Early Detection of Spontaneous Combustion in Pellet Mills

Spontaneous heating and spontaneous combustion pose risks in any location that handles and processes woody biomass.
By Derek Stuart | July 26, 2019

Wood pellets are increasingly being adopted as a fuel for both domestic and industrial applications at all scales, from small space heating to a 600-MW power plant. Pellets are considered a renewable fuel, provided the wood comes from sustainably managed forests. Using them instead of fossil fuels such as coal helps reduce emissions of carbon dioxide, which contributes to climate change.

Wood pellets are by their very nature combustible and can be ignited by a range of sources. Stored bulk piles of wood pellets tend to oxidize, which leads to self-heating and, potentially, spontaneous combustion. Additionally, the dust associated with the pellets, when dispersed and ignited, can give rise to a dust explosion under appropriate conditions of containment.

Fortunately, there are techniques to detect the early stages of spontaneous heating and combustion, allowing operators the chance to prevent the problem and avoid costly damages.

Gas Detection
Carbon monoxide (CO) monitoring is the most effective method of detecting the presence of spontaneous heating or combustion in an enclosed space, such as a silo or pulverizer. CO detection gives a fast and unambiguous indication that spontaneous combustion is taking place.

The concentration of CO in ambient air is very low, and a lot of this gas is produced as spontaneous combustion begins, so a rapid increase in CO concentration is a sure sign that action is needed. Measurement of CO in pulverizers is especially important. Along with the risk that burning material could be introduced, the mill performs a great deal of mechanical work in crushing the fuel that can lead to a fire or explosion.

The risk is small when the mill is in operation because the particle concentration is above the upper explosive limit. However, whenever the mill is started or stopped, the concentration inevitably passes through the explosive range and, if burning material is present, a potentially serious incident is extremely likely.

Temperature Measurement
The actions of bacteria and fungi cause an increase in the temperature of a storage pile, whether open or enclosed. Temperature measurement is the most effective option for outdoor locations.
The simplest method for scanning a storage pile is use of a handheld thermal imager. Such devices are relatively inexpensive, but the intermittent measurement means that spontaneous heating can go undetected. A fixed imaging system is preferable, since it allows operators to store and compare images over time. Additionally, image processing software measures the temperature over different zones of the storage pile and can also exclude short-term fluctuations, such as a vehicle passing through the field of view.

Temperature measurements can be taken on a conveyor using a line-scanning infrared pyrometer that uses a single detector with a high-speed scanning mirror to make up to 1,000 discrete readings across the width of the conveyor. The movement of the conveyor allows the scanner software to build up a two-dimensional image of the material on the belt and show any hot spots associated with burning material.

Two instruments from Ametek Land enable the monitoring of gas and the measurement of temperature—the Silowatch extractive CO monitors, which are widely used in pellet silos at one of the largest biomass electrical generating facilities in the United Kingdom—and the HotSpotIR infrared line scanner, which have become the industry standard for a variety of conveyors from wood pellets to petcoke. In all cases, the analyzers, which all have variants approved for use in hazardous area locations, have provided valuable information to the plant operator while maintaining high levels of reliability and safety at industrial sites. This allows the plant to provide evidence of automated detection systems if needed in insurance situations.

Spontaneous heating and spontaneous combustion pose risks in any location that handles and processes woody biomass. Choosing the right detection method can significantly improve site safety by reducing fire risks.

Author: Derek Stuart
Ametek, Global Product Manager