1. Introduction
Since the end of 19th century, the earth’s average surface temperature has increased by around 1.8 °C [
1]. This global warming is mainly due to the increase of greenhouse gases content in the atmosphere, which has been linked to the extensive usage of fossil fuels. Today, our energy systems are largely dependent on fossil resources. Among other fossil fuels, coal plays a major role in global energy production [
2]. According to IEA energy statistics, global energy production from coal in 2019 was around 163 EJ, i.e., 26.8% of the total [
3]. In the same year, the share of coal in the global CO
2 emission production was around 44% [
3]. At a regional level, the role of coal in energy production can be much higher than the global average. For example, in China, more than half of the primary energy is still produced from coal [
4]. Realizing the role of coal in greenhouse gas emissions, under COP26, several countries agreed to phase out its use from their energy production [
2]. One alternative to coal could be biomass. However, the characteristics of biomass fuel are not comparable to those of coal [
5]. Thus, there is a need to pretreat biomass prior to its application in energy production; torrefaction is one such pretreatment. Torrefaction is a mild pyrolysis which is carried out in the temperature range of 200–300 °C [
6]. Although torrefied biomass pellets can be compared with coal in terms of their fuel characteristics, they are not yet economically competitive. The main reason for this is the high price of the woody biomass. According to Doddapaneni et al. [
7], the raw biomass itself accounts for 80% of the total production costs of torrefied pellets. Thus, producing torrefied pellets from low cost or no-cost organic residues could be helpful to improve the economic feasibility of the torrefaction process.
On the other hand, pulp industries produce large quantities of sludge during wastewater treatment, commonly known as primary, secondary and tertiary sludge, depending on the stage and waste water treatment approach. Pulp industry sludge contains undigested wood fibers, fines, fillers, clays, plastics, and metals, depending on the type of pulping process, i.e., virgin fibers or reused waste paper [
8,
9]. Because of strict environmental regulations, sustainable methods to handle such large quantities of sludge are needed. Today, pulp industry sludge is disposed of through composting, landfill combustion, agricultural field application, and anaerobic digestion [
10]. Considering its organic fraction and availability in large quantities, alternatively, pulp industry sludge could be used as a raw material for the production of torrefaction pellets and, subsequently, in energy production.
Previously, Mondoza Martinez et al. [
11] studied the hydrothermal carbonization (HTC) of chemical and biological pulp mill sludges and reported that the physiochemical properties of the pulp sludge improved with HTC treatment, where dehydration and demethylation were the dominant reaction pathways. In the same study, the authors observed that the heating value of the sludge increased by 1 MJ/kg and 5 MJ/kg for primary and biological sludges, respectively. Recently, Liu et al. [
12] studied the applicability of paper mill sludge char as a catalyst to improve the quality of pyrolysis gas; those authors observed that the Ca present in pulp sludge had a significant effect on the composition and energy content of the pyrolysis gas. Tarelho et. al. [
13] studied the slow pyrolysis of biological sludge from pulp from the paper industry and reported a biochar, dry sludge yield in the range of 0.40 to 0.73 kg/kg. Grimm et. al. [
14] studied the co-combustion of pulp industry chemical sludge and Scots pine bark and observed increasing NO
x and SO
2 emissions in the flue gas and reduced ash slagging and fouling tendency. Latva-Somppi et al. [
15] studied ash related issues during pulp sludge combustion in an industrial scale bubbling fluidized bed and circulating fluidized bed. They concluded that pulp and paper industry sludge can be combusted in fluidized beds at a temperature range of 800–900 °C. Huang et al. [
16] studied the torrefaction of pulp sludge from a pulp factory in Taiwan and observed that torrefaction treatment improved the fuel properties of the sludge. The authors reported that the heating value of pulp sludge increased by 25% following 60 min. of torrefaction at 300 °C.
A considerable number of studies are available on the combustion and pyrolysis of pulp sludge, but very few are available on low temperature thermal treatments such as torrefaction, hydrothermal carbonation, and hydrothermal liquefaction. Notably, not much data is available on the torrefaction of pulp industry sludge, and to the knowledge of the authors, only one study is available on the torrefaction of pulp industry sludge. Thus, the main aim of the present study is to understand the influence of torrefaction treatment on the fuel characteristics of pulp sludge. Torrefaction experiments were carried out using a continuous torrefaction reactor setup at 250, 275 and 300 °C. The influence of torrefaction on the chemical composition of biomass fibers is also discussed. Pulp sludge is known for its high ash content. Thus, the ash melting behavior of the dried and torrefied sludge was also studied. Finally, a SWOT analysis on the torrefaction of pulp industry sludge is presented.
4. Summary
Our study clearly showed that torrefaction treatment improved the fuel characteristics of the pulp industry sludge. In order to give a comprehensive perspective of the opportunities and challenges of pulp sludge torrefaction, a SWOT analysis is presented in
Figure 5. In the authors’ opinion, producing torrefied pellets from pulp sludge could be a win-win strategy for both the torrefaction and pulp industries. For the torrefaction industry, the production cost of torrefied pellets could be reduced compared with the use of woody biomass because of the reduced material costs. For the pulp industry, sludge can be handled in an effective way and ultimately, the resource efficiency and sustainability of the industry could be significantly improved.
In this study, it was observed that torrefied pulp sludge has better fuel characteristics compared with other biomasses, especially in terms of its energy density and ash melting behavior. Although the studied pulp sludge contained more ash, the concentrations of problematic compounds, such as chlorine, phosphorous, and potassium, were lower in comparison with those of other biomasses, especially agricultural waste, which is also an organic residue that, like pulp sludge, is available at low cost.
Although the torrefied sludge showed better fuel characteristics, there could also be some challenges. The authors foresee two immediate difficulties, i.e., the high energy input requirements during drying and the high ash content. As the sludge contained around 20% dry weight, drying requires higher energy input compared with biomass. Alternatively, pulp sludge, after mechanical dewatering, can be air-dried in subtropical and tropical regions. However, air-drying may not be practically possible in other regions. The excess heat energy produced in pulp mills could also be used to dry the sludge. It is worth noting that at present, industrial energy recovery processes are well established, and thus, part of the required energy could be recovered from drying volatiles. The higher ash content of the torrefied sludge could create operational issues and increase ash handling costs. However, torrefied sludge is comparable with low grade coals such as lignite, which have been used in power plants for a long time. At the same time, high ash agricultural waste, like straw, is already being used for energy production in countries like Denmark. Thus, the authors believe that torrefied pulp sludge could be effectively used in existing power plants.
The authors also want to stress that the properties of pulp sludge vary significantly based on the operating conditions from which the material is sourced. As such, the properties of the torrefied sludge would also vary. At the same time, future studies are required to determine the overall feasibility of the torrefaction of pulp sludge. Although the theoretical indices showed moderate values for ash fouling and slagging, there is a need for the experimental evaluation. The energy balance of the overall process needs to be established. These will be the topics of our future study.