HAZOP for thermal oxidisers – What are thermal oxidisers and why do they need a HAZOP?

What are thermal oxidisers?

Thermal Oxidisers are a process that is used for pollution control to decompose unwanted pollutants emitted from industrial processes to prevent them from being directly emitted into the atmosphere, with the potential added benefit of generating heat for use in the process. They typically operate as a combustion principle using high temperatures to oxidise and remove volatile organic compounds (VOCs), hazardous air pollutants (HAPs) and other contaminants which are found in industrial exhaust gases.

There are four main types of thermal oxidisers: Direct-Fired Thermal Oxidisers (DFTOs), Regenerative Thermal Oxidisers (RTOs), recuperative thermal oxidisers and Catalytic thermal oxidisers (CTOs).

• Direct-fired Thermal Oxidisers (DTFOs) burn the pollutants directly in a combustion chamber at very high temperatures (>1200 ^oC) to convert contaminants into harmless by-products (Figure 1).
• Regenerative Thermal Oxidisers (RTOs) use a regenerative heat exchanger to preheat pollutants enhancing energy efficiency by recovering heat from the combustion process (Figure 2).
• Recuperative Thermal Oxidisers heat is recovered by passing the high-temperature exhaust gases through a non-contact (indirect) air-to-air heat exchanger, to heat the incoming cold air to the oxidiser (Figure 3).
• Catalytic Thermal Oxidisers (CTOs) use catalysts to facilitate lower-temperature combustion reducing energy consumption and operational costs (Figure 4).

Figure 1: Schematic of Directed Fired Thermal Oxidiser

Figure 2: Schematic of Regenerative thermal oxidiser

Figure 3: Schematic of Recuperative thermal oxidiser

Figure 4: Schematic of Catalyst thermal oxidiser

Reference: Working Principle of Thermal Oxidizers and its Applications · CFD Flow Engineering

Thermal oxidisers are being used more frequently due to the increasing global pressures to reduce the amount of pollution entering the atmosphere and therefore it is fundamental that the design of these systems is suitable for all the upstream and downstream conditions for the industrial process which it is being installed for. For uniformity in design, installation and operation of thermal oxidiser plants across Europe, a European standard has been published – EN 12753:1999 (Ref.28) entitled ‘Thermal cleaning systems for exhaust gas from surface treatment equipment Safety requirements.’

Why do thermal oxidisers need a HAZOP?

Thermal oxidisers require a Hazard and Operability Study (HAZOP) due to the risk of fire and explosions and potentially complex interaction with upstream and downstream processes. Over the years, several serious fire and explosion incidents across industry involving thermal oxidisers have occurred due to the failures of managing the initial design and changes to the process. The best way to ensure the design of the thermal oxidiser is done correctly is to carry out a HAZOP to ensure no critical matters have been missed especially for upstream and downstream processes since these can greatly affect the fuel-to-air ratio and other key parameters of the thermal oxidisers which could lead to a fire/explosion.

Any changes to the upstream processes can cause major issues with the fuel burning within the thermal oxidiser which can lead to an increase in flammable atmospheres which could lead to a fire/explosion if an ignition source is found. The HSE strongly recommend that a HAZOP is carried out on the initial design and at least every 5 years of thermal oxidiser use, or when any part of the process (including upstream) changes, to ensure there have been no significant changes carried out, which can affect the thermal oxidiser performance and efficiency to ensure process safety risks are as low as reasonably practicable (ALARP). The HAZOP can also help ensure that operational risks such as system failure and process variability are evaluated, ensuring these risks are addressed during the design phase of the project. The other operability issues such as maintenance of the thermal oxidisers can be addressed in the HAZOP process to ensure the risk to personnel is kept to a minimum.