On November 25, 2020, an explosion and fire occurred at an industrial facility in Houston BC. The facility utilizes a system incorporating a large natural gas-fired air heater to dry wood fiber material for the purpose of wood pellet production. The facility had recently been experiencing issues with operating conditions with the dryer system. The system was producing abnormal temperature spikes, irregular responses to operator controls, and pulses of positive pressure in areas automated to maintain negative pressure conditions. The issues were communicated within the operations team and were being investigated by facility staff and management. Plant operators made decisions to stop and restart the gas burner 3 times within an 18-hour period to alleviate the upset system conditions. During the third burner restart, an explosion occurred approximately 5 minutes after the main burner ignition. The explosion originated at the natural gas-fueled air heater and created a secondary explosion and fireball in the dust collection equipment. Three employees were injured, and the facility sustained major damage during the explosion.
Technical Safety BC conducted an investigation in coordination with WorkSafeBC and other parties.
Incorrect governing of the air and fuel volumes led to upset pressure conditions and an unbalanced air/fuel ratio in the combustion chamber. Unburnt natural gas may have collected in the system and ignited causing an explosion. The initial explosion disturbed combustible dust downstream causing a
secondary dust explosion and fireball.
Failure to have the gas train, air control components and combustion inspected, tested, and maintained on a regular basis, along with the operator and management decisions to repeatedly restart and continue operation of the air heater when known upset conditions continued to occur were the cause of the incident.
Major findings identified by this investigation are summarized below:
- The incident was found to be due to ignition of natural gas that collected in the system due to incorrect governing of air and fuel volumes and pressure conditions in the combustion chamber.
- The cause of the incident was found to be due to the following:
- Failure to conduct a routine inspection, testing and maintenance of the gas train, air control components and combustion systems, and
- Management decisions to repeatedly restart and continue operation of the air heater when known upset conditions continued to occur were the cause of the incident.
- Combustible dust is present in the ductwork and collection system. The initial explosion disturbed the combustible dust causing it to be airborne within the duct. The airborne dust then ignited causing a secondary dust explosion and fireball. The ancillary dust explosion and fireball may have been prevented through the use of explosion isolation systems including isolation dampers, valves, or barriers.