Goals and scope of task
Two types of thermal pre-treatment processes were developed and tested to improve the overall chain from energy crops to solid bio-energy carrier. The hydro-thermal pre-treatment TORWASH was used with grassy energy crops because of their relatively high moisture and alkali content. Torrefaction, another thermo-chemical pre-treatment route suited for drier types of biomass was tested on woody energy crops, with relatively lower moisture and alkali contents. Both TORWASH and Torrefaction can improve the supply chain of biomass, by increasing the energy content of the fuel, and improving its fuel characteristics. This leads to the production of sustainable solid bioenergy carriers. In addition, TORWASH can reduce the salt content of herbaceous biomass.
The overall objective of this task was to test the behaviour of the feedstocks used in LogistEC during TORWASH and Torrefaction, and to evaluate their suitability as raw materials for the production of solid bioenergy carriers.
Both TORWASH and torrefaction were found to improve the fuel characteristics of biomass, for all the feedstocks tested, by increasing the energy density, while simultaneously making the material more brittle and easier to mill.
Detailed results can be found in project Deliverable D3.4 – while a summary is provided below.
Briquetting and pelletization of thermally pre-treated material was also conducted. The results are reported in Deliverable D5.4 (pp. 79 – 83) – while the main conclusions are given below.
Main findings from pre-treatement and briquetting trials
Miscanthus, Fibre sorghum, Triticale and Arundo donax (giant reed) were subjected to washing and TORWASH conditions in order to identify the differences between these biomass feedstocks. A similar set of conditions were used to identify any differences between Miscanthus samples grown under different conditions. Two selected types of biomass were evaluated at 2-liter scale, after which a series of 20-liter tests were performed to analyse all inputs and outputs, and to produce a liquid TORWASH residue to perform fertilizer tests.
The experiments with Miscanthus and Arundo donax demonstrated the highest solid matter recovery. Washing tests remove already a significant amount of salts from the biomass. The 20-liter tests showed that Arundo donax can be converted to a solid bioenergy carrier that complies with the ENplus A1 standard, which is the most stringent standard for biomass pellets. The extent of ash removal was such that the fuel can be used in any thermal conversion process without the need for any further treatment.
Exploratory torrefaction tests using thermogravimetric analysis (TGA) were performed using a total of 10 different samples from various species of biomass: Poplar, Willow, Eucalyptus, Triticale and Fibre sorghum. Significant differences were observed between non-woody biomass (Triticale and Fibre sorghum) and woody biomass, which means that the non-woody biomass must be torrefied at a different temperature than the woody biomass.
Batch tests at kilogram scale were performed using Triticale, Fibre sorghum and Willow. As expected, an increase in torrefaction temperature yielded an increase in energy content of the solid bioenergy carrier produced. The herbaceous biomass samples showed relatively high ash contents in excess of 5 wt%, but also Willow, containing also small branches and bark, showed a relatively high ash content of 2 wt%. Clean wood typically has an ash content below 2 wt%.
Regarding the densification of torrefied material by briquetting,the following conclusions applied after tests were run on willow and triticale:
- Willow is a suitable material for torrefaction and subsequent briquetting. A high die temperature is preferable. Power consumption is lowered and briquette quality increases
- Briquetting of willow was achieved at 101-110 kWh/t power consumption and a die temperature of 215°C.
- Willow briquettes were produced with a specific density of around 1200 kg/m3 and a bulk density of about 600 kg/m3
- Torrefaction temperature variance (250°C and 260°C) did not have a significant effect on performance or quality of the briquettes of willow.
- Briquetting of triticale pellets was not possible even tough different parameters were tested.