RTO/RCO Design and Thermal Energy Recovery Efficiency

Regenerative Thermal Oxidizers (RTO) and Regenerative Catalytic Oxidizers (RCO) are prized not only for their effectiveness at abating hazardous materials, but also because they can operate at remarkable efficiencies.

But, getting maximum performance out of RTO/RCO equipment takes serious know-how. A common mistake is to assume that fuel cost based on Thermal Energy Recovery (TER) efficiency only is a sufficient indicator of actual fuel use. In addition to available energy corrections, a true RTO/RCO energy consumption calculation requires that mass balance also be taken into consideration.

The RTO/RCO is a counterflow regenerator in which heat energy is alternately accumulated for the length of the hot period (RTO/RCO exhaust flow, or energy recovery) and then released from the heat exchange media for the duration of the cold, counterflow period (RTO/RCO inlet flow, or preheat). Since the RTO/RCO is unable to accumulate mass flow, these periods are basically the same in each direction of flow. Therefore, the energy released on the inlet flow (preheat) is closely linked to the energy absorbed (recovered) on the outlet flow.

Unless the process solvent concentration is high enough to sustain the RTO/RCO (typically 3% of the Lower Explosive Limit [LEL] for a 95% TER design), or a Natural Gas Enhancement (NGE) system is included, the combustion chamber burner will have to provide the additional energy to maintain the desired purification temperature. Thus, additional mass (burner combustion products) is added to the combustion chamber and RTO/RCO exhaust flow, causing an unbalanced mass flow in the heat exchange media, which in turn causes a lower TER and an increase in fuel consumption.

A common mistake is to assume that fuel cost based on Thermal Energy Recovery (TER) efficiency only is a sufficient indicator of actual fuel use.

TER Calculations:

Q_i = M_i (h_ic-h_i) preheat   =   Q_o = M_o (h_oc-h_o) recovery

Q_i ……. Heat energy released on the RTO/RCO inlet flow

Q_o ……. Heat energy absorbed on the RTO/RCO outlet flow

M_i……. Mass flow on the RTO/RCO inlet  

M_o ……Mass flow on the RTO/RCO outlet (process flow [M_i] + burner combustion products [M_C])

h_i………RTO/RCO inlet enthalpy

h_ic…….Combustion chamber inlet enthalpy

h_oc…..Combustion chamber outlet enthalpy

h_o…… RTO/RCO Outlet enthalpy

In the case shown below, a 95% TER design, the actual mass balance fuel consumption and associated fuel cost is actually 66% greater than the amount indicated by a straight thermal energy recovery calculation. The takeaway is clear: when it comes to efficiency, TER is only part of the story.

Mass Balance Fuel Consumption

Typical mass balance operating conditions:

• Self-sustaining: VOC (Volatile Organic Compound) auto-ignition energy release sufficient to maintain the purification temperature
• NGE (Natural Gas Enhancement): Sufficient additional NG mixed into the process inlet flow to maintain the purification temperature
• And/or a combination of VOCs and NG enhancement sufficient to maintain the purification temperature

These factors go a long way in clarifying the efficiencies of an RTO/RCO process, but others play a role in overall fuel consumption as well, such as radiation losses, uniform air flow through the heat recovery media, and cycle time.

For example, the following chart shows the effect of cycle time and the potential reductions in TER:

When operating personnel understand and apply the concept of RTO/RCO mass balance, available energy, uniform air flow, and radiation losses, a true RTO/RCO energy consumption can be estimated.  

Points to Remember

• The RTO/RCO design thermal energy recovery (TER) efficiency is only achieved in applications that have a mass balance flow
• Radiation loss is an energy requirement in addition to the energy required for the temperature rise across the RTO/RCO
• VOC energy contributions due to oxidation will have an approximate 10% loss due to latent heat loss  
• Non-uniform airflow distribution through the media can also have a major effect on fuel consumption.

NESTEC offers several energy saving features to provide energy savings, many of which can be applied to existing RTO systems.

If you want to maximize your RTO energy conservation and reduce your energy bill, contact NESTEC for a free process analysis.

Office: 610.323.7670

Jim Nester, CEO: jnester@nestecinc.com

Rick Reimlinger, Vice President: rick.reimlinger@nestecinc.com

Rodney L Pennington, PE, Vice President of Key Accounts: rpennington@nestecinc.com

William Holden, Aftermarket & Services Manager: wholden@nestecinc.com