Figure 1: Bacterial metabolic product of typical storage lagoon/chest. (Rowbottom,1993).
A recent explosion at a pulp and paper mill had left a victim suffered an eye injury and amputation of his left leg. Chemical analysis confirmedthe presence of combustible gas, most likely arose from bacterial activity within the slurry content of the broke chest.
According to various researchers (Rowbottom, 1993; Kapdan and Kargi, 2006; and Salgar and Constantacos, 1999) bacterial activity in white water system of a pulp mill can produce combustible gases. The bacterial biological activity can be divided into 3 stages, depending on its location within the strata of the tank. Figure 1 summarizes the product of bacterial activity within a chest. If methanogenic bacteria is present, the hydrogen product will be converted to methane. Therefore, depending on the types and the condition of the bacteria (which makes them active), the degree of combustible gas production will also increase. Otherimportant factors areacidity and holding time, as indicated in Figure 2.
Figure 2: Two of the factors studied by Rowbottom (1993) influencing microbial activity in production of H2.
Hydrogen, methane and hydrogen sulphide are all combustible gasses. Thus, it is important that those involved in the pulp and paper mill industry be able to appreciate therisk involved due to bacterial activity in the pulp tank. Particular attention should be given to the following points :
- Hot work activity, particularly near the openings or ventilation of a pulp tank, should be monitored and sanctioned by relevant personnel.
- The process and maintenance workgroup need to be aware of this potential combustible hazard, especially if the plant is being shutdown for a few hours.
- Promoting the awareness of combustible hazard should include proper warning signs and instruction to staffs, as well as contractors.
- Relevant HIRARC exercise may need to be done, either for activities during normal operation or maintenance periodto systematically reduce the risk ofexplosion and implement the necessary improvement (where applicable). Some examples of suchimprovementsmay be:
- the review of startup procedure to include the reduction/elimination of combustible gas from the system, e.g. verification of operation of agitators in tank, and functionality of tank ventilation system,
- the consideration of equipment, electrical motors and fixtures near the chest opening/vent-off to be spark-proof
- categorization/zoning area hazard system
- hot work permit system to be implemented and managed for all hot work involving welding near to the broke chest
- specific training ,instruction and supervision on combustible gas hazard to ensure that they are free of any harm or risk arising from any work activity that has been carried out
- The above recommendation should be of utmost importance if the paper mill utilises nonacid-based papermaking technology.
References and Further Readings.
- Rowbottom, R.S. (1993). Risk of Bacterial Hydrogen Generation in White Water Systems. Vol. 76. No. 1. Technical Assoc. of the Pulp and Paper Industry (TAPPI).
- Kapdan, I.K. and Kargi, F. (2006). Bio-Hydrogen Production From Waste Materials. Enzyme and Microbial Technology 38 (2006). pp. 569-582. Elsevier
- Salgar, S. and Constantacos, J. (1999). Non-BiocidalVFA and H2S Control Technology for Pulp/Paper and Recycling Mills, EfluentWastewater, Industrial Sludge and Storage Lagoons.1999 International Environmental Conference. pp. 103 – 109. Technical Assoc. of the Pulp and Paper Industry (TAPPI) Proceedings.
- Lower and Upper Explosive Limits for Flammable Gases and Vapors (LEL/UEL). Reference information. Matheson Tri-Gas. www.mathesongas.com