Keeping Your Cool

Enclosed Combustor Temperature Requirements

The skin temperature and exhaust temperature on enclosed combustors are measurements that are controlled and limited. In most jurisdictions, upper-temperature limits must not be exceeded for reduced spacing of enclosed combustors.

For combustor designers, this presents a challenge. How do you efficiently combust waste gasses while controlling the heat generated?

 Skin

When designing for temperature, there are several considerations, but the main concerns are personnel safety and fire prevention.

The external shell (skin) temperature needs to be below the autoignition temperatures of any substances that may contact it. For example, organic debris (i.e. leaves, grasses, tumbleweeds) may settle and maintain contact with the exposed surfaces of an enclosed combustor. The heat exposure must not reach a limit that can produce ignition.  Several studies have recorded the autoignition temperature of wood to be between 200-300ºC (Vytenis Babrauskas, August 2002). These relatively low temperatures are a major consideration for manufacturers.

The other, equally important, consideration is Occupational Health and Safety Codes. Most codes require protection for personnel when exposed to surfaces hot enough to cause a burn or blister (Occupational Health and Safety Codes, AR87/2009). The National Institute of Standards and Technology states that temperatures of 55C can cause blistering and second-degree burn injuries, and temperatures of 72C can cause tissue to be instantly destroyed (James Randall Lawsone, 2009).

The point is, it doesn’t take a whole lot of heat to burn someone or cause a fire, so you’d better make sure that your enclosed combustor is designed properly to eliminate these risks.

So, how are combustor designers controlling the heat distribution? One way is to prevent the skin temperature from ever getting too hot by using high-temperature refractory insulation on the inside of the combustor shell. Another way is to protect personnel or debris from contact with the hot combustor shell by adding insulation on the outside of the combustor shell. A third way is to actively cool the outer shell to prevent the shell from reaching dangerous temperatures. Emission Rx combustors, for example, use a patent-protected double-shell system that pulls air between the two shells, actively cooling the outer shell (Figure 1).

If none of these methods are employed and high temperatures surfaces exist, additional guards or shields or both should be used.

No matter how you achieve it, some form of protection for personnel and the environment should always be part of the combustor design..

Figure 1: Photo showing a thermal image of an operating Emission Rx E-Series Combustor.

Exhaust

The exhaust temperature limits are based on the autoignition temperature of the gases that may be present in the area, at a concentration above the lower explosive limit (LEL). This temperature is determined by examining the gas composition of the source gas, identifying the gases above the LEL concentration, then using the autoignition temperature of these gases to set the temperature limits.

For example, methane has an autoignition temperature of 600C (National Fire Protection Report, 2019). If this is the primary gas type on-site, a combustor designer should set the upper exhaust temperature below this temperature (include a safety factor).

(include a statement about exhaust gas temp monitoring and monitor location)

Wrap up

It is important for combustor designers and manufacturers to be familiar with the local and federal manufacturing requirements for the protection of people and the environment.  A good manufacturer will always satisfy these requirements to ensure that their equipment is designed safely. Emission Rx combustors are proudly engineered to meet or exceed all provincial and federal safety requirements.

References

National fire code https://www.nfpa.org/assets/files/AboutTheCodes/497/497_A2020_EEC_AAA_SD_PCResponses.pdf

James Randall lawson, 2009, Fire Facts, National Institute of Standard and Technology

Occupational Health and Safety code – AR87/2009 (effective June 1, 2018), Part 22 Safeguards

Vytenis Babrauskas, August 2002, Ignition of wood: a review of the state of the art, Fire Science and Technology Inc.

Occupational Health and Safety code – AR87/2009 (effective June 1, 2018)