Achieving Combustion Control
Understand the role of systems for burner management and combustion control: When properly selected and designed, the control systems provide safety and operational efficiencies to thermal processing.
Like many of the process industries, thermal processing is a critical necessity to the oil-and-gas industry. Applications range from simple line heaters being fed from a single well to safety instrumented systems at a large-scale refinery. Regardless of the application, the thermal process is measured based on how safe, environmentally friendly and efficient it is.
Both burner management and combustion control systems are designed specifically to help ensure the safe, reliable operation of any critical thermal process. Given the inherent dangers associated with high temperature thermal processing in such applications, safety should never be a compromise.
Systems for Industrial Burner Management and Combustion Control
Safety is the common denominator of any equation associated with designing, building, installing and operating a thermal processing unit. Engineering firms, consultants, fabricators, system integrators and manufacturers need to be mindful of the current regulatory and safety aspects when discussing burner management and combustion control. Burner management covers the majority of the safety aspects though combustion efficiencies contribute to safety as well. Known safety parameters are defined during a hazard and operability study (HAZOP) or layer of protection analysis (LOPA). A process hazard analysis (PHA) may be added to this list where applicable. Such analysis helps identify the required controls needed to safely operate and recognize the potential impact if a problem were to occur. This management of change is designed to mitigate known safety concerns and react to those conditions.
Burner and combustion equipment manufacturers play a crucial role when specifying burners, instrumentation and control equipment based on application requirements. Typical factors to consider include characteristics such as:
- Indirect versus direct heating.
- Upstream fuel gas pressure feeds.
- Temperature requirements.
- BTU of the fuel gas feed.
- Heat flux.
- Environmental constraints.
Each relates to the environment in which the equipment will be installed and what will be coming out of the stack. Missing one calculation on the above requirements could result in the unsafe operation of your thermal processing unit.
This 12 burner 30 MM BTU amine heater is used for thermal processing.
For example, elevation can affect burner performance, resulting in unreliable and environmental inefficiencies. Gulf Coast installations look at the ambient pressures at 14.7 psi while installations in the Rockies calculate for 12.6 psi. The difference is because typical capacity change to the BTU output decreases by 4 percent for every 1,000’ elevation increase.
Another consideration is the BTU of the fuel gas feeding your thermal processing equipment. Continuants of the fuel influence the effective heat release, pressure input settings and volume requirements.
Combustion control is sometimes confused with burner management. A burner management comprises the safe and reliable safe start checks (limits), ignition, fuel gas valves operation, flame detection/supervision, monitoring I/O and shutting down the appliance in the event of an upset. Burner management systems (BMS) are designed for the supervisory role of managing the flame (hence the clever naming).
Combustion control provides a more complex introduction to safety, environmental impact and operational efficiency. Temperature elements, processor and temperature control valves make up the primary control elements of combustion control. The PID settings determine the reaction to the fuel gas feed based on load requirements of the appliance/vessel.
Combustion control can be monitored, tuned and localized to the combustion equipment installed on the thermal processing equipment. The combustion control can also be controlled by the plant SCADA, DCS or plant PLC. Combustion control looks specifically at the fuel-and-air ratios, the PID tuning parameters and the operational temperatures. It then provides a firing rate to control or operate a temperature control valve (and air actuator if fuel/air ratio control is present). Tuning parameters associated with combustion control consist of the speed of the temperature reaction to the burner (indirect vs. direct heating) output and the operation of the end-device controls (temperature control valve or air actuator). These operational conditions can be fast or slow, which will be dependent on the design of the equipment and process requirements.
Understanding the environmental constraints or localized requirements is based on the air permitting and agencies governing the operation of the equipment. These include the Environmental Protection Agency (EPA), those having local jurisdiction or self-imposed guidelines. These constraints — as they relate to thermal processing — define the carbon footprint allowable by site or client.
Now that the parameters or those that monitor the parameters have been identified, we can look at the factors that can contribute to better operational efficiencies and environmentally friendly results.
Independent combustion and burner management control is provided for each burner.
Operational Efficiencies and Environmental Impact
There has been a global shift by manufacturers to focus on the operational efficiencies and environmental impact of their equipment. Environmental concerns revolve around the unspent hydrocarbon release, NOX output and greenhouse gases such as CO2. Specific to flares, combustors and thermal oxidizers, the EPA has created the EPA 40 Code of Federal Regulations, Part 60, Subpart OOOO, also known as “Quad Oa.” This regulation specifically targets the destruction efficiency of the fuels being sent to the appliance. Unspent hydrocarbons — or volatile organic compounds (VOCs) — can contribute to ozone, smog and unsafe operational conditions. Most of these values can be monitored with a continued emissions monitoring system (CEMS) or through a localized gas analysis (a spot check of the stack output). Based on these values, minor or major adjustments can be applied to reach desired results.
Factors associated with combustion control include:
- The BTU of the fuel gas, which is approximately 1,100 BTU at 0.65 specific gravity — methane (CH4 ) typical.
- The inlet fuel gas pressure and air requirement. (Natural gas or forced draft; forced draft being the more efficient and most costly option).
A slight change in one of these conditions can result in the production of NOX, CO2, unspent hydrocarbon release and unsafe operating conditions. The size of the fuel train, burner front pressure and type of fuel being used for combustion also contribute to the operation of the appliance.
Typical operational efficiency of a single burner natural-draft heater is sub-80 percent (with the parameters mentioned above considered). A forced-draft heater can see sub- to mid-90 percent efficiencies. These efficiencies equate to better operational control, gas savings and, of course, environmental friendliness. Properly designed systems run on lower gas pressures, low gas volumes and lower operational temperatures — allowing for better thermal exchange and longer life expectancy of an appliance or vessel.
The design and control of any thermal processing unit will widely vary by geography. Those operating in the Rockies will contend with the Bureau of Land Management (BLM), and those operating in Texas will contend with the air-permitting agencies such as the Rail Road Commission, Texas Commission on Environmental Quality (TCEQ) or the EPA. Each agency will help guide or offer insight to the combustion control parameters.
This retrofit in south Texas includes a 12-burner amine heater combustion and burner management system. The 12 independent NFPA 87 built fuel trains include ignition coils.
Combustion control can be the cause of or the cure to gaining published operational efficiencies. These efficiencies typically are reserved to define operational conditions although they can be tied to cost savings. Most times, gas savings is a byproduct of properly sized fuel trains, inlet fuel gas pressures and PID-tuned parameters — not as a selling point. Safety and return on investment (ROI) are rarely used in the same sentence. With safety as our common denominator, however, having an ROI is recognized by appliance/vessel life expectancy and cost of ownership. Combustion control is a significant contributor to cost savings.
Combustion control gives operators the opportunity to reliably operate and produce product to their specifications, all while maintaining prescribed safety mandates and regulatory standards. Knowing your operational limits allows industrial product and solution providers to better understand your application and provide a safe, reliable, repeatable solution. Whether your burner management and combustion controls are specified internally or you rely on third-party consultation, be mindful of the parameters of your operational conditions and the environmental constraints associated with the location and output of your appliance/vessel. Combustion and burner manufacturers stay up-to-date with changing standards and regulatory mandates, so consider seeking a reputable combustion company to assistance in your next project. Remember, it is your responsibility: do not let an event be your fault.