Development of product, safety and emission characteristics of pellets

Aim and goal

The overall objective of the project is to develop new knowledge and contribute to the development of new technologies for storage, combustion, drying and pelleting of established and new raw materials to enable the pellet and power industries to meet future product quality and emissions from storage and combustion and reduce the energy requirements of pellet production.

Specific goals are:

• To minimize / prevent self-heating and emissions during large-scale storage (industrial storage) through changes in production conditions and additives.

• Test whether selected additives can be used without significant (negative) impact on full-scale combustion.

• Document combustion disturbances in small-scale pellet boilers.

• Extend the available basic knowledge of the impact of the physical pellet parameters on ignition, burning behaviour and coke structure.

• Suggest measures in physical pellet quality and / or combustion systems with potential for reducing combustion disturbances.

• Verify a new full-scale drying technology and show how the quality of the mechanically dewatered material varies depending on the season, composition and incoming moisture content, and how the pellet quality is affected by the drying technique.

Challenge

Pellets are an advantageous and environmentally-friendly solid biofuel because of high energy density and better-defined qualities compared to many other solid biofuels, such as wood chips and forest residues.

Emissions and self-heating

In the storage/transport of biofuel pellets, both in industry and at consumers, there are problems with heat generation (self-heating) and odourless degassing (CO, CO2, CH4) and aldehydes (VOC) that smell strongly. This can lead to discomfort, poisoning and self-ignition with a high risk of personal injuries and deaths, and large fires that are difficult to extinguish, shutdown in production and high costs as a consequence. There are several different suggested causes for these phenomena, but the most important appear to be different auto-oxidative processes in the wood raw material and moisture absorption from ambient air. Results from previous research show that you need to have more control over different parameters, such as production parameters, to be able to say with certainty what is the most important for differences in the tendency to self-heat between different types of pellets. One major problem with self-heating is that there are many different parameters that could affect the final self-heating behaviour of a pellet.

Pellet quality and combustion

Pellets from different raw materials and with different manufacturing conditions get different hardness. The problem that is experienced about hard pellets is one of the industry's biggest problems. Previous research suggests that the hardness of the pellets should originates from either the combustion system or the ignition phase.

New drying technique

During pellet production, raw material with about 10 % moisture content is required. Fresh wood chips with a moisture content of about 50 % are dewatered today by thermal drying to a moisture content of about 10 %. Drying is very energy consuming process and accounts for about 30 % of the pellets price, which strongly limits the economy of pellet production. The most common type of driers used for pellet production is drum dryers. This is a proven technique where hot gases dry the material and the evaporated water vapor is transport away. Belt dryer is another commonly used technique whose advantages are that it is simple in its construction and can use a low-temperature energy source below 150 °C. In a pneumatic dryer there is a simultaneous drying and transport of sawdust. However, the pneumatic dryer is unsuitable for sawdust with high moisture content.

New patented technology that enables wood chips and sawdust to be pressed under high pressure without moisturizing allows up to 50 % of the water to be dewatered in a few seconds to a fraction of the energy cost compared with thermal drying. This technique can be used as a pre-treatment before the material is transported to a conventional dryer. It is therefore very important to test and validate this new drying technique in an industrial environment and to investigate how this technique affects pellet quality in full scale production.

Solution

RISE Fire Research has in its research developed a method for studying self-heating in lab scale using isothermal calorimetry (micro calorimetry). This makes it possible to study the impact of different production, drying and storage parameters, which can then be used for verifying relevant parameters in full scale. This method is therefore very suitable for answering a lot of questions that have not be answered in the past. Factors that affect emissions of VOC, carbon monoxide, carbon dioxide and methane, and reduction of oxygen in connection with storage of pellets will be identified. Furthermore, the correlation between self-heating and emissions during storage of pellets will be thoroughly investigated.

The project will investigate how permitted variations in the European pellet standard, with respect to density and mechanical strength, impact combustion, with specific focus on circumstances typical of the ignition process in small-scale combustion equipment.

The project will test and validate a new patented technology that allows wood chips and sawdust to be mechanically pressed under high pressure without moisturizing. This allows up to 50 % of the water to be removed within a few second to a fraction of the energy cost compared with thermal drying. This will be done in an industrial environment, to investigate how this technology affects pellet quality in full scale production.

Effect

The research will generate new knowledge of the basic factors/mechanisms that affect the combustion properties and emissions and self-heating during storage of pellets. The applied research will use results from basic research to develop new knowledge that is used to significantly improve existing products and processes as well as reduce storage problems and emissions when burning pellets. The experimental development will verify whether newly developed drying technology meets the results obtained in pilot scale.

The utility for the client and society is a more efficient and safer production and handling of biomass pellets. This means benefits economically.