Waste wood as a substitute fuel and alternative to coal

Substitute fuels drive climate-friendly energy production

Sun, wind and water – these three energy sources are largely synonymous with renewable fuels. But there is another sustainable energy source with largely untapped potential: waste wood. The term describes wood that has already been used for other purposes, for example to build houses or models.

The use of waste wood in coal-fired power plants is becoming hugely important in our current age. But experience has shown that coal-fired power plants place strict requirements in the alternative fuel when converting from coal. The criteria extend beyond merely the calorific value, the ash melting behaviour or the chemical composition. Other key aspects include availability on the marketplace, suitability for storage, processing or grindability, as well as consistency and particle size distribution. The availability of machinery is also a vital factor. Production waste from the wood-processing industry can only be integrated economically and cost-efficiently to create fuel for current power plant processes if we manage to create similar properties to coal through processing the fuel.

Energy production using solid fuels: of grate firing and dust firing

The type of fuel vessel has a major influence on the various treatment steps. Two methods are used in this context: the grate firing and dust firing methods.

Grate firing describes a process using solid fuels that are placed on a grate and then exposed to air flow. It is mostly sufficient in this case to shred the various biogenic fuels coarsely. Power systems based on grate firing can be operated using wood, bark, peat, coal or biomass.

In the case of dust firing, it is necessary to prepare the material as finely and homogeneously as possible for pneumatic transport to the burner. The particle belt should be as narrow as possible to ensure even firing in the vessel and a low level of slagging caused by unburned material. Fine grinding is therefore necessary for dust firing.

At present, coal is predominantly used in the dust firing method. Grinding modules by Tietjen have been used for many years to treat suitable production wastes from the wood-processing industry.

The potential of waste wood in the production of energy

Waste wood is a coveted resource and always accrues when wooden products have reached the end of their service lives. It is mainly obtained from construction sites, demolitions and conversions, as well as from packaging and shredded waste whose pollutant content satisfy the guide values for material recycling or for waste wood firing.

Waste wood is usable and therefore does not require disposal. Instead, it represents, in the right category, a valuable resource for manufacturing industry and a raw material for the production of energy. Waste wood is divided into the following classes

A1 for untreated wood
Pallets, for instance, are a source of untreated wood with little contamination.

A2 and A3 for treated wood
This material is obtained from interior environments. Waste wood in classes A2 and A3 usually comes from inside doors or work surfaces.

A4 for treated wood used outdoors
A lot of waste wood accumulates on building sites and during demolitions, renovations or conversions. It comes mainly in the form of structural timber (e.g. roof trusses) and impregnated timber (e.g. lattice fences, windows). Energy can also be obtained from this waste wood, provided it is processed suitably.

After pre-grinding and metal separation, waste wood in class A1 is usually suitable for grate firing. It can also used in dust firing methods after a process of fine grinding. Hammer mills by Tietjen are often used in these settings. The aim is harness the complete energetic potential of waste wood as a raw material.

Waste wood in classes A2 and A3 can also be used in the same way, provided they posses the same combustion characteristics.

Plant design for grinding waste wood with the VDK high speed mill

The VDK high speed mill is a completely versatile system and particularly suitable for achieving particularly fine grinding results. The special 6-axis rotor design delivers an optimum beater configuration and guarantees maximum material contact for the efficient production of finest qualities in the wood industry.

We redesigned and configured the processing of production waste, returns and waste wood in class A1 on behalf of a partner in the wood industry. Production waste accumulates in the cutting process. In the past, leftovers were transported to a central shredder by hand. This manual process is no longer necessary with our new plant design. The production waste is shredded directly using two shredders on the automatic sawing machines and transported pneumatically to the mill feed container. A third processing line shreds the returns and waste wood. This material is also transported pneumatically to the storage container. It is then ground on the high speed mill – the plant’s centrepiece – and transported pneumatically into the feed silo for introduction to the burner. The entire system is designed to be pressure-shock resistant and flame-proof; in addition, the pneumatic delivery lines are equipped with spark detection and extinguishing systems.

To the VDK product page

Obtaining energy from wooden pellets using the GDL big chamber mill

Over the next two years, we will collaborate with a partner to upgrade a large thermal power plant in Europe with new burners and the necessary grinding lines. The aim is to use wooden pellets for the production of energy. 

Power plants have successfully ground wooden pellets as an exclusive or substitute fuel to coal for many years. The fineness before pelleting process is crucial here, as the pellets are often merely dissolved on the coal mills. Many of our grinding lines are operating in the wooden pellet industry and achieve the necessary degrees of fineness. But there are scenarios in which the fineness is not adequate for the burner manufacturer. The pellets are ground on a Tietjen GDL 12–25 big chamber mill in these cases. Characteristic features of these machines include high availability, excellent particle belt distribution and low energy consumption.

To the GDL big chamber mill product page

From waste wood to energy

Waste wood is a special resource that needs to be harnessed. Using manufacturing waste from the wood-processing industry in coal-fired power plants is an important step towards needs-based and climate-friendly energy production. “Recycling of waste wood in all classes and quantities is close to our hearts, as this raw material is an important source of energy that must not be dumped in landfills.” It can be used to generate green electricity and heat. “We are careful to ensure perfect processing”, explains Thomas Runde, managing director at Tietjen.

Burning wood only releases as much CO₂ as the plant obtained from the atmosphere during its growth process. Here, selecting the right processing for these waste-based substitute fuels is essential to enable their economical and cost-efficient integration within current power plant operations. Tietjen grinding modules have been used successfully for many years in these scenarios. We are delighted by every single project to harness the energy of waste wood as an important resource and work tirelessly to improve our grinding plants for the timber industry. If: the better the grinding, the higher the yield.