On the Challenges Facing the Handling of Solid Biomass Feedstocks
D. Ilic
1
, K. Williams
1
, R. Farnish
2
, E. Webb
3
and G. Liu
4
1
The University of Newcastle, Australia,
2
The University of Greenwich, U.K.,
3
National Oak Ridge Laboratory, U.S.A and
4
DuPont, U.S.A
Abstract
With major global emphasis on the management of waste, alternative resources and a shift to
advanced, environmentally sustainable technologies, demand for large volumes of heterogeneous
solid biomass feedstocks for use in energy, heating and as a biofuel or biochemical is expected to
significantly rise. In transforming sporadic supply of a low value, highly variable product, to continuous
and controlled high throughput systems, a thorough understanding of the feedstock physical
properties will increase in importance. Appropriate characterisation tests are necessary to define
technical specification and selection criteria for handling equipment and to adequately appraise the
requirement and location for additional processes or pre-treatment (additional energy and cost) to be
integrated into the handling chain. Such tests may also be of influence on the material characteristics
to be used in the process of converting the feedstock to energy or fuel. This paper discusses the main
parameters and the approach to obtain them associated with a number of handling phases. A holistic
approach to the design of biomass feedstock handling systems for further development and practical
implementation is also proposed.
1. Introduction
The utilisation of waste is becoming a significant global issue. Landfill space has long been exhausted
and poor disposal practices including burning result in detrimental effects such as green house gas
emissions and soil contamination. Concurrently, high use of traditional resources required for energy
and transportation and reliance on exhaustible fuels have increased demand for renewable and
sustainable solutions. Never before observed global climate patterns, air quality and acceptance of
environmental degradation directly impacting quality of life have lead to increased governmental
policy aimed at incentivising a transition to more environmentally focused technologies and
processes. This also includes a significant shift towards second-generation biofuel or feedstock
supplies that do not compete with arable land or potentially vital food resources.
Much emphasis is placed on the technologies and process used to obtain energy and quality biofuel
from feedstocks, as well as integration of the biomass/renewables into existing energy and fuel supply
chains. In order to facilitate competitiveness through the valorisation of biomass, effective application
and integration of equipment, technology and systems as well as optimising processes within the
handling chain is essential. This will provide improved integration, increased system efficiency and
reduce handling chain costs, making the process financially viable.
It has been recognised that agricultural and forestry waste, by-products and residues will alone not be
sufficient to meet the level of demand necessary and dedicated sustainable production systems will
be required. There is also recognition that agriculture and forestry biomass feedstock production will
need to significantly increase in order to meet demand and requirements of a commercial scale
bioenergy and biofuel sectors.
In developing countries, lack of access to affordable, reliable, safe and environmentally sustainable
energy sources has seen an increase in the development of biogas applications (anaerobic digesters).
Such developments also may be used for secondary benefits such as natural fertiliser as a by-product.
Globally, in 2016, renewable energy sources in power generation have reached 2.8% of global energy
consumption. This is significantly lower compared to oil (32.9%), coal (39.2%) and gas (23.8%) [BP
Statistical Review of World Energy]. However, according to the same report, renewable energy
consumed in power generation grew by 15.2%, which is approximately equal to the entire increase
globally. Renewable energy consumption is highest in Europe and Eurasia (39.2%), followed by Asia
Pacific (30.4%) and then North America (22.6%). Similarly, the U.S. Energy Information Administration,
International Energy Outlook 2016 cites renewables as the worlds fastest-growing energy source, with
consumption estimated to increase by an average of 2.6% per year from 2012 to 2040 [USEIA].
Through increased government incentives, according to the International Renewable Energy Agency
(IRENA), renewable energy could account for up to 36% of the global energy mix by 2030 with biomass
accounting for up to 60% [IRENA]. This is perhaps offset by generally high costs associated with set-up
and operation for plants using the latest conversion technologies. Such biomass statistics, and many
more referenced in a variety of current research, from a sustainability perspective, have the potential
to impact demand of other commodities.
Justification in shifting to a renewable energy technology may satisfy moral and environmental
requirements, significant barriers are also evident in transforming from pilot scale to
commercialisation (termed pioneer plants in North America). For example, a proven and effective,
second generation bio-ethanol pilot plant, unavoidably reaches an obstacle that requires an
influential, well established supporting organisation, willing to take on existing federal and/or state
legislation. Such an ordeal in turn is related to lack of purpose built infrastructure for on-time, cost
effective, efficient, continuous and reliable supply of high throughput, high density feedstocks. This
journey is also heavily intertwined with societal dependence on traditional, well established fossil fuels
and resources.
Previous studies into the research necessities of biomass characterisation have been presented by
[Ramirez-Gomez]. That work outlined a number of areas requiring further research related to
understanding biomass behaviour during storage, flow and handling including methods for measuring
PSD, flowability and strength, durability, self-ignition/oxidation and explosibility. Such parameters
effectively drive equipment and plant layout design, as well as selection of most appropriate mode of
conveying, transportation, storage and feeding. This paper looks to further elaborate on these aspects
and proposes a framework to facilitate the transition to a higher volume system.
While characterisation is necessary to define the materials handling equipment and processes,
characteristics of the feedstock are effectively defined by the conversion process requirements with
respect to producing highest energy or sugar yield. According to [Sharma et al], biofuel refineries may
be classified into four types: starch, sugar, oil and lignocellulosic based. Energy from feedstocks is
utilised by direct combustion or converting to solid, gas or liquid with main conversion technologies
including thermo-chemical processes (pyrolysis and gasification), bio-chemical processes (anaerobic
digestion and fermentation) and physicochemical processes. In addition to this, depending on
feedstock characteristics, development of cost-effective pre-treatment processes may also be
required for conversion. Taking these aspects into consideration, it is important to note that material
feed and handling systems have been cited as the most common causes of downtime in
thermochemical conversion processing [Craven et al].
A shift towards reducing environmental footprint and sustainability has resulted in utilisation of
advanced second-generation biofuels from lignocellulosic materials (cellulose, hemi-cellulose and
lignin) utilising products that do not compete with land for growing food crops. This is in contrast to
first-generation commercial cellulosic feedstock sources and processes, primarily at the pioneer scale.
In North America, corn stover biomass is the primary feedstock of choice for first-generation cellulosic
bio refineries [Shah, Darr]. Lignocellulosic feedstock sources include agricultural residue, herbaceous
crops, invasive weeds, short rotation woody crops, urban woody waste, sawmill residue and forestry
biomass [Sharma et al].
With respect to handling aspects, the biomass feedstock supply chain consists of storage, feed
(loading/unloading) and transportation, all requiring appropriate and favourable principles for design
in order to maintain the low emission benefit of using biomass. To provide a means of effectively
designing systems to handle the heterogeneous biomass feedstocks, existing procedures to obtain
representative samples, prior to the characterisation process, will need to be reviewed.
Representative sampling represents the first crucial step in accurate characterisation test work.
Physical characterisation is then required, which will become the basis to drive equipment and facility
specification criteria and selection based on the properties of the handled individual feedstocks, in
view of the operational requirements for a economically viable conversion process. Characterisation
would also form the basis for evaluating if processing or pre-treatment (ease of handling) and
conditioning (influencing specific attributes) is required and at which stage of the supply chain is most
optimal – in order to facilitate handling storage and transportation of the biomass material. This is a
vital criteria that also will influence the location and the delegation of responsibility of the
processing/treatment, ultimately also directly impacting financial considerations included in the
business development model of the enterprise and required to necessitate trade of the valuable
commodity. In turn, such information will also decide the location where the feedstock should be
processed, who should process it (producer or user), in which form it is to be supplied in and which
pre-defined characteristics must be met. Characterisation will also allow determining if integration or
application of existing equipment and facilities available is not possible. The feedstock characteristics
such as particle size for example, may then be cross-correlated to those specifically required by the
conversion process technology (i.e. at the bio refinery).
Due to the extremely low energy density per unit mass of raw biomass, pre-treatment of the feedstock
is necessary. Some of the different approaches include torrefaction, carbonization, pelletizing,
chopping, shredding and grinding [Ramirez-Gomez]. Densification or increasing the density of the
handled product is also required to reduce transportation costs by allowing transportation of larger
quantities of bulk solids and trade more economically feasible. It is important to note that the process
used to transform raw biomass feedstocks into more handleable bulk solids needs to be taken into
account when assessing the overall carbon footprint (life cycle analysis) and possible impact on the
environment in utilising the feedstock end-product. This for example also includes other factors
required for their modification including type of binder used [Kaliyan]. Globally, wood pellet
production and demand has significantly increased over the previous five years. This has eventuated
through incentives driven by European Commission’s 2020 plan for reducing greenhouse gas
emissions and increasing renewable energy percentage of total energy generation. Drax power station
in the U.K., imports wood pellets from North America and Canada, contributing to the U.K. being the
world’s largest importer of wood pellets. The U.S. is the largest exporter of wood pellets globally with
exports increasing almost 40% (2013-2014). In 2014, around three quarters of these exports were
supplied to the U.K, with Drax power plant accounting for over 80% [USIEA].
2. Storage, Handling and Transportation
In a recent paper on cornstover feedstock supply in the Midwestern U.S., [Shah, Darr] estimate that
fuel and labour amount to around 50% and 60% of the total cost of feedstock transportation (based
on trucking) respectively. One critical aspect of biomass valorisation is reduction of transportation
costs through the development of efficient logistic systems. This includes reducing capital costs
through utilisation of existing logistics for feedstock transportation. Considering the generally low
density per unit mass of biomass feedstocks, long distance transportation is not economically feasible
unless efficient handling transportation and storage systems are implemented.
A number of researchers have commenced with assessing biomass feedstocks by relating them to
traditionalist flow parameters and either using well established characterisation equipment or
defining new characterisation equipment based on what is most appropriate for the tested feedstock.
Undoubtedly, the procedure envisioned must involve a performance comparison to some existing
resource such as coal for example (or other well utilised bulk solid) as a means of establishing
limitations and applicability of traditional tests and procedures, prior to verifying the suitability of a
design or equipment required for transportation and handling purposes. For example, in discussing a
dedicated dry handling bulk terminal facility [Wu et al], tested feedstocks including wood pellets, chips
and torrefied pellets. The programme involved assessment of particle size distribution (PSD), internal
angle of friction and degradation. Similarly, [P.S. Lam] has identified engineering properties of biomass
based on application and lists density and angle of repose as design parameters for handling, storage
and transportation as well as discussing characterisation methods for their determination.
Specification of the applicable test and most appropriate procedure to classify and assess feedstock
physical properties including particle characterisation, degradation and moisture effects is also
required.
Investigation into application of fundamental principles associated with traditional flow properties
and arching behaviour has previously been studied by [Miccio et al, Wu et al, Hinterreiter and Khan].
[Miccio et al] indicate that, shear cells could be used to assess flow properties if particle size is below
2mm (for the biomass solids tested). Importantly, that work also noted particle elasticity, shape and
internal shear as properties that influence flow towards unexpected behaviour, generally related to
achieving steady state during compaction (consolidation).
It appears evident that feedstock bulk density will be one of the most critical characterising
parameters of influence on the economics of biomass trade. Symbolically, [Shah, Darr] cite feedstock
(in this case bale) density as being of most influence on the cost to deliver stover to the bio refinery.
Of particular significance, due to elasticity, research into compacted storage beyond traditional stress
analyses is required, where focus needs to be measurement of tensile force due to the bulk solids
“springiness”. Such test work will need to quantify un-restrained, un-consolidated dilation, which goes
beyond current compressibility assessment methods. This may be obtained through development of
a test apparatus to establish the relationship between compaction, dilation and the applied stress.
Such a relationship is demonstrated on granulated wattle and wheat straw in Figure 1 to Figure 3. The
test was conducted in a 63.5mm diameter, 19mm deep cell, which is typically used for mined material
assessment of the finer sized fractions (typically less than 5 mm). For the wheat straw, the sample was
first cut into small pieces in the order of 5 mm to fit in the cell in a loose poured state. Loads were
applied by a means of a lid and weight carrier, and the consolidation/dilation of the sample measured
via a linear variable displacement transducer (LVDT). Knowing the sample volume, mass and applied
loads allows for the consolidation/dilation relationship to the normal consolidation pressure to be
determined.
Figure 1: a) Granulated Wattle and b) Wheat Straw
Figure 2: Consolidation/Dilation response of (a) Granulated Wattle and b) Wheat Straw
Figure 3: Corresponding Stress/Strain response of (a) Granulated Wattle and b) Wheat Straw
The data from the compaction/dilation test allows for a hysteresis relationship of the material to be
established: consisting of the total energy to compress, plastic and elastic energy response. In the
above case, the plastic energy response to compression, the area between the two curves, for
granulated wattle and wheat straw, was calculated to be 2.0kJ/m
3
and 3.0kJ/m
3
. The corresponding
maximum compaction (compressibility) observed is approximately 22% and 42% respectively.
However, upon removal of consolidation, the materials dilate, resulting in a compaction loss in the
order of 6-19% respectively. This means that the maximum effective compaction, for the applied load
tested, is 15% for the granulated wattle and 20% for the wheat straw. An alternative relationship
describing the ability of the feedstock to deform under load can also be observed through the strain
response to normal pressure (stress) plots presented in Figure 3. These results demonstrate greater
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![Figure 4: Complete Design Methodology [Ilic]](/figures/figure-4-complete-design-methodology-ilic-1txj9pwy.webp)