Torrefied versus conventional pellet production – A comparative study on energy and emission balance based on pilot-plant data and EU sustainability criteria

Torrefaction is an emerging technology which enables greater co-firing rates of biomass with coal. To date however there has been a lack of real production data from pilot-scale torrefaction plants. Without such data any environmental benefits of torrefied pellet production are difficult to quantify.

In this study data on consumable inputs from a semi-industrial torrefaction plant and the physical properties of produced pellets are used to analyse energy input and air emissions of torrefied pellet production and product transport. EU sustainability criteria are used to compare CO₂-equivalent emissions from torrefied and conventional pellet production starting from harvesting of logging residues to end use of co-firing the pellets with coal. A production scenario is examined in which raw material supply and production takes place in Finland with co-firing in Spain. The influence of transport distance (by lorry, rail and ship) on environmental impact is analysed for European and representative North American production sites.

Results indicate about 4.5 GJ of energy is consumed for each tonne of torrefied pellets compared to 3.3 GJ for conventional pellets; representing 23% and 21% of the energy content of the pellets respectively. If electricity from renewable sources can be used, the fossil fuel fraction of input energy can be reduced to 37% and 41% respectively. Production and use of both pellet types in co-firing have similar environmental impact generating CO₂-equivalent emissions from electricity in the range 43–45 g MJ⁻¹. An emission savings of 77% can be realised by co-firing torrefied pellets with fossil coal. Pellet production amounts correspond to 4.3 and 4.2 MJ generated electricity for each kilogram of feedstock (dry mass) and co-firing ratios (energy basis) in the range of 1.4–1.8%.

If only one mode of product transport is used torrefied pellet production and co-firing is found to generate fewer emissions than its conventional counterpart when transport distance is more than 400 km (lorry), 1850 km (rail) and 25,500 km (ship).