The U.S. Environmental Protection Agency and local air boards require DRE values from 95% upward, and CO and NOx emissions measured in the “tons per year” range. Fuel usage is up to the operator but more fuel means higher operating cost and more greenhouse gases, so lower is better.
For good DRE values, furnace temperature must be high enough, residence time of the flue gas in the furnace must be long enough and 2% to 3% O2 must remain in the flue gas leaving the stack. Stack temperature doesn’t matter — furnace temperature is all that’s important.
For most incinerators, furnace temperature is 1,400°F to 1,600°F. Higher temperatures require more expensive refractory to avoid heat damage. Furnace residence time typically is 0.5–1 seconds. Hydrocarbons that are hard to burn, like pesticides, may require more time. Oxygen content usually is about 3% or more by volume; as low as 2% O2 might be OK. If the waste gas is “dirty air,” it will contain all the oxygen needed. Otherwise outside air has to be added.
If the waste gas is relatively rich with hydrocarbons, a simple “direct fired” thermal oxidizer with a small burner will do the job. In this case, heat recovery is unnecessary but you can add a boiler if you need steam for process heating or to generate electricity.
Lean waste gas, in this situation, may require a large burner. That means some sort of heat recovery will make sense, because the cooler your stack gas, the lower your net operating costs. An RTO recovers heat very efficiently.
While an RTO is the most fuel-efficient oxidizer, it doesn’t suit all applications:
• A waste gas with entrained particles or droplets may cause fouling of the heat exchange media. In-place cleaning of the media is difficult or impossible. Fouled media means pressure-drop and efficiency problems. A direct fired oxidizer would be better.
• An intermittent waste gas requires idling or shutting down the RTO when waste isn’t flowing. It can take several hours to heat an RTO system for operation. A direct fired oxidizer probably would be better.
• Too rich a waste gas may lead the RTO to be too efficient, resulting in furnace temperatures that can cause refractory damage. If the waste-gas heating value significantly exceeds 20 Btu/ft, an RTO is a bad choice. Most RTO designers want waste gas no richer than 25% of the lower explosive limit.
• A waste gas containing chlorinated hydrocarbons, like methyl chloride, or sulfur bearing compounds, like hydrogen sulfide, will form a stack gas that might produce acid droplets if cooled enough — keep in mind that an RTO provides cooler stack gas than other types of oxidizer. If acid droplets are expected, you may need special construction, driving up cost.
Avoid Common RTO Mistakes (near the end of the article) summarizes some common mistakes in RTO selection and operation.
The Role of Packing
Directing the lean waste gas through a packed bed enables its temperature to be brought close to the target furnace temperature using only residual heat left in the bed by the hot flue gas. Sometimes this results in the waste gas hydrocarbons igniting on their own, achieving a further rise in temperature. While an RTO furnace always has a fuel gas burner, with good design fuel gas consumption might be zero during normal operation.
Without heat exchange packing, an operating RTO would perform like an ordinary incinerator — fuel usage to reach the needed furnace temperature would be high with a lean waste gas. More bed packing lowers fuel gas needed or raises furnace temperature reached.
If the beds were never switched, an RTO would perform like an ordinary incinerator — the hot flue gas would heat the bed it’s flowing through to the flue gas temperature and the lean waste gas would draw all of the heat out of the other bed. Fuel consumption would be high with a lean waste gas. Shorter bed switching time reduces fuel gas needed or increases furnace temperature reached.
The flow and composition of the waste gas determine bed size and packing type. You must use enough packing to absorb the heat from the full flow of stack gas — once a layer of packing is heated to combustion chamber temperature, it can’t pick up any more energy so another layer must be added. The designer sets the pounds of packing in each bed according to the rate of stack gas flow and the time the bed is absorbing heat before it’s switched. The type of packing used is an economic decision — random packing is cheaper but structured packing (Figure 2) takes up less room for the same amount of heat transfer.
Figure 2 -- Typical ceramic block:
Bed construction and the flow and composition of the waste gas dictate bed switching time. For greater heat-transfer efficiency, switching time needs to be shorter. But with larger beds switching time can be extended because there’s more packing to absorb the heat. Every time the beds switch, a small burst of unburned waste gas flows to the stack; a high required DRE mandates longer switching times, which results in larger packed beds.
Varying Quality Gas
A waste gas that usually is lean but occasionally can be richer demands special attention, as waste gas with more hydrocarbons requires less heat recovery to maintain low fuel-gas consumption. To reduce the heat recovery efficiency of the system, you can remove bed packing — but this is difficult. So, in such situations, units generally rely on either cold gas bypass (CGB) or hot gas bypass (HGB) to divert some gas around the heat recovery section of the RTO. With CGB, part of the cold waste gas is ducted directly to the furnace; with HGB, part of the hot furnace exhaust is ducted directly to the stack.