Torrefaction of lignocellulosic residues

Challenges

Using wood on an industrial scale offers a great opportunity to replace crude oil and natural gas. The challenge is coming up with innovative ways to treat biomass so as to allow for sustainable products and production processes. Normally, forestry waste, weak wood and other inferior wood is ground into wood chips, which are an important feedstock for the wood processing industry. They are also used as fuel in thermal power plants and woodchip heating systems. However, the high moisture content of fresh wood chips presents two problems: it is a lot of weight to transport, and it is difficult to store the damp cargo. If it is not protected from rain, the material quickly rots.

A number of problems need to be solved for the industrial use of biomass, including aspects of material quality, stability, storage and energy density. This is why Fraunhofer IGB has developed an innovative technology, e.g. within the framework of the EU SteamBio project, which addresses all of these problems by offering efficient treatment of biomass residues and at the same time an alternative to the production of green chemicals. The technology is not limited to biomass products, but can be used for various solid residues such as manure, fermentation residues or sludge.

Our solution

For the improved energetic and material use of lignocellulose-containing biomass, we have successfully demonstrated torrefaction up to pilot scale. The technology consists in drying or torrefying solid residues with superheated steam (SHS) under atmospheric pressure. The material to be dried or torrefied is introduced to the SHS atmosphere where it is supplied convectively with heat and its moisture content evaporates.

Thermal  separation of residual biomass

In the superheated steam atmosphere, and thus with the exclusion of oxygen, the woody material is treated at temperatures of 220–300 °C. After the water present in the material is evaporated, decomposition of lignocellulosic compounds takes place; firstly hemicellulose and then a part of cellulose and lignin. The aim of the torrefaction is to enhance the mass-related energy density and thus the heating value of the raw material, to increase the transport capability and storage stability, and to reduce the mechanical works required for subsequent grinding or pelletizing. The resulting product is considered to be an ideal additional fuel for power stations using coal dust firing as well as raw material for biotechnical refinery for the production of chemical products. Volatile compounds evolving during torrefaction can be pre-separated and further used as feedstock for the production of chemical building blocks.

Applications

• Supply of high energy density and microbiologically stable biofuels
• Supply of green chemicals and fertilizers
• Efficient transport and storage of solid residues
• Treatment and valorization of biomass products
• Treatment and valorization of solid manure / digestate
• Disposal of industrial solid waste

The use of superheated steam drying (SHSD) / torrefaction of biobased raw and residual materials has many advantages over conventional methods with hot air.

• The volatile compounds liberated during torrefaction are primarily recovered in the condensate fraction in a concentrated form for onward transport to a biorefinery.
  
• This technology permits significant energy savings in comparison to the conventional drying process with hot air.
  
• Due to the absence of oxygen there are no oxidative reactions resulting in higher product quality and there is no risk of explosion.
  
• The process is free of sound and odor emissions.

The properties of the obtained solid output are significantly upgraded:

• Reduced volumetric density and increased hydrophobicity for optimal transport and storage
• Improved grindability, allowing reduction in power consumption due to milling
• Increased calorific value 
• Minimized decomposition of cellulose and lignin, enabling lignocellulosic biomass to be exploited in biorefinery processes

We accompany you from the first preliminary investigations through the realization of the developed plant concepts to the commissioning of a plant:

• Scientific characterization and specification, research on tasks related to drying and heat transfer
• Development of specific process and plant concepts according to the individual needs of customers
• Layout and specification of process by an interdisciplinary team with expertise in process engineering, design, chemistry, microbiology and electric engineering
• Laboratory and pilot plants for test trials
• Product-related analysis of the drying process with a wide range of analytic equipment and competences
• Design specification of process unit and components, e.g. by integrated combination of 3D CAD design and numerical modeling of fluids, heat transfer etc., with latest software releases
• Assisting and monitoring clients from first test trials up to commissioning of a plant

Not only virtually all of the supplied latent heat can be recovered from the condensed excess steam, but the condensate itself can be used as demineralized water. If the products to be dried contain toxic or expensive organic liquids, superheated steam drying avoids the danger of fire or explosions, as no oxidative or combustion reactions are possible in the absence of oxygen.

Moreover drying with superheated steam allows separation of volatile organic compounds (VOCs) at a condenser. Thus, the environmental problem of emission of VOCs produced during drying is also eliminated and no waste air treatment is required. The organic volatile components are condensed out with the excess steam. Standard separation processes allow recovery of the condensable organic substances, which can be considered as value-added products.