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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

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