner to modulate, which helps avoid abrupt cycling. Effective turndown capability will allow smoother process temperature trends and increased efficiency as well as lessened equipment fatigue.
Improperly selected, installed and set burners can lead to multiple inefficiencies, pollution, carbon buildup, firetube stress, poor light-off and safety concerns. Burner choice, coupled with nozzle selection, can affect flame characteristics such as length, envelope, stability and turbulence. Some burners offer limited capability to modify flame length and pattern due to flame instability. They tend to have lazy yellow flames — representing incomplete combustion — which are prone to impinge on firetubes. The capabilities of such burners minimize the amount of primary air that can be brought in prior to the flame becoming unstable. Performance burners can accept 80 to 90 percent of primary air required for combustion and, therefore, can more easily achieve and maintain a stable, linear flame pattern. It can be shaped to match firetube diameter and length. Despite the number of natural-draft burners that exist and are used every day in the oil-and-gas industry, the expertise surrounding the principles of combustion is relatively low. This augments the need to select an easy-to-use (and maintain) burner because this will lessen risks related to safety and asset protection. In conclusion, natural-draft appliances abound in many industries, including the upstream, midstream and downstream transmission oil-and-gas sectors. With an appropriate burner selection, these appliances have the ability to be operated safely, efficiently and cost effectively. Understanding options and considerations is paramount to achieving these goals.