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Belts for Conveying Biomass – The Safety and critical Importance of Conveyor Belt Technology

Written by Williams, L. edited by mhd on 19. Feb. 2019
For a great many reasons, the conveyor belts used to convey coal or other bulk materials are highly unlikely to be suitable for carrying biomass. Sadly some very expensive lessons are already being learnt. Here Les Williams of Dunlop Conveyor Belting discusses the essential and very specific characteristics that biomass-carrying belts must have in order to perform safely and efficiently.
The EU is determined to achieve 63% of heat generation by biomass by 2020.
The EU is determined to achieve 63% of heat generation by biomass by 2020.

The growing pressure to reduce CO2 emissions has seen an enormous growth in the transhipment of biomass. With the Kyoto Protocol and the EU’s continuing intention to reduce carbon footprints of its member states, more and more countries are making the push towards biofuel. In fact the EU is determined to achieve 63% of heat generation by biomass by 2020. In the Port of Amsterdam alone, the boom in biomass volumes means that the port could see handling of biomass products rise from its current 1.5 million tonnes per annum to 6 million tonnes by 2020. At the same time, there continues to be a parallel decline in the volume of coal transhipment.

What is Biomass?

To the uninitiated, a commonly held belief is that biomass is simply compressed wood waste that is formed into pellets. Actually, it is not nearly as simple as that because biomass can be made up of a combination of several different resources. Apart from the several different types of wood and wood waste (more will be explained later), biomass can include agricultural crops and their waste by-products, municipal solid waste, animal wastes, waste from food processing and even aquatic plants and algae. Nowadays, bulk terminals often receive shipments that contain a mixture of different biomass pellets.

It is this multitude of different organisms plus other characteristics of biomass that provides such a wide range of challenges to conveyor belt manufacturers and those who operate the conveyors that carry it. Apart from the usual considerations of adequate tensile strength, tear strength, elongation and cross-rigidity, the type of rubber compound used on the belt covers must be able to cope with the differing demands created by different forms of biomass. There are four essential characteristics that biomass-carrying belts must have.

Risk of Explosion (Anti-Static Properties)

Safety should always be the first priority and one of the biggest issues concerning belts that carry biomass is dust emission and the prevention of biomass dust explosion. In the production process of biomass wood pellets, wood chip and similar renewable resources, the materials are continually broken down. This results in high levels of combustible dust. The dry flammable dust found in biomass can easily be ignited by static electricity created by abrasion within the conveyor system. In fact the energy required to create ignition is as low as 17mJ. Worse still, biomass dust can also be highly prone to self-ignition, especially if it has become damp. A chemical reaction can take place that causes self-heating and what is referred to as “off-gassing” (carbon dioxide, carbon monoxide and methane emissions).

There should be no more than 35 grammes of dust in a cubic meter of air (lower explosive limit) in the atmosphere immediately surrounding a conveyor carrying biomass. Put into perspective, that is approximately the volume of a small tube of popular brand of children’s sweets. This means that conveyor design including dust extraction systems and chutes takes on far greater importance. Strict conformity to Directive 2014/34/EU should be a pre-requisite. This is also known as 'ATEX 95' or 'the ATEX Equipment Directive' and is applicable to potentially explosive atmospheres of zones 20, 21 and 22 where combustible dust is present.

The build-up of dust has to be kept to an absolute minimum, which means almost constant cleaning. When carrying out any form of maintenance or repair, the dust must be completely removed within several meters of the working area to prevent the possibility of ignition. From a conveyor belt point of view, it is absolutely essential that the electrostatic dischargeability (anti-static) properties of the conveyor belt cover rubber (according to ISO 284 test methods) do not exceed the maximum resistance value of 300 MΩ.

Despite the claims of some manufacturers, not all rubber belts are sufficiently anti-static. When sourcing conveyor belts for use in ATEX regulated areas it is very important to ask potential suppliers for a copy of a certificate provided by an appropriate independent testing authority such as the German Institute Dekra Exam GmBH. It is important to bear in mind that it is not possible to obtain an ATEX certificate for a conveyor belt because belts are classified as a component. ATEX certification only applies to the whole conveyor.

Fire Safety (Self-Extinguishing Properties)

Factual evidence (gained from extensive laboratory testing) as well as anecdotal evidence certainly indicates that some of Europe’s biggest users of conveyor belts, including some major ports, are using belts that are not as resistant to fire as they are claimed to be. Only the best quality fire resistant belting for conveyors carrying biomass should ever be considered.

The important thing to remember about fire proof rubber conveyor belts is that they do not exist. By their very nature, rubber and the fabric inner ply material (usually polyester & nylon) used to make conveyor belts is combustible. They will burn – end of story. The two descriptions used by conveyor belt manufacturers are “fire retardant” and, most commonly, “fire resistant”. However, in truth, a more accurate description would be “self-extinguishing”. This is because the ability of a conveyor belt to ‘resist’ fire is achieved by adding special chemicals and additives to the rubber compound during the mixing process. What actually happens when the rubber is ignited is that it emits gases that effectively starve the source of the flames of oxygen, thereby extinguishing the fire.

If poor quality and/or insufficient amounts of the additives are used in the rubber compound then the ability to self-extinguish is slower and less effective. The time it takes for the belt to self-extinguish is enormously important because the conveyor is doing what it is meant to do, which is to convey at speed; only this time it is conveying fire. This means that literally every second counts.

There are numerous safety classifications and international standards for which there are many different tests used to measure the self-extinguishing properties of conveyor belts. The basis of virtually all fire testing for belting used in the cargo industry is ISO 340.

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