Research PaperRheology of fibre suspension flows in the pipeline hydro-transport of biomass feedstock
Introduction
Fossil fuel-based generating facilities continue to be the primary supplier of energy worldwide. However, adverse environmental effects, as well as issues associated with the secure supply of fossil fuels, have increased the interest in renewable sources of energy (Vakulchuk, Overland, & Scholten, 2020). Among those, biomass often holds a top rank primarily because of its carbon neutrality over its life cycle as well as its utility as a feedstock for direct conversion to fuels and chemicals (Pootakham & Kumar, 2010). However, ‘as-received’ biomass has low bulk and low energy density. As a result, the costs associated with transporting biomass by vehicle can contribute 12–80% to the delivered cost (Vaezi, Nimana, & Kumar, 2015). This is the main barrier towards enabling biomass-based facilities to compete on grounds of capacity and scale with fossil-fuel based plants (Kumar, Cameron, Flynn, 2004, 2005a, 2005b).
Truck delivery is the most common mode of biomass transport. However, there are issues caused by the number and frequency of large capacity trucks (Kumar, Cameron, & Flynn, 2003; Kumar et al., 2005a). Using alternative modes of delivery of biomass could be socially, environmentally, and economically more beneficial (Vaezi, Katta, & Kumar, 2014). Pipeline hydro-transport (the transportation of solids in a liquid carrier stream) is a reliable and economic mode of solid transport at large scales and over long distances and it has recently attracted attention in the bioenergy sector (Kumar & Sokhansanj, 2007; Morey, Kaliyan, Schmidt, & Tiffany, 2009; Vaezi et al., 2015). Research studies on the pipeline hydro-transport of biomass feedstock has demonstrated the overall cost of delivery is less than by truck beyond certain scales and distances (Vaezi et al., 2015). Moreover, Vaezi et al. in various studies (Vaezi et al., 2014; Vaezi & Kumar 2014a, 2014b; Vaezi, Verma, & Kumar, 2018) demonstrated the mechanical and chemical feasibility of the pipeline in transporting agricultural and forest residue biomass.
In the design and operation of pipelines to hydro-transport energy commodities, the rheological and transport properties (density, apparent viscosity, concentration, temperature and velocity) of the solid–liquid flow mixture (slurry) directly impact the size and cost of pipeline components (e.g., pipes, pumps, instrumentation) (Abulnaga, 2002; Wilson, Addie, Sellgren, & Clift, 2006). It is therefore critical to determine the apparent viscosity (for non-Newtonian fluids the ratio of shear stress to shear rate) before the pipeline to hydro-transport solid materials can be specified (Chhabra & Richardson, 2011; Chin, 2001).
Several studies have been conducted to evaluate the effects of biomass slurry concentrations as well as biomass fibre characteristics (size and geometry) on slurry apparent viscosity in biofuel production applications (Ghosh, Holwerda, Worthen, Lynd, & Epps, 2018; Pimenova & Hanley, 2003; Pimenova & Hanley, 2004; Rosgaard, Andric, Dam-Johansen, Pedersen, & Meyer, 2007; Viamajala, McMillan, Schell, & Elander, 2009). The rheological properties of pre-treated corn (maize) stover (plant residue left after removal of kernels) suspensions were experimentally studied by Pimenova and Hanley (2004) over a range of concentrations from 5 to 30 wt% dry-matter. The yield stress of acid-hydrolysed corn stover suspensions was approximated using three different non-Newtonian viscosity models – Bingham (Bingham, 1917), Casson (Casson, 1959), and Herschel-Bulkley (Herschel-Bulkley, 1926). The results indicated that the Herschel-Bulkley model agreed better than the others for the experimental measurements.
The rheology of two different batches of dilute-acid pre-treated corn stover was also studied by Knutsen and Liberatore (2009). Their study showed that pre-treated corn stover suspensions at concentrations of 5–17 wt% dry-matter behave as soft solids and exhibit elastic deformation at low strains prior to yielding. Moreover, strong shear thinning behaviour was reported even at solid concentrations as low as 5 wt% dry-matter.
Viamajala et al. (2009) investigated the rheological characteristics of treated and untreated corn stover using plate–plate viscosity measurement geometry over a range of concentrations from 10 to 40 wt% dry-matter. They reported the behaviour of both treated and untreated mixtures to be shear thinning and the Casson model was used to describe it well. They found the viscosity and yield stress of treated corn stover slurries to be less than those of untreated slurries.
Using a torque rheometry method, Ehrhardt et al. (2010) investigated the rheological properties of acid hydrolysed corn stover slurries at high concentrations (20–30 wt% dry-matter) in batches. The results indicated that acid hydrolysed corn stover slurries behaved as Bingham plastic fluids in which yield stress decreased with increasing acid concentration, rheometer temperature, and hydrolysis reaction temperature but increased with increasing solids concentration. These results demonstrated that the Bingham model was capable of describing the rheological behaviour of acid hydrolysed corn stover slurries.
In other research, Klingenberg et al. (2017) designed and fabricated a rheometer in order to measure the rheological properties of lignocellulosic biomass slurries at high temperatures and solids contents (>25 wt% dry-matter). The results confirmed the shear-thinning behaviour occurred with chopped and milled corn stover-water mixtures and they also demonstrated an inverse relationship between apparent viscosity and temperature.
Research has been carried out on the rheological characteristics of biomass slurries where the apparent viscosity was either measured in a batch using a torque rheometry method (Klingenberg et al., 2017; Novotna, Landfeld, Kyhos, Houska, & Strohalm, 2001; Pimenova & Hanley, 2003, 2004) or where samples were directly made from the batch and tested in rheometers (Bhattacharya & Bhat, 1997; Ehrhardt et al., 2010; Grigelmo-Miguel, Ibarz-Ribas, Martı́n-Belloso, 1999; Knutsen & Liberatore, 2009; Viamajala et al., 2009). In the pipeline hydro-transport of biomass feedstock, however, slurries are pumped over a range of velocities, pressures, and temperatures (depending on the feedstock type, pipe material, design and topography of the pipeline, and the climate) these factors can change the rheological characteristics of the biomass slurries. Hence, it appears that if the sample under test is made from a batch and not in situ, i.e., right drawn off from the pipeline, it will not properly represent the rheological specifications of the slurry that is being pumped through a pipeline. Therefore, in this research, an experimental facility comprised of a 25 m long and 50 mm diameter closed-circuit pipeline was fabricated, the biomass slurry was prepared and circulated, longitudinal pressure drops were measured, and samples were made directly from the pipeline (Vaezi et al. 2014, 2018; Vaezi & Kumar, 2014b). In addition, while all previous research studies have investigated the rheological characteristics of agricultural residue biomass slurries, the present research focused on the slurries made from forest residue biomass, e.g., wood chips which is commonly used in biofuel industries (Ragauskas et al., 2006; Sahoo, Bilek, Bergman, & Mani, 2018) and paper production (Duffy, 2006; Pécora, Ruiz, & Soriano, 2007).
In this study, the rheology of untreated wood chips slurries, sampled across a broad range of slurry temperatures and solids concentrations from 3 to 15 wt% dry-matter was experimentally studied. In addition, to evaluate the rheological behaviour of biomass slurries (e.g., Newtonian, shear-thinning, etc.) and to approximate the corresponding rheological parameters (e.g., yield stress, consistency index, fluid behaviour index), three different well-known viscosity models – Bingham (Bingham, 1917), Casson (Casson, 1959) and Herschel-Bulkley (Herschel & Bulkley, 1926) were applied to the experimental data in order to ascertain the best representative model. Finally, a novel approach for approximating the apparent viscosity of biomass slurries based on longitudinal pressure drop data was introduced and validated. This technique makes it possible to estimate the in situ apparent viscosity of the slurries through pipelines at large Reynolds numbers.
Section snippets
Material preparation
The wood chips (aspen and poplar) were supplied by Weyerhaeuser Company Ltd. (Pembina Timberland, Alberta, Canada). Large chips were hammer-milled using a Gisiger–Technik 2301 hammermill (GT Zesor AG, Messen, Switzerland) with a 3 mm round hole screen and refined into fine chips. The size distribution and shape analysis were performed using a Camsizer P4 (Microtrac (2014) Retsch GmbH, Haan, Duesseldorf, Germany). The measurements indicated a number of characteristics such as cumulative
Experimental results
To evaluate the rheological behaviour of the biomass slurry, the change in shear stress with respect to shear rate was experimentally measured over a wide range of concentrations (3, 5, 6, 8, 10, 11, 13 and 15 wt% dry-matter). At low slurry concentrations of 3 and 5 wt% dry-matter, the results showed a linear relation between shear stress and shear rate, i.e., the results demonstrated Newtonian behaviour of the slurry (Fig. 8). However, experimental results showed a nonlinear relation between
Conclusions
In this research, the rheological behaviour of untreated fine wood chip biomass slurries at various slurry concentrations and temperatures was investigated. Samples were taken from a closed-circuit pipeline facility, and the apparent viscosity of the slurries was investigated using a rotational viscometer with vane-in-cup geometry as well as from slurry pressure drop data. The results indicate Newtonian, power-law, and Bingham plastic rheological behaviours at different slurry concentrations.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the National Sciences and Engineering Research Council of Canada (NSERC), Canada, University of Alberta Future Energy Systems research initiative, Canada and the Division of Research and Innovation Partnerships (RIPs) at Northern Illinois University, USA for their financial support. They also thank Astrid Blodgett for editing the paper. All the results, justifications, and conclusions are solely the authors' and have not been endorsed by any other party.
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