Detailed Document
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IntroductionA large number of flows encountered in nature and technology are a mixture of
phases. Physical phases of matter are gas, liquid, and solid, but the concept of
phase in a multiphase flow system is applied in a broader sense. In multiphase
flow, a phase can be defined as an identifiable class of material that has a particularinertial response to and interaction with the flow and the potential field in which it
is immersed. For example, different-sized solid particles of the same material can
be treated as different phases because each collection of particles with the same
size will have a similar dynamical response to the flow field.
Multiphase Flow RegimesMultiphase flow regimes can be grouped into four categories: gas-liquid or liquid-
liquid flows; gas-solid flows; liquid-solid flows; and three-phase flows.
Gas-Liquid or Liquid-Liquid Flows
The following regimes are gas-liquid or liquid-liquid flows:
Bubbly flow: This is the flow of discrete gaseous or fluid bubbles in a
continuous fluid.
Droplet flow: This is the flow of discrete fluid droplets in a continuous gas.
Slug flow: This is the flow of large bubbles in a continuous fluid.
Stratified/free-surface flow: This is the flow of immiscible fluids separated
by a clearly-defined interface.
Gas-Solid FlowsThe following regimes are gas-solid flows:
Particle-laden flow: This is flow of discrete particles in a continuous gas.
Pneumatic transport: This is a flow pattern that depends on factors such as
solid loading, Reynolds numbers, and particle properties. Typical patterns
are dune flow, slug flow, packed beds, and homogeneous flow.
Fluidized bed: This consists of a vertical cylinder containing particles, into
which a gas is introduced through a distributor. The gas rising through the
bed suspends the particles. Depending on the gas flow rate, bubbles appear
and rise through the bed, intensifying the mixing within the bed.
Liquid-Solid Flows
The following regimes are liquid-solid flows:
Slurry flow: This flow is the transport of particles in liquids. The
fundamental behavior of liquid-solid flows varies with the properties of the
solid particles relative to those of the liquid. In slurry flows, the Stokes
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number (see Equation 23.2-4) is normally less than 1. When the Stokes
number is larger than 1, the characteristic of the flow is liquid-solid
fluidization.
Hydro-transport: This describes densely-distributed solid particles in a
continuous liquid.
Sedimentation: This describes a tall column initially containing a uniform
dispersed mixture of particles. At the bottom, the particles will slow down
and form a sludge layer. At the top, a clear interface will appear, and in the
middle a constant settling zone will exist.
Three-Phase Flows
Three-phase flows are combinations of the other flow regimes listed in the
previous sections.
Figure: Multiphase Flow Regimes
Examples of Multiphase Systems
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Specific examples of each regime are listed below:
Bubbly flow examples include absorbers, aeration, air lift pumps, cavitation,
evaporators, flotation, and scrubbers.
Droplet flow examples include absorbers, atomizers, combustors, cryogenic
pumping, dryers, evaporation, gas cooling, and scrubbers. Slug flow examples include large bubble motion in pipes or tanks.
Stratified/free-surface flow examples include sloshing in offshore separator
devices and boiling and condensation in nuclear reactors.
Particle-laden flow examples include cyclone separators, air classifiers, dust
collectors, and dust-laden environmental flows.
Pneumatic transport examples include transport of cement, grains, and metal
powders.
Fluidized bed examples include fluidized bed reactors and circulating
fluidized beds. Slurry flow examples include slurry transport and mineral processing.
Hydro-transport examples include mineral processing and biomedical and
physiochemical fluid systems
Sedimentation examples include mineral processing.
Choosing a General Multiphase ModelThe first step in solving any multiphase problem is to determine which of the
regimes provides some broad guidelines for determining appropriate models for
each regime, and how to determine the degree of interphase coupling for flowsinvolving bubbles, droplets, or particles, and the appropriate model for different
amounts of coupling.
Approaches to Multiphase ModelingAdvances in computational fluid mechanics have provided the basis for further
insight in to the dynamics of multiphase flows. Currently there are two approaches
for the numerical calculation of multiphase flows: the Euler-Lagrange approach
and the Euler-Euler approach.
The Euler-Lagrange ApproachThe Lagrangian discrete phase model in FLUENT follows the Euler-Lagrange
approach. The fluid phase is treated as a continuum by solving the time-averaged
Navier-Stokes equations, while the dispersed (to spread widely) phase is solved by
tracking a large number of particles, bubbles, or droplets through the calculated
flow field. The dispersed phase can exchange momentum, mass, and energy with
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the fluid phase. A fundamental assumption made in this model is that the dispersed
second phase occupies a low volume fraction, even though high mass loading
(mparticles>= mfluid) is acceptable.
The particle or droplet trajectories are computed individually at specified intervals
during the fluid phase calculation. This makes the model appropriate for the
modeling of spray dryers, coal and liquid fuel combustion, and some particle-laden
(to put (something) on or in, as a burden, load, or cargo; load) flows, but
inappropriate for the modeling of liquid-liquid mixtures, fluidized beds, or any
application where the volume fraction of the second phase is not negligible.
Limitation on the Particle Volume FractionThe discrete phase formulation used by FLUENT contains the assumption that the
second phase is sufficiently dilute that particle-particle interactions and the effects
of the particle volume fraction on the gas phase are negligible. In practice, these
issues imply that the discrete phase must be present at a fairly low volume fraction,usually less than 10-12%.
Limitation on Modeling Continuous Suspensions of ParticlesThe steady-particle Lagrangian discrete phase model is suited for flows in which
particle streams are injected into a continuous phase flow with a well-defined
entrance and exit condition. The Lagrangian model does not effectively model
flows in which particles are suspended indefinitely in the continuum, as occurs in
solid suspensions within closed systems such as stirred tanks, mixing vessels, or
fluidized beds. The unsteady-particle discrete phase model, however, is capable ofmodeling continuous suspensions of particles.
Limitations on Using the Discrete Phase Model with Other FLUENT
Models Stream-wise periodic flow (either specified mass flow rate or specified
pressure drop) cannot be modeled when the discrete phase model is used.
Only non-reacting particles can be included when the premixed combustion
model is used.
The Euler-Euler ApproachIn the Euler-Euler approach, the different phases are treated mathematically as
interpenetrating continua. Since the volume of a phase cannot be occupied by the
other phases, the concept of phasic volume fraction is introduced. These volume
fractions are assumed to be continuous functions of space and time and their sum is
equal to one. Conservation equations for each phase are derived to obtain a set of
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equations, which have similar structure for all phases. These equations are closed
by providing constitutive relations that are obtained from empirical information,
or, in the case of granular lows, by application of kinetic theory.
In FLUENT, three different Euler-Euler multiphase models are available: the
volume of fluid (VOF) model, the mixture model, and the Eulerian model.
Solution Guidelines
All multiphase calculations:
Start with a single-phase calculation to establish broad flow patterns. Eulerian multiphase calculations:
Copy primary phase velocities to secondary phases.
Patch secondary volume fraction(s) as an initial condition.
For a single outflow, use OUTLET rather than PRESSURE-INLET;
for multiple outflow boundaries, must use PRESSURE-INLET.
For circulating fluidized beds, avoid symmetry planes (they promote
unphysical cluster formation).
Set the false time step for under-relaxation to 0.001.
Set normalizing density equal to physical density.
Compute a transient solution.
The Eddy Dissipation Model
The eddy dissipation model is based on the concept that chemical reaction is fast relative to the
transport processes in the flow. When reactants mix at the molecular level, they instantaneouslyformproducts. The model assumes that the reaction rate may be related directly to the time
required to mix reactants at the molecular level. In turbulent flows, this mixing time is dominated
by the eddy properties and, therefore, the rate is proportional to a mixing time defined by the
turbulent kinetic energy, k, and dissipation
BIOMASS
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Introduction
Solid biomass can be grown for use as fuel on farms and for sale. This Factsheet outlines thephysical and chemical characteristics of solid biomass fuels, explains their significance and
includes a table containing detailed information on the properties of 22 common biomass fuels in
Ontario.
What is Biomass?
Biomass refers to any organic material derived from plants that use sunlight to grow. When
burned, the energy stored in biomass is released to produce heat or electricity. Common forms ofsolid biomass include agricultural crops, crop residues and forestry products. Switchgrass
(Figure 1) is an example.
Using biomass for energy offers potential advantages:
Biomass is an abundant and renewable source of energy. Using biomass for energy would diversify the energy supply and reduce
dependency on fossil fuels. Biomass production may create new jobs for the local economy in Ontario.
Figure 1. A field of switchgrass.
Energy Content of Biomass
The heating value of a fuel indicates the energy available in the fuel per unit mass MJ/kg
(BTU/lb). The net heating value is the actual energy available for heat transfer. The difference inavailable energy is explained by the fuel's chemical composition, moisture and ash content. For
comparison, the energy content of fuels is reported on a dry basis. For example, most agricultural
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residues have heating values that fall in the range of 1419 MJ/kg (6,0408,200 BTU/lb); coalranges from 1730 MJ/kg (7,3001,3000 BTU/lb).
Moisture
Moisture content is the key factor determining the net energy content of biomass material. Drybiomass has a greater heating value (or net energy potential), as it uses little of its energy toevaporate any moisture. Figure 2shows this relationship and illustrates the correlation between
energy and moisture contents. Increased moisture means less energy available for the boiler.
Figure 2. Typical net heating value (BTU/lb) as a function of moisture content. Moisture levelsshown (w.b. = wet basis) are the fraction (%) of raw biomass material that is water.
Text equivalent
Table 1. Ultimate analysis for a variety of biomass fuels in Ontario(all values reported on a dry matter basis)
Biomass Type MJ/kg
BTU/lb
Typical Values1
Ash
%
Carb
on
%
Hydrog
en
%
Nitrog
en
%
Sulph
ur
%
Oxyg
en
%2
Total
Chlorine
(g/g)3
Off-spec (non-food) grains
Beans 19 7,996 4.7 45.7 6.3 4.3 0.7 38.8 193
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Corn 17 7,350 1.5 42.1 6.5 1.2 0.1 48.9 472
Canola 28 12,220
4.5 60.8 8.3 4.5 0.5 21.4 163
Dried distillers grain 22 9,450 4.9 50.4 6.7 4.7 0.7 32.6 1,367
Grass/forages
Big blue stem 19 8,020 6.1 44.4 6.1 0.8 0.1 42.6 1,880
Miscanthus 19 8,250 2.7 47.9 5.8 0.5 0.1 43.0 1,048
Sorghum 17 7,240 6.6 45.8 5.3 1.0 0.1 42.3 760
Switchgrass 18 7,929 5.7 45.5 6.1 0.9 0.1 41.7 1,980
Straw/residue
Alfalfa 17 7,435 9.1 45.9 5.2 2.5 0.2 39.5 3,129
Barley straw 17 7,480 5.9 46.9 5.3 0.7 0.1 41.0 1,040
Corn cobs 18 7,927 1.5 48.1 6.0 0.4 0.1 44.0 2,907
Corn stover 19 7,960 5.1 43.7 6.1 0.5 0.1 44.6 1,380
Flax straw 18 7,810 3.7 48.2 5.6 0.9 0.1 41.6 2,594
Wheat straw 18 7,710 7.7 43.4 6.0 0.8 0.1 44.5 525
Processing by-product
Oat hulls 19 7,960 5.1 46.7 6.1 0.9 0.1 41.1 1,065
Soybean hulls 18 7,720 4.3 43.2 6.2 1.8 0.2 44.3 266
Sunflower hulls 20 8,530 4.0 47.5 6.2 1.0 0.2 41.2 3,034
Wood
Bark 19 8,432 1.5 47.8 5.9 0.4 0.1 45.4 257
Willow 19 8,550 2.1 50.1 5.8 0.5 0.1 41.4 134
Hardwood 19 8,300 0.4 48.3 6.0 0.2 0.0 45.1 472
Coal
Low sulphur subbit 25 10,52 6.0 55.0 3.7 0.9 0.4 11.5 35
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coal PRB4 0
Lignite 22 9,350 22.0
58.8 4.2 0.9 0.5 13.6 25
1 The content level of ash, chlorine and other elements can be lowered through crop selectivity,
growing conditions, plant fractionation, harvest time and harvest method.
2 Calculated by difference. Percent by difference refers to the difference between two numbers asa percent of one of them. For example, the percentage difference from 5 to 3 is: 2/5 = 0.4 = 40%.
3 A microgram (g) is a unit of mass equal to 1/1,000,000 of a gram (1 x 106), or 1/1,000 of amilligram. It is one of the smallest units of mass commonly used.
4 PRB Power River Basin
Data compiled from AURI, 2005; BIOBIB; Preto, 2010.
Generally, the moisture content of a solid is expressed as the quantity of water per unit mass.
Moisture content is usually reported on an as-is or wet basis (w.b.) in which the watercontent is given as a fraction of the total weight. All biomass materials contain some moisture,
from as low as 8% for dried straw to over 50% for fresh-cut wood.
A high moisture content adversely affects the collection, storage, pre-processing, handling andtransportation of biomass. In addition, transporting wet material costs more.
The moisture content of raw biomass can be reduced by:
leaving biomass in the field to dry for several weeks storing biomass, sheltered from precipitation commercial drying
Biomass Composition
The composition of biomass varies significantly among biomass types. Fuel performance is
related to the composition of the biomaterial. Important factors include ash, carbon, hydrogen,
nitrogen, sulphur, oxygen and chloride content. The elemental composition of various fuels inOntario is indicated in Table 1. All values are reported on a dry basis.
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Figure 3. Clinkering results in jamming of furnace elements. Source: CanmetENERGY.
Ash
The non-combustible content of biomass is referred to as ash. High ash content leads to fouling
problems, especially if the ash is high in metal halides (e.g., potassium). Unfortunately, biomassfuels, especially agricultural crops/residues tend to have a high ash with high potassium content.
As a result, the ash melts at lower temperatures, resulting in clinkers that can jam furnaceelements (Figure 3). Alternately, slagging and fouling occur when ash is vapourized and
condensed in the boiler, resulting in the production of hard formations on the heat transfersurfaces (Figure 4).
Wood (core, no bark) has less than 1% ash. Bark can have up to 3% ash. Agricultural crops have
higher ash content, from 3% and higher (Figure 5). Some boilers/stoves cannot handle fuels with
high ash content. More ash means more maintenance.
Carbon
The carbon content of biomass is around 45%, while coal contains 60% or greater (Demirbas,2007). A higher carbon content leads to a higher heating value.
Hydrogen
The hydrogen content of biomass is around 6% (Jenkins, 1998). A higher hydrogen content leads
to a higher heating value.
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Nitrogen
The nitrogen content of biomass varies from 0.2% to more than 1% (Jenkins, 1998). Fuel-bound
nitrogen is responsible for most nitrogen oxide (NOx) emissions produced from biomass
combustion. Lower nitrogen content in the fuel should lead to lower NOx emissions.
Figure 4. Boiler tube fouling leads to decreased efficiency. Source: CanmetENERGY.
Sulphur
Most biomass fuels have a sulphur content below 0.2%, with a few exceptions as high as0.5%0.7%. Coals range from 0.5%7.5% (Demirbas 2007). Sulphur oxides (SOx) are formedduring combustion and contribute significantly to particulate matter (PM) pollution and acid rain.Since biomass has negligible sulphur content, its combustion does not contribute significantly to
sulphur emissions.
Chloride
Combustion of biomass with high chloride concentrations (over 1,000 g/g) can lead toincreased ash fouling. High chloride content leads to the formation of hydrochloric acid in the
boiler tubes, resulting in corrosion that can lead to tube failure and water leaks in the boiler.
Fuels where this has been observed include corn stover and corn cobs.
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Figure 5. Typical ash content for selected biomass on a dry basis. Source: AURI, 2005; Preto,2010.
Text equivalent
Properties of Biomass
The ultimate analyses for a variety of biomass materials are presented inTable 1. All results are
displayed on a dry matter basis for comparison. Use the compiled data only as a generalcomparative guide.
It is important to note that biomass materials naturally contain variability, which depends on:
geographical location variety climate conditions harvest methods
Processes to Reduce Ash, Chloride and Other Elements
Various management strategies exist to reduce the ash and primary elements that interfere with
the combustion process, including crop selection, growing conditions, plant fractions, harvestingtime and minimizing soil contamination.
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Crop Selection
Ash is found in lower levels in warm-season grasses, such as big bluestem, switchgrass and
annuals such as corn, compared to cool-season grasses, such as orchard grass, fescues and
perennial ryegrass (Mehdi & Samson 1998).
Growing Conditions
Soil type highly influences the ash levels of biomass. Higher ash levels are found in crops
produced on clay soils than in crops produced on sandy soils.
Plant Fractions
The major components of ash are silica and potassium. The distribution and composition of ash
varies among different plant fractions. Ash levels are lowest in grass stems and highest in leaves
(Samson et al. 1999b). Harvesting biomass with higher stem content will reduce the plant's ashconcentration, thus improving the biomass quality forcombustion. See Table 2.
Harvest Timing to Allow for Leaching
Ash, chloride and potassium content are minimized by leaving the cut biomass in the field to
overwinter. Overwintering switchgrass in the field reduces ash levels to as low as 3.5%, due toleaching and loss of plant components that are higher in ash (i.e., leaves). However, harvesting in
the spring comes at a cost, with biomass losses of between 20% and 50%.
Component Switchgrass Ash Contents (%)
Leaves 7.0
Leaf sheaths 3.0
Stems 1.0
Seed heads 2.4
Minimizing Soil Contamination
It is important to minimize soil contamination of the crop residue, since soil particles greatly
increase the ash concentration of the biomass. Select mechanical harvesting techniques that
avoid digging up the soil (e.g., cut the biomass with a higher stubble height).
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Summary
Biomass materials are very diverse, ranging from wood, bark, straw and other agriculturalresidues, grasses and forages, and off-spec grains, etc. Despite this diversity, the composition of
most biomass materials is relatively uniform, especially after moisture is removed. The energy
content (on a mass basis) of most dry biomass fuels is in the 1719 MJ/kg (7,3008,000BTU/lb) range. Differences in energy content are due to differences in density and moisturecontent.
For most biomass fuels, nitrogen and sulphur levels are quite low, resulting in relatively low SOx
and NOx emissions. Biomass outside the normal range of these categories is mostly in the off-
spec, nonfood grain category.
The major difference in the composition of biomass fuels is ash content. Wood, the traditional
biomass fuel, generally contains less than 0.5% ash. With bark, this increases to 2%3% andjumps to above 5% for most grasses and agricultural residues. The increased ash content can
cause significant fouling, clinkering and handling issues.
Take care when using these fuels. Design conversion systems specifically for the target fuels.
Systems designed for wood (or coal) may not be suitable for other biomass fuels.
Conversion:
From to Multiply by
MJ/kg BTU/lb 430
BTU/lb GJ/ton 0.00233