Cut the Cost of Waste Gas Incineration

A regenerative thermal oxidizer (RTO) burns very lean waste gases without using much fuel. If an RTO can be used, it’s always more fuel-efficient than any other type of oxidizer. Operating temperature is about the same as a normal thermal oxidizer (say, 1,600°F) but the hot flue gas passes through a heat exchange module before reaching the stack.

The module is an insulated box full of heat exchange media, usually ceramic packing. At least two modules are used — one absorbs heat from the flue gas while the other sheds heat into the waste gas (Figure 1). When a box has absorbed all the heat it can, it’s taken offline; waste gas then passes through it backwards until the box is cool again. Once cooled, it’s returned to handling hot flue gas. Two boxes are needed so the flue gas always has a path to the exhaust stack — specialized valves set on a timer switch each box from heating to cooling every 5 minutes or so.

Figure 1 -- Two-bed RTO: Heat exchange media alternate between heating and cooling. Diagram courtesy of CMM Group.

 

In this way, if one pound of waste gas enters at 70°F, one pound of flue gas exits at 200°F. With other thermal oxidizer designs, the pound of flue gas may exit at 500°F or 1,600°F — a lot more heat is lost up the stack. If the waste gas is lean, most of this heat comes from firing auxiliary fuel. The popularity of RTO units stems from the desire to cut such fuel costs.

The Basics

Waste gas incinerators react oxygen with waste hydrocarbons at high temperature to produce a clean flue gas. A perfect incinerator would have a destruction and removal efficiency (DRE) of 100%, zero fuel usage and zero emission of carbon monoxide and nitrogen oxides. A small amount of the original hydrocarbons always remains, though. If 1% is left, the DRE is 99%. Some CO and NOx always are produced, too. However, NOx emissions are lower for an RTO than for almost any other type of thermal oxidizer. Table 1 compares various options.

Thermal Oxidizers

Type	Waste Types	Heat recovered?	Stack temperature (°F)	Maximum DRE (%)	Heat recovery method
Direct fired	Any gas or liquid	No	1,200–2,200	98–99.99+	None
Catalytic	Lean waste gases	Yes	≈500	95–99+	Metal gas/gas heat exchanger
Recuperative	Lean waste gases	Yes	≈500	95–99.9+	Metal gas/gas heat exchanger
Boiler	Any gas or liquid	Yes	350+	98–99.99+	Boiler and economizer
RTO	Lean waste gases	Yes	200–300	95–99+	Packed beds