Pelleting torrefied biomass at pilot-scale – Quality and implications for co-firing

David A Agar,Magnus Rudolfsson,Simon Lavergne,Thierry Melkior,Denilson Da Silva Perez,Capucine Dupont,Matthieu Campargue,Gunnar Kalén,Sylvia H Larsson

doi:10.1016/j.renene.2021.06.094

Abstract

The co-firing of solid biofuels in coal plants is an attractive and fast-track means of cutting emissions but its potential is linked to biomass densification. For torrefied materials this topic is under-represented in literature. This pilot-scale (121–203 kg h−1) pelleting study generated detailed knowledge on the densification of torrefied biomass compared to untreated biomass. Four feedstock with high supply availability (beech, poplar, wheat straw and corn cob) were studied in their untreated and torrefied forms. Systematic methods were used to produce 180 batches of 8 mm dia. pellets using press channel length (PCL) and moisture content (MC) ranges of 30–60 mm and 7.3–16.6% (wet basis) respectively. Analysis showed that moderate degrees of torrefaction (250–280 °C, 20–75 min) strongly affected pelleting behaviour. The highest quality black pellets had a mechanical durability and bulk density range of 87.5–98.7% and 662–697 kg m−3 respectively. Pelleting energy using torrefied feedstock varied from −15 to +53 kWh t−1 from untreated with increases in production fines. Optimal pelleting MC and PCL were reduced significantly for torrefied feedstock and pellet quality was characterised by a decrease in mechanical durability and an increase in bulk density. Energy densities of 11.9–13.2 GJ m−3 (as received) were obtained.

Highlights

The push to limit global temperature rise to one and a half degrees above pre-industrial levels signals the phase out of coal within a generation [1]
The durability (DU) and bulk density (BD) of produced pellets as a function of feedstock moisture content are presented for the four investigated feedstock (Fig. 2)
Returning to the question posed by the International Energy Agency (IEA) report: What needs to be understood in order to achieve a good quality product when pelleting torrefied lignocellulosic biomass? The results have shown that moderate degrees of torrefaction (250e280 C, 20e75 min) had a large influence on pellet durability yet a large effect on bulk density

Summary

Introduction

The push to limit global temperature rise to one and a half degrees above pre-industrial levels signals the phase out of coal within a generation [1]. The realisation of European Union energy policy, for instance, will cut emissions by 40% by 2030 [2] and China has committed to carbon neutrality by 2060 [3]. With such short time spans, sustainable transitional fuel use in existing coal power plants makes a good economic argument. Bio-coal technology takes the second approach and takes advantage of existing pulverised-fuel power plants, which are plentiful [5,6] This is the primary application of torrefied pellets; to co-fire them with coal in existing power plants. Large reductions in net carbon dioxide emissions are enabled [7,8] because energy sector emissions make up about one third of anthropologic emissions [9]

Objectives

Methods

Results

Conclusion