Integrating Torrefaction in the Wood Pellet Industry: A Critical Review

Abstract

As an established bioenergy industry, the global wood pellet sector is ideally positioned to adopt biomass torrefaction technologies. In this review, potential benefits and challenges of integrating torrefaction within the pellet industry are reviewed and the required technological interventions to enable this integration are analyzed. It is apparent that integration would accelerate the commercialization of biomass torrefaction and benefit the wood pellet industry, torrefaction developers, as well as coal-based utilities. Torrefied pellets are expected to have distinct applications in power generation and help coal-based utilities to reduce their emission levels and comply with increasingly stringent regulations. Co-firing coal with black pellets requires little-to-no retrofit of the existing power plant and, therefore, can offer a low-cost solution. Two potential pathways, in either an upstream or downstream configuration, for retrofitting torrefaction within a white pellet facility are assessed. A successful upstream integration can potentially produce highly dense and durable torrefied pellets. However, the current limitations with this approach are (a) greater reactor volume required for torrefying wood chips and consequently high capital expenses (CAPEX); (b) difficulty in densification; (c) frequent maintenance requirements due to the abrasive nature of torrefied biomass; and (d) safety concerns from fine generation. Torrefaction as a downstream operation, wherein white pellets are subsequently torrefied to produce black pellets, has the benefit of being a simple bolt-on integration. Downstream integration eliminates the need for additional grinding and pelletizing capacity, while minimizing plant-wide contamination due to dust generated from the processing of torrefied biomass. Furthermore, CAPEX can be reduced, as a typical torrefier has significantly higher throughput when processing pellets compared to wood chips. The limitation of downstream integration includes the potential compromise in pellet quality, especially a loss in strength and density. While the benefits of integrating torrefaction are numerous, the techno-economics for each pathway should be further examined.

Physical and Compression Properties of Pellets Manufactured with the Biomass of Five Woody Tropical Species of Costa Rica Torrefied at Different Temperatures and Times

“…However, pellet properties had been determined for similar advice used by Mani et al [50], Rhen et al [51], and Carone et al [33]; they had demonstrated the reproduction of the conditions of the industrial process of pellet fabrication, and they can be implemented in an industrial application [52,53]. …”

Influence of torrefaction after densification on the fuel characteristics and the inherited gasification kinetics of Erianthus arundinaceus energy grass

“…However, the bulk density of the torrefied biomass is low, even lower than the raw biomass itself, which makes the transportation and handling economically challenging . Consequently, integrating torrefaction with densification process such as pelletizing can solve challenges in storing and transporting biomass and facilitate a feasible use of the torrefied biomass as bioenergy commodity . In this context, the first pathway involves torrefaction followed by pelletizing process (TOP process) .…”

“…Here, pelletizing power consumption is increased due to the reduced water content and the brittle nature of the torrefied biomass while the grinding power consumption is decreased when it is used as pulverized solid fuel . In the second pathway, pelletizing is followed by torrefaction (TAP process) . In this process, pelletizing of raw material is possible without adding binding agents, the torrefied pellets stay intact and could facilitate handling, feeding, and safe torrefaction operation .…”

“…11 Consequently, integrating torrefaction with densification process such as pelletizing can solve challenges in storing and transporting biomass and facilitate a feasible use of the torrefied biomass as bioenergy commodity. 12 In this context, the first pathway involves torrefaction followed by pelletizing process (TOP process). 13 In this process, using a binding agent is necessary due to the weakening of bonding forces between biomass particles and the loss of natural bonding characteristics of lignin after torrefaction.…”

Properties of Torrefied U.S. Waste Blends

“…Although torrefaction technology is well developed, it has not yet moved to the commercial market. The consensus is that the main hindrance to the commercialization of this technology is the use of high-cost woody biomass as a feedstock (Kumar et al, 2017;Radics et al, 2017).…”