Biofuel companies can minimize the risks of spontaneous heating and spontaneous combustion through effective temperature measurement, write Derek Stuart and Richard Gagg
Biomass fuels can potentially be dangerous to transport and store
Biomass is used increasingly as a fuel for power generation, because it is both a sustainable fuel source and considered as relatively carbon neutral over the near term.
In some instances, biomass fuels are co-fired with coal. But a number of electricity generating units have converted to run on 100 per cent biomass fuels.
Although these fuels have many desirable characteristics, biomass fuels, such as wood chips and pellets, have a number of properties that make them difficult and potentially dangerous to store and transport.
Among the most serious challenges with some biomass fuels is their susceptibility to spontaneous heating and spontaneous combustion.
Fortunately, a number of techniques are available that can detect the early stages of spontaneous combustion, allowing the problem to be detected in time to take preventative action and avoid a costly and potentially damaging fire. The best technique depends on the measurement location, and the most appropriate choice is the one that best balances the advantages and drawbacks of each technique.
Hog fuel (coarsely shredded wood waste) can be used for power generation, but it has a number of drawbacks. Its high moisture content increases transport cost and reduces the amount of heat available to produce steam, and hence generate electricity.
That is because the moisture must be evaporated in the boiler, and the latent heat of vaporization is lost when the water vapour exits through the stack. Hog fuel also has variable size distribution and often contains foreign material, such as soil, rocks, grit and even metal. As a result, it is best burned on a moving grate or fluidized bed combustor rather than a pulverized fuel boiler.
Generally speaking, wood pellets are preferred for large-scale electricity generation. In many ways, they are easier to handle than hog fuel, though they do tend to disintegrate if they receive rough handling.
They have a low, predictable moisture content and high energy density, which reduces transport costs. They are generally free of contaminants and have predictable mechanical properties, so they can be ground in a pulverizer, which allows co-firing with coal or even direct substitution for coal.
Torrefied wood pellets are made from wood chips that have been roasted at a temperature of around 300°C in an oxygen-deficient atmosphere. The process removes moisture and volatile organic compounds and allows the production of pellets with low moisture content and with an energy density close to that of coal. Through the torrefaction process, about 15 per cent of the heat content of the raw wood is lost, but the process results in a material that is sometimes referred to as “biocoal.” It is less expensive to transport than either hog fuel or conventional pellets, and it requires fewer modifications to a coal-burning plant’s fuel handling system.
There are two principal mechanisms that lead to spontaneous heating in woody biomass – oxidation and biological action. Biological agents include both bacteria and fungi. Their actions are generally the first stage in spontaneous heating. Aside from the hazards associated with spontaneous combustion, spontaneous heating consumes the fuel and reduces the amount available for use in the boiler. Some reports indicate that as much as 1 per cent of the fuel in an open storage pile can be consumed every month through bacterial action alone.
A number of factors determine the rate of heating. High moisture content encourages both fungal and bacterial activity. Bark and leaves provide nutrients for the fungi and bacteria. Solar heating on the surface of a storage pile also encourages bacterial action. All of these factors are more likely to be present in hog fuel stored in the open than in pellets stored in an enclosed silo.
Bacterial and fungal action give rise to carbon dioxide (CO2) and methane (CH4). The amount of gas produced is an indication of the extent of the spontaneous heating. The early stages of combustion produce a large amount of carbon monoxide (CO), so the presence of this gas is an indication that spontaneous combustion is taking place.
Once the temperature is high enough, oxidation becomes the principal heating mechanism. Poor air circulation in the middle of the pile allows heat to build up, which can lead to a runaway effect as the higher temperature increases the rate of oxidation. A fire can begin to smoulder slowly in the middle of the pile, gradually working its way to the surface, where it will burst into flames, when exposed to a plentiful supply of oxygen from the air.
Fuel stored in the open is more susceptible to bacterial action
Spontaneous heating and spontaneous combustion have tell-tale signs. The most obvious of these is an increase in temperature. It may take a long time for a temperature increase to become apparent if the reaction is taking place deep inside a storage pile, but elevated temperatures are much more apparent once the material is loaded onto a conveyor.
The most effective method for detecting the presence of spontaneous heating or spontaneous combustion depends on the location.
Gas detection: CO
Carbon monoxide (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.
In biomass applications, most CO monitors use electrochemical sensors. These are compact, specific and very sensitive, with typical detection limits in the region of 2 ppm. However, these sensors do have drawbacks. The most serious is they give zero output when they fail, so a faulty sensor is indistinguishable from a safe condition.
It is critically important with a CO monitoring system to perform regular calibration verification to ensure that the sensor is functioning correctly. A weekly verification is generally adequate. Although this can be done manually, an automatic check ensures that it is done consistently. This also removes the possibility that a faulty sensor can be neglected because plant personnel have other priorities.
Continuous exposure to the target gas leads to a reduction in the sensor response, so some systems use a pair of sensors that are alternately exposed to the sample and to ambient air, allowing continuous measurement without degradation of accuracy.
Because it measures gas concentration, CO monitoring is only effective in enclosed spaces, such as silos and pulverizers. It cannot be used in open storage areas because wind and other air movements disperse the gas before the concentration becomes high enough to measure.
Both in situ and extractive systems are available. Although in situ systems are the simplest and least expensive, calibration verification is more difficult than for an extractive system, which allows coal gas to be injected at the sample probe. The high concentration of dust in the headspace of a silo or in a pulverizer, especially, means that plugging of the sample port can cause problems.
Regular inspection and cleaning of in situ probes are essential. For extractive systems, a blowback system employing compressed air can be employed to clean the sample probe automatically, and a flow sensor in the analyzer can provide an indication that manual cleaning may be 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, in itself, can lead to a fire or explosion. The explosion risk is small when the mill is in operation because the particle concentration is above the upper explosive limit but, whenever the mill is started or stopped, the concentration inevitably passes through the explosive range. If burning material is present in the mill at this stage, an explosion is extremely likely.
Gas detection: CO2 or CH4
The presence of carbon dioxide and methane is an indication that biological action is taking place, and the concentration of those gases is an indication of the extent of such heating. As with CO measurements, those gases can only be measured effectively in enclosed spaces.
Most sensors commonly use infrared absorption to detect CO2 and CH4. Compact IR sensors are less sensitive than electrochemical cells, but detection limits of a few hundred parts per million are adequate to show whether spontaneous heating is taking place. The measurements are most effective when combined with a CO sensor, since they can be used to show when spontaneous heating becomes spontaneous combustion.
An extractive analyser that measures CO, CO2 and CH4 allows a common sample system and control electronics to be used for all three gas sensors.
Bacteria and fungi actions cause the temperature of a storage pile to increase, whether the pile is open or enclosed. It is generally impractical to detect this temperature rise by direct measurements with thermometers or thermocouples because the material is regularly moved to the combustor and because of the size of the piles. So a non-contact infrared temperature measurement is preferred. Although this type of measurement only looks at the surface of the stored material, it does provide an indication of its temperature and, therefore, of the heat being generated inside the storage pile.
The simplest method for scanning a storage pile is by using a hand-held thermal imager. Such devices are relatively inexpensive, but intermittent measurement means that spontaneous heating can go undetected. A fixed imaging system is preferable, since this allows images to be stored and compared over time.
Image processing software further allows the temperature to be measured over different zones of the storage pile. The software also can exclude short-term fluctuations, such as a vehicle passing through the field of view.
A highly effective conveyor temperature measurement can be made using a line-scanning infrared pyrometer with a single detector wand a high-speed scanning mirror that can make up to 1000 discrete temperature measurements across the width of the conveyor. The conveyor’s movement 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.
Thermal imaging also can be used inside a silo to measure the surface temperature of stored biomass fuels. Although this method cannot measure the temperature of hot material deep within the silo, the hot gases produced by spontaneous heating carry heat to the top of the pile, allowing a thermal imager to detect an abnormal temperature profile.
Once spontaneous heating or spontaneous combustion is detected, appropriate remedial action must be taken. The best course of action depends on the location and severity of the problem. For example, a storage vessel can be inerted with nitrogen or steam. Burning material can be diverted from a conveyor so that it does no more harm. In some cases, the best action can be to burn the fuel in the boiler in order to empty a storage vessel.
Spontaneous heating and spontaneous combustion pose risks at all sites that handle and process woody biomass. An appropriate choice of detection methods can significantly improve site safety. Gas measurements are most effective in enclosed spaces, while infrared temperature measurements are the preferred choice for outdoor locations.
Derek Stuart and Richard Gagg are Global Product Managers at Ametek Land.
AMETEK Land has deployed a variety of measurement methods to detect spontaneous heating and spontaneous combustion in fuel handling and storage systems used for traditional fuels, such as coal, as well as for woody biomass. Its Silowatch extractive CO monitors are installed in pellet silos at one of the largest large biomass electrical generating facilities in the UK. ARC thermal imagers have been used inside storage domes and silos in the UK and the Middle East. The Land HotSpotIR line scanning pyrometers have become the industry standard on a variety of conveyors from wood pellets to pet coke.