Review pubs.acs.org/EF
Integrating Torrefaction in the Wood Pellet Industry: A Critical Review Linoj Kumar,† Alexander A. Koukoulas,*,‡ Sudhagar Mani,§ and Jagannadh Satyavolu∥ †
Herty Advanced Materials Development Center, Savannah, Georgia 31408, United States ANL Consultants LLC, Savannah, Georgia 31410, United States § University of Georgia, Athens, Georgia 30602, United States ∥ Conn Center for Renewable Research, University of Louisville, Louisville, Kentucky 40208, United States ‡
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.
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coal.7 As a result, co-firing biomass with coal requires greater volumetric flow rates to achieve the equivalent energy output. The transport capacity of conveyors and reclaimers for biomass must be multifold larger than that of coal. In addition, crossing points, chutes, and openings must be designed to minimize the potential plugging of fibrous biomass. Size variation and fiber content add complexity to flow properties and complicate handling. The fibrous structure of biomass makes it difficult to copulverize with coal and obtain similar particle size distribution.8 Moreover, the inherently high hygroscopic nature of biomass restricts outdoor storage.9 It is apparent that the existing infrastructure available within a coal fired power plant can only be used if the energy density, uniformity, hydrophobicity, flow and grinding properties, and other processing characteristics of biomass are substantially improved and made closer to those of coal.7,10 Torrefaction can overcome the heterogeneity among different types of cellulosic feedstock, thus producing a uniform-quality energy carrier with coal-like properties.11−15 Torrefaction, a thermal treatment of biomass at mild temperatures typically ranging between 200 and 300 °C in an inert environment,
INTRODUCTION Renewable energy is projected to be the fastest growing energy sector, as it has the ability to meet the ever-increasing global demand for energy while minimizing the consequences of carbon emissions that contribute to climate change.1,2 Among the emerging alternatives (e.g., solar, wind, and biomass), biomass is an attractive option as a sustainable carbon carrier3,4 that can replace fossil-based coal. In the context of increasingly stringent EPA (Environmental Protection Agency) regulations, biomass co-firing has been recognized as a means to extend the life of close to 100 GW of “soon-to-be-retired” coal plant generating capacity in the United States.5 Similarly, the European Union’s target of reducing greenhouse gas emissions by 40% by 2030 is driving large scale initiatives for biomass combustion and cofiring in Europe. Despite its potential, there are several challenges for the effective use of biomass as a substitute for coal. For example, forest biomass, such as logs and chips, sawdust, shavings, bark, understory biomass, and deadwood, as well as various types of residues from agricultural crops, presents many logistical and conversion challenges. Low density, high moisture, and the heterogeneity of cellulosic biomass make on-site delivery, storage, and handling substantially more complex as compared to coal.6 Due to its relatively low bulk density and heating value, the energy density of biomass is approximately one-tenth that of © XXXX American Chemical Society
Received: October 26, 2016 Revised: December 6, 2016 Published: December 12, 2016 A
DOI: 10.1021/acs.energyfuels.6b02803 Energy Fuels XXXX, XXX, XXX−XXX
Review
Energy & Fuels
An efficient approach to accelerate the commercialization of torrefaction technologies is to integrate torrefaction within an existing bioenergy value chain. The wood pellet industry, being an established modern bioenergy industry, represents an ideal avenue for integrating torrefaction. The demand for low-carbon fuels, especially wood pellets in Europe and other regions, and concurrent advancements in the wood pellet industry have created a global biomass energy industry, with the United States now the world’s largest exporter of wood pellets.33−37 White pellets are typically produced by pelletizing pulpwood-quality wood chips and sawmill residues.33−36 Pelletization improves flow and handling characteristics, reduces shipping costs, and renders the effective processing and storage of biomass.33 Wood pellets with premium quality (90 More hydrophobic Minimal; Classic milling similar to coal
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