Cutting the Risk of Landfill Fires

With the average landfill site having a 60% statistical risk of fire each year, it's critical that operators have an emergency plan in place. So if disaster strikes, just how do you cut your losses?

With the average landfill site having a 60% statistical risk of fire each year, it's critical that operators have an emergency plan in place. Immediate and decisive action can be the difference between a minor incident and a multi-million dollar environmental disaster. So if disaster strikes, just how do you cut your losses?

by Patrick Foss-Smith

It's probably obvious that the cost of preventing a landfill fire is far less than extinguishing it. The treatment costs go well beyond the physical works if one includes, for instance, loss of profits and environmental and residual healthcare costs. Also obvious, but worth saying, is that the longer a fire burns the more difficult it is to put out. Left to its own devices an underground fire will burn for a long time.

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Landfill fires cannot be left to burn (smoulder) due to dangerous emissions, including dioxin, given off by slowly combusting PVC, among other substances. In July 2012 Dioxin contamination from a landfill fire resulted in local food products being condemned in Palermo, Italy.

To better understand the issues, let's consider two types of landfill fire - surface, or shallow fires and deep seated fires.

Surface fires

Surface fires can start unexpectedly in defiance to good planning. Examples of ignition sources include lightning, hot coals entering the site in the waste stream and a wide range of often unexpected events. One example includes the cascade of hot cartridge cases ejected from an Apache helicopter's 30mm cannon which recently caused a landfill fire in the Middle East.

If the fire is noticed quickly enough an immediate response will normally do the trick but only if the operation is planned in advance. Emergency planning is essential as some basic resources need to be pre-positioned in advance of the big day.

One often successful response to a surface fire is the rapid placement of a layer of fine granular material on the fire, which is spread out and compacted with a landfill compactor before significant heat has built-up. This reduces exposure to oxygen and, crucially, closes the voids within the burning waste reducing the surface area of the waste particles exposed to heat, as well as the oxygen available to the fire. Various materials have been tried – topsoil works well, clay and sand no so well.

Another technique is to quench the fire with a 10 tonne deluge of water, delivered from a site tipper. Although this works well, the driver needs to have a good aim and have practised it once or twice. A badly aimed 'shot' has been known to make matters worse by disrupting and enlarging the fire. There are, of course, dangers in operating any plant so close to the fire.

Failing this, the operator may have to resort to the local Fire and Rescue service. However, for this to be successful the operator must identify or provide a water supply and have briefed both the Fire and Rescue service and site staff. The fire brigade may never have visited a landfill site and the staff may never have witnessed a fire. The attendance of Fire and Rescue is not an alternative to good planning.

Neither of these methods work on demolition landfills containing timber, or tyre fires. These fires are much more difficult to extinguish as they go underground very quickly. A technique which was tried in Canada used diesel powered blowers to hasten the fire towards burnout. It certainly hastened the progress of the situation to an uncontrolled emergency involving every available fire appliance in the area - and a multi-million dollar bill for the contractor's insurers.

In short, lay on water, topsoil, plant, arrange cross briefings and develop an action plan.

Underground fires

Treating deeply seated landfill fires often defies common sense. The first method attempted often fails - for instance injecting the site with water or placing a clay cap over the fire. However, it should be noted that neither method is reliable and can make matters worse by causing a fire to retire into pyrolysis (oxygen starved).

It is unlikely that injecting water will extinguish a deep seated fire, however infiltrating an inert gas such as CO2 or nitrogen into the hot zone can be successful

For uncapped sites, often found in illegal landfills in the Middle East, the only viable method is to excavate the fire, known as overhauling. The contractor's primary concern will be to stop the underground fire from spreading during treatment. To stop this from happening, a horse-shoe shaped cut-off curtain is formed in a deep trench surrounding the fire. The cut off curtain is created from any fine granular material to hand. Topsoil, with any degree of cohesiveness, works very well, while sand works quite well but needs a larger excavation to avoid a collapsed cut-off curtain.

Although this is a simple concept, the execution is challenging and involves significant on-site resources including, a hydraulic excavator, a bulldozer, fire pumps, lighting towers, closed circuit breathing apparatus and much more. Once the fire is exposed, it will immediately flare up and must be suppressed by water jets playing on both the burning waste and the hydraulic excavator.

A number of safety risks need to be carefully identified and resolved. An example is the risk of the hydraulic excavator falling into the fire void. This can be resolved by attaching the excavator to an idling bulldozer, using a steel wire cable, kept under constant tension by an alert bulldozer operator.

This operation needs expert detailed planning to ensure a safe outcome and considerable personal courage on behalf of the plant operators.

For capped landfills the treatment options are somewhat simpler but take longer to execute. The operator will be concerned that the fire might impact the liner system and result in perforation and an environmental release. The operator might consider slightly raising the level of leachate to reduce this risk. Gas abstraction should be slowed until the operator is certain that the cap is not leaking air into the system.

Cutting the cost of fire

To reduce the cost of treatment, the location of the fire must be defined in three dimensions using well known techniques.

It is extremely unlikely that water injection can be used to extinguish an underground fire since the water will take its own direction dictated by gravity and the hydraulic conductivity of the waste.

Similarly, capping is not usually successful since a surface outbreak at the epicentre of the fire is only a symptom of the problem below. Air cannot be prevented from penetrating a temporary cap due to changes in air pressure.

One method of extinguishing deep seated fires at capped landfills, which has proved successful, is infiltrating an inert gas into the hot zone. Either CO2 or Nitrogen can be used although nitrogen is marginally cheaper. In either case the gas is delivered through an array of perforated pin wells located around the hot zone. The gas is delivered a few millibars above the landfill pressure at ambient temperature.

The gas is delivered to the site as a super cool liquid in a bulk tanker and expanded through a separate evaporator. A small fire might use around 40 tonnes of gas in 10 days and the gas will linger for a further five days. Confirmation that the fire is out is based on monitoring of all the site observables – temperature, gas concentration, odour, etc.

The cost of treating a fire at a regulated landfill in 2012 ranges from around £100,000 to £2.5 million. The cost can be dramatically reduced by forward planning. Since the statistical risk of a fire is 60%, a plan to deal with the problem makes good sense.

Patrick Foss-Smith is an environmental consulting engineer specialising in landfill and underground fires.

This article is on-line.

Tyre Fires

Tyre fires are a particularly worrying event since they are difficult to extinguish and produce emissions with serious, acute and chronic health implications. Further to this, the risks posed by the emissions are not completely localised and can extend for many kilometres downwind of the fire.

Tyre fires spread quickly and produce an extremely pungent smoke filled with dioxins and furans

Most surface tyre fires are caused by either by lightning strikes, tyre shredding or arson. Once the fire starts it will spread quickly becoming uncontrollable within a few minutes. Tyres burn by the incomplete combustion of the vapour they give off when heated. The tyre will also melt, forming a burning liquid, which flows under gravity to the bottom of the pile. Pools of tyre oil will then spread laterally over the ground. Adding water to the pile merely hastens the flow of burning liquid away from the original seat as it floats on top of the water.

Smoke from tyre fires has some interesting and very unattractive properties. Any fireman will tell you of the dangers of smoke inhalation, but smoke from tyre fires is in a class of its own. The smoke is formed from particles of (mainly) unburned carbon. A combination of hot carbon and atmospheric moisture has the effect of slightly activating the unburned carbon. Commercially produced Granular Activated Carbon (known as GAC) is used to mop-up cooking odour in cooking hoods and nerve agents in chemical warfare.

In tyre fires the fugitive activated carbon mops up toxic emissions, including dioxins and furans, on its way to freedom by adsorbing toxins onto the surface of the particle. The particles are extremely small but with a very large internal surface area of perhaps 60m2 per gram upon which to load the toxins.

Very small smoke particles, of less than 2.5µ (2.5 millionths of a meter in diameter), can be inhaled and pass directly into the bloodstream. A significant proportion of tyre smoke falls within this category, known as Pm2.5. By this means, the worst emissions are carried far from the fire.

Because the particles are so small and may have been boosted high into the atmosphere by the heat of the fire, the time taken for the particles to settle out of the atmosphere can amount to days rather than minutes. A tyre fire in Ontario in 1990 produced a smoke plume which was seen, by satellite imagery, to have encircled the earth.

Tyre oil has about the same fuel value as heating oil. From now on you might think of tyres as fuel, just waiting for the big day…