Text by Prof. Jean-Bernard Michel (HEIG-VD) and Ms. Claire Tansley (Granit Technologies SA)
Biomass torrefaction has the potential to bring significant additional biomass resources to the market and thus provides business opportunities in fuel production and utilization. Previous work done at HEIG-VD has shown that torrefaction of wood and low cost trimmings and forest residues yields a solid fuel having improved storage and combustion properties. The Industrial Bioenergy Systems group of the HEIG-VD has been conducting pioneering R&D in the field of torrefaction at the small scale since 2008 when they completed a first study on the combustion and lifecycle evaluation of torrefied wood pellets. The work has been reported elsewhere and the main conclusions of this theoretical and experimental study where that:
- There was no need to adjust the operating conditions of a boiler designed for normal pellets. The combustion behavior of the torrefied pellets was found very similar to that of the normal pellets with an improvement in the combustion characteristics (warm-up period, thermal efficiency)
- The overall life-cycle-impact from wood harvesting to useful energy production may be reduced by 50% as compared to normal pellets.
In 2012, a new project was started aiming at the design and construction of a 500-600 kg/h demonstration plant using biomass residues, which are otherwise incinerated or used for compost.
At first, laboratory experiments were carried out on a small scale batch reactor (above, left) with 500 g samples of various biomass types and varying operating conditions (flow temperature, heating time and biomass residence time). Continuous torrefaction runs were then carried out in order to define the effect of various parameters on the mass loss of raw biomass and the resulting calorific value and other main properties of the torrefied material. The maximum increase of HHV of 28% was obtained for wood chips at 245°C temperature during 13 minutes. Low value biomass resources such as garden trimmings were also successfully upgraded by torrefaction. The effect of chip size was not found significant. An innovative pilot plant aiming at the torrefaction of woody residues was then designed, developed and commissioned in Orbe at the Environment Technopole TecOrbe. The so-called TORPLANT project (2013-2016) received financial support from the Swiss state of Vaud within the framework of their program “100 millions for renewable energy and energy efficiency”. The project aims at upgrading various local sources of biomass residues in order to produce pellets, heat and electricity, based on the concept developed by HEIG-VD. Granit Technologies, the HEIG-VD and Ökozentrum are partners in the construction and exploitation of the system:
- Granit Technologies SA is in charge of the development, engineering, construction, and commissioning of the pilot-plant.
- Ökozentrum, is responsible for supplying the heating system using a FLOX burner coupled to a gas-turbine/compressor to recover thermal and mechanical energy from the torrefaction gases.
- The University of Applied Sciences Western Switzerland is leading the project and is in charge of pilot plant testing with the support of the other partner.
The installation, having a nominal output capacity of 100 kg/h, is designed to process locally produced biomass residues which are currently not integrated into recovery chains such as: wood waste from logging, green waste from the waste collection centres, and waste from anaerobic digestion units. The plant can run in automatic mode with remote supervision in order to reduce the cost of production of the solid biomass fuel. The pellets produced from torrefied biomass have an energy density that is one third more in comparison to the classical pellets. This enables a reduction in the storage and delivery costs. In addition, the global energy requirement for the process is reduced due to the very effective recovery of energy from the gases produced during the process (Torgas). Economic calculations have shown that a commercial plant of small capacity (500 kg/h output) can achieve good profitability and a ROI of 2-3 years with the current cost of biomass residues and fuel prices in Switzerland.The commissioning of the pilot plant took place in May 2016. Necessary adaptations have been made to the plant to ensure safety during automatic operation. Test runs are expected to continue until December 2016; these runs are organised in 4 day, 24h/24 operation. The tests will consist in recording all input, process, and output parameters thereby enabling the calculation of the energy efficiency of the various stages, in particular (not exhaustive):
- Raw Biomass supply with a conveyor belt
- Grinder to produce smaller size chips (<30 mm)
- Tumbling dryer heated both by thermal oil and Torgas combustion gases
- Flox® burner and gas turbine/compressor system
- Thermal oil heat exchanger.
In addition, several combustion tests to evaluate the torrefied products as a substitute fuel for biomass chips and pellets will be carried-out on various types and sizes of boilers and stoves.