Case Studies » Wight Farm Energy
The challenge
A visit was made to the Wight Farm Energy site at Gore Cross, Isle of Wight on the 04th of March 2015 to consider compliance in respect of the Dangerous Substances and Explosive Atmosphere Regulations 2002 (DSEAR). The visit also included a review of the ongoing fire safety arrangements on-site in line with the Regulatory Reform (Fire Safety) Order 2005.
The site footprint extends to over 3.5 hectares although plant infrastructure is restricted to around one-fifth of this area. The biogas process utilizes established anaerobic digestion (AD) technology to convert organic matter into Bio methane for injection into the national gas network – with some gas converted to electricity for use on-site.
The solution provided
DSEAR imposes a requirement to eliminate or reduce risks to safety from fire, explosion or other events arising from the hazardous properties of any dangerous substance used in connection with a work process.
AD operations mimic natural processes to breakdown organic material. Methodology is well established and purpose-designed plant incorporates extensive controls to minimise ignition. Methane-rich product supports combustion despite major presence of inert Carbon Dioxide.
The AD process involves significant formation of Hydrogen as an intermediate product – this is consumed during subsequent process stages though some residual H2 is typically present.
Biogas is Methane-rich and properties of gas produced using AD are similar to Methane itself. Flammability at even low concentration is a major concern when designing plant and electric supply arrangements etc. As Biogas is increased in Methane content the similarities become less distinguishable, and under stress conditions a worse-case scenario approach is advised. Hence biogas will readily form an explosion hazard, but the large volume of flammable Methane-rich gas present is contained within confines of equipment by design
Methane has a specific flammable or explosible concentration range – the lower explosive limit (LEL) is the lowest concentration in air which will burn and hence LEL of 5.0% should not be exceeded in process areas to avoid risk of explosion. The upper explosive limit (UEL) is the maximum concentration of ‘fuel’ in air which will burn or explode – above this concentration there is insufficient atmospheric Oxygen available. Hence Methane will not burn where it achieves a concentration in excess of its UEL of 15%.
By its nature, the AD process reduces Oxygen levels in air within digester tanks so that Methane gas is outside of its flammable range, but there is some overlap before this falls to safe levels during plant start-up or shutdown.
A critical aspect of this substance is the density and gas distribution is likely both above and below a point release source due to the rapid dispersion in air with only minimal ventilation flow. The very low flashpoint of Methane is also a key consideration as operations will always be above this temperature and hence ignition is always a real threat.
Avoidance and control of ignition sources is critical for operations – particularly maintenance or upset conditions.
A full site survey covering all areas of the operations was undertaken and any gaps in control identified in a fully detailed action plan. The report was supported by clear indication of hazardous area classification.
