Taylor Made: Smokeless fires
While wandering around the various tailgates surrounding Grange Grove before the Homecoming game last month, I noticed that several tents had cylindrical, stainless steel fire pits with wood-burning fires inside. The vapors rising from these pits were clear and virtually odorless, leading me to wonder at first whether the logs were hiding a gas fuel source. After all, the smoke coming from each fire was, essentially, smokeless.
At its most basic, our combustion reaction can be written CH4 + 2O2 >> CO2 + 2H2O. Hydrocarbons contained in fuel react with oxygen to produce carbon dioxide and water vapor. This is the ideal reaction—in reality, combustion exhaust can also contain carbon monoxide, soot, and various other products. Combustion begins once the required energy threshold has been achieved by sufficient input into the equation, such as current from a spark plug or friction from the strike of a match.
Upon closer inspection, I confirmed that real wood was indeed being burned. The pit design is comprised of two nested chambers, with circular inlets around the bottom of the outer chamber. Air from the inlets can rise in the space between the chambers and escape through outlets around the top perimeter of the inner chamber, as well as enter the inner chamber through a large hole in its bottom. A metal grill suspends the wood over a pan that prevents ash from falling into the hole. The grill-and-pan system allows for airflow between the upper and lower sections of the inner chamber, which is essential for good performance.
While burning wood in a cold pit of this type would still produce typical smoke, the characteristics of the combustion reaction change once the pit is warm. Exhaust vapor flowing upward through the pit creates a natural draw of ambient air through the inlets. The warm environment inside the pit heats the air as it flows to the burning wood, creating a feed of pre-heated oxygen that contributes to a more efficient flame. Air flowing between the chambers creates an insulating layer that helps manage heat loss. As it leaves the outlets, this airflow also helps direct the exhaust vapor to flow out the center of the pit. A lip around the rim of the inner chamber further directs the exhaust, ensuring a steady draw through the pit.
The pit is made of 304 stainless steel and weighs approximately 20 pounds. For those who love materials, 304 has a yield strength of 215 MPa, Young’s modulus between 193-200 GPa, and melting point between 1400-1455 degrees Celsius. The design achieves a hotter flame than that of a typical open-style fire, resulting in more complete combustion and less soot. In a sense, the pit acts as a catalyst to produce a more desirable reaction with less carbon-based product. Think about running your car fuel-rich vs fuel-lean—too rich, and unused hydrocarbons burn into soot. Too lean, and excess oxygen overheats the engine. In this case, the flame is essentially burning lean, a phenomenon that is not easily achieved in more traditional fire pit configurations.
Several different smokeless pit designs are commercially available. The type of pit I saw was 14” tall with a diameter of ~20”, but the heat being produced from the few logs burning inside was impressive—something we could use even more now that the cold hand of late fall has gripped Champaign.