Wood Pellet Storage

Wood pellet and wood chip boilers are used in homes, community buildings and businesses as a renewable energy alternative to oil or gas fired boilers. They are also increasingly being considered for use in large-scale power generation.

Most wood pellets are produced by milling wood chips, shavings, or sawdust into a fine powder which is then dried and compressed under high pressure. The high pressure causes heating of the wood, and the natural lignin present in the wood acts as a glue holding the pellet together. Typically pellets are 6 or 8 mm in diameter and contain less than 10% moisture with no additives or binders.

Wood chips are produced by processing timber through a wood chipper. They are typically 20-50 mm long, but need to be compatible with heating system feed mechanisms e.g. augers. The composition of the wood chips depends upon the feedstock e.g. virgin or recycled wood. Wood pellets and chips need to be kept dry not only to maintain their calorific value and ease of ignition but also to prevent microbiological growth.

Wood pellets and wood chip biofuel can produce dangerous, toxic atmospheres in unventilated, enclosed spaces producing Carbon Monoxide and Carbon Dioxide, and depleting Oxygen concentrations.

The stores and their adjacent areas should be assessed to determine if they should be classed as confined spaces.

There are a number of health and safety hazards associated with the transportation and storage of wood pellets and wood chip fuels, including airborne dust, gaseous toxic emissions, spontaneous combustion and asphyxiating and explosive atmospheres. As they are enclosed and there is a reasonably foreseeable risk of serious injury or death, these storage areas should be considered to be confined spaces, as defined by the Confined Space Regulations 1997 (HSE 1997).

Wood Pellet and Chips Storage Hazards

Under suitable conditions of temperature and moisture, wood fuel can provide the nutrients for micro-organisms to grow and proliferate. The heat from microbiological proliferation may raise the temperature sufficiently to start the thermal oxidation, generating further heat and eventually leading to increased off-gassing and combustion.

There are also a number of fuel store hazards associated with any connected boiler system. The mechanism for feeding fuel to the boiler may also provide a route for exhaust fumes (e.g. Carbon Monoxide) to permeate back to the fuel store if the flue system is not working adequately or a continuous path of fuel for ‘burn back’.

In a study of wood pellets stored in industrial warehouses and domestic store rooms, Svedberg et al (2004) found high levels of Carbon Monoxide and Hexanal. In another study of wood pellets in ships holds, storage of pellets was shown to deplete Oxygen and produce Carbon Monoxide, Carbon Dioxide, Methane and a range of Volatile Organic Compounds (VOCs).

Fresh wood chips can produce Carbon Monoxide, but not in the same quantities as wood pellets; however, there is still a hazard from Carbon Dioxide emissions and depleted Oxygen. Artificially drying the chips decreases the rate of Carbon Dioxide production but increases the rate of Carbon Monoxide production.

It can be concluded that the decomposition of wood chip is much more complicated than that of wood pellets. Carbon Monoxide and Carbon Dioxide were produced from both fuels, but wood pellets produced more Carbon Monoxide than wood chips, whilst these produced more Carbon Dioxide than wood pellets.

Workplace Exposure Limits

Carbon Monoxide and Carbon Dioxide are both asphyxiants. In the UK, Carbon Monoxide has workplace exposure limits (WELs) of 30 ppm (8 hour time weighted average (TWA)  and 200 ppm (15-minute short-term exposure limit (STEL)). Carbon Dioxide has corresponding WELs of 5000 and 15,000 ppm.

The Approved Code of Practice (ACOP) to the Confined Space Regulations 1997, states that there are substantial risks if the concentration of Oxygen in the atmosphere varies significantly from normal (i.e. 20.8%); and very low Oxygen concentrations (i.e. below 16%) can lead to unconsciousness and death (HSE 1997).

Methane and Carbon Monoxide are both explosive. The lower explosive limit (LEL) for methane is 5% by volume, and 12% by volume for Carbon Monoxide.

Gas Monitoring

Both wood pellet and wood chip biofuel can produce dangerous atmospheres when stored in an unventilated enclosed space. Both fuels can produce Carbon Monoxide and Carbon Dioxide. Wood pellets in particular produce Carbon Monoxide, and wood chip Carbon Dioxide. Both fuels can deplete air of Oxygen.

Before entering the wood storage areas, air quality checks and measurements are to be made using a portable gas detector and gas sampling probe. When entering the store, a personal gas alarm should be worn. Some portable gas detectors can perform both jobs using an internal sampling pump with an attached sampling probe. 

When the wood store is opened e.g. for ventilation, access should still be controlled. Air quality checks should include Oxygen measurement as well as Carbon Monoxide and Carbon Dioxide.

For gas monitoring in a ship's hold, consider the PS500 which can detect up to five gases with its toxic and catalytic sensors, PID (VOCs) and IR capabilities (Carbon Dioxide)

When pellets are stored in a store room, a Scott Safety AVIVA half mask respirator fitted with P3 filters can be worn when entering the store (if worn properly and face fitted, this provides protection against wood dust, but would provide no protection against Carbon Monoxide or an Oxygen deficient atmosphere).

Do you work in the wood storage sector and need some advice about gas detection? Contact a1-cbiss to arrange a site survey or talk to our technical specialists for help with your gas detection needs