Fig. 1. Typical gas-fired furnace with low-NOx burners at a commercial heat treaterm(Photograph courtesy of Metals Engineering, Green Bay, Wis.)

When most of us think about health and safety in the heat-treat shop, we tend to focus on things that are hot, heavy or dangling overhead. The threats we can see, smell, hear or perhaps even taste are most often dealt with quickly. The old adage, “what you can’t see, won’t hurt you,” doesn’t apply in our industry. Let’s learn more.

Lesson Learned

I recall as a young engineer that when called upon to inspect the inside of a cold, well-ventilated batch integral-quench or pusher-type furnace that had been running endothermic or exothermic gas atmosphere, I would often experience mild headaches after an hour or so inside. I remember carrying aspirin with me for just such an occasion. Little did I realize at the time that furnace atmosphere was slowly being released from the brickwork and that, in this confined space, I was experiencing the early symptoms of gas poisoning.

Dangers of Carbon Monoxide

Carbon monoxide (CO) is an odorless, colorless, toxic gas present in many heat-treat shops from furnace atmospheres and products of (incomplete) combustion. The symptoms of carbon-monoxide-gas poisoning, defined as levels of 10% or higher absorbed gas in the bloodstream, include in descending order of danger:
  • Headaches
  • Dizziness
  • Irritability
  • Confusion/memory loss
  • Disorientation
  • Muscle aches/poor reaction time
  • Balance/coordination problems
  • Nausea and/or vomiting
  • Difficulty breathing
  • Chest pain
  • Swelling of the brain (cerebral edema)
  • Convulsions/seizures
  • Coma
  • Death
Carbon monoxide has powerful effects (Table 1) even at very low-dosage exposures and often begins with “flu-like” symptoms such as being tired, having achy muscles, a headache that just won’t go away, eye strain and even a runny nose.

When carbon monoxide enters the body, it is absorbed into the bloodstream, preventing oxygen absorption and the transfer of oxygen to vital organs such as the heart, central nervous system and brain. The heart responds by beating more rapidly and irregularly and by decreasing blood pressure. In extreme exposure conditions, a life-threatening neurological condition results due to the destruction of brain cells.

Monitoring the heat-treat environment coupled with proper fresh-air ventilation is the key to combating this problem. One of the most important considerations in the treatment for carbon-monoxide exposure is the immediate recognition of the problem. That’s why CO detectors should be installed in every heat-treat shop. Does yours have one?

Once detected or suspected, the following actions can be taken:

1. Move the victim(s) to fresh air.
2. If the victim(s) are experiencing any gas poisoning symptoms call for trained paramedics and activate your company’s emergency plan.
3. Warn others and ventilate the affected area.
4. Monitor the victim(s) for respiratory problems.
5. Try to ascertain the source and shutdown suspect equipment until trained professionals can assess what is wrong.

Dangers of NOx

NOx (pronounced “knocks”) emissions is the generic term for a group of highly reactive gases that contain combinations of nitrogen and oxygen in varying amounts – nitrogen monoxide (NO) and nitrogen dioxide (NO2) being prime examples. NOx is responsible for a wide variety of long-term health and environmental problems, so it is important for the heat treater to understand that most combustion processes, like those that take place in our gas-fired furnaces (Fig. 1), can be a source for NOx emissions. These pollutants spread out over great distances, which means that problems associated with NOx are not confined to just the heat-treat shop. These effects include:
  • Smog (ozone) – formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight.
  • Dust (particulate material) – NOx reacts with ammonia, moisture and other compounds to form solid particles that permeate the air we breathe.
  • Acid rain – NOx and sulfur dioxide react with other substances in the air to form sulfuric, nitric and other acids that fall back to earth as rain, fog, snow or dry particles.
  • Oxygen-depleted bodies of water – NOx emissions increase the absorbed nitrogen level in water, upsetting the chemical balance of nutrients used to support life.
  • Toxic compounds – NOx reacts in the air with common organic chemicals and ozone to form a wide variety of toxic chemicals and by-products.
  • Greenhouse gases – Nitrous oxide (N2O), a member of the NOx family, is a greenhouse gas contributing to climate change.
  • Global dimming – Nitrate particles and nitrogen dioxide block sunlight.

Fig. 2. Low-NOx burner

Reducing NOx Emissions From Furnaces

The amount of NOx emissions is a function of the type (composition) of the fuel being burned, combustion conditions, burner design and flame temperature. “Thermal NOx” is the result of the conversion of atmospheric nitrogen and oxygen and is dependent on the reaction temperature, residence time and gas chemistry (stoichiometry). Long residence times at high temperature contribute to thermal NOx formation as does rapid mixing of combustion air (oxygen) with the fuel. “Fuel NOx,” by contrast, is the conversion of chemically bound nitrogen in the fuel with oxygen. It also increases with rapid mixing.

Incomplete combustion products can usually be held to industry minimums by the proper operation of modern burner equipment (Fig. 2). There are also several strategies for reducing NOx that rely on either post-combustion treatment of the gas or upgrading to different types of burners. Do you know your shop’s total NOx emissions? You should. Here are some methods to better control how much NOx is being emitted:
  • Flue gas recirculation (FGR)
  • Selective catalytic reduction (SCR)
  • Selective noncatalytic reduction (SNCR)
Upgrading burner designs (Fig. 2) is yet another way to maintain control over NOx emissions. Options include the use of:
  • Staged air burners
  • Staged fuel burners
  • Ultra-low NOx burners
  • Low excess-air burners

Other Safety Threats

When dealing with any gas (including nitrogen), always be aware of its inherent dangers and understand the causes of asphyxia. For example, oxygen may be displaced to a level of around 16% of the gas/air mixture before symptoms begin to appear (Table 2). Health and safety are everyone’s responsibility and avoiding accidents in the heat-treat shop will make it a better place for all of us. IH