An effective degradation of biomass and the gasification of intermediate prod­ucts of thermal degradation into lower-molecular-weight gases like hydrogen require the SCW reactor to operate in the high-temperature range (>600 °C). The higher the temperature, the better the conversion, especially for production of hydrogen, but the lower the SCW’s energy efficiency. A lower gasification temperature is therefore desirable for higher thermodynamic efficiency of the process.

Catalysts help gasify the biomass at lower temperatures, thereby retaining, at the same time, high conversion and high thermal efficiency. Additionally, some catalysts also help gasification of difficult items like the lignin in biomass. Watanabe et al. (2003) noted that the hydrogen yield from lignin at 400 °C and 30 MPa is doubled when a metal oxide (ZrO2) catalyst is used in the SCW. The yield increases four times with a base catalyst (NaOH) compared to gasification without a catalyst. The three principal types of catalyst used so far for SCW gasification are: (1) alkali, (2) metal, and (3) carbon-based.

An important positive effect of catalysts in SCWG is the reduction in required gasification temperature for a given yield. Minowa et al. (1998) noted a significant reduction in unconverted char while gasifying cellulose with an Na2CO3 catalyst at 380 °C. Base catalysts (e. g., NaOH, KOH) offer better performance, but they are difficult to recover from the effluent. Some alkalis (e. g., NaOH, KOH, Na2CO3, K2CO3, and Ca(OH)2) are also used. They, too, are difficult to recover.

The special advantage of metal oxide catalysts is that they can be recovered, regenerated, and reused. Commercially available nickel-based catalysts are effective in SCW biomass gasification. Among them, Ni/MgO (nickel sup­ported on an MgO catalyst) shows high catalytic activity, especially for biomass (Minowa et al., 1998).

Metal catalysts have a severe corrosion effect at the temperatures needed to secure high yields of hydrogen. To overcome this problem, Antal et al. (2000) used carbon (e. g., coal-activated and coconut shell-activated carbon and maca — damia shell and spruce wood charcoal). The carbon catalysts resulted in high yields of gas without tar formation.

18 16

14

9>

12

о

10

T3

8

>.

6

CO

4 2 0

H2 —ch4 — A — CO — X — CO2

FIGURE 7.8 Effect of residence time on the gasification of 2% rice husk in supercritical water at 650 °C and 30 MPa in a batch reactor.

As we can sense from Figure 9.1, bio-oil is free-flowing. Its low viscosity is due to its high water content. Also, it has an acrid, smoky smell that can irritate eyes with long-term exposure. With a specific gravity of ~1.2, bio-oil is heavier than water or any oil derived from petroleum. A comparison of its physical and chemical properties with those of conventional fossil fuels is given in Table 9.3.

An important feature of bio-oil not reflected in Table 9.3 is that some of its properties change with time. For example, its viscosity increases and its volatility decreases (Mohan et al., 2006) with time. Some phase separation and

deposition of gums may also occur with time, primarily because of polymeriza­tion, condensation, esterification, and etherification. This feature distinguishes bio-oil from mineral oils, the properties of which do not change with time.

Bio-oil is not soluble in water, although it contains a substantial amount of water. However, it is miscible in polar solvents, such as methanol and acetone, but immiscible with petroleum-derived oils. Bio-oil can accept water up to a maximum limit of 50% (total moisture). Any more water results in phase separation. Table 9.3 shows that bio-oil has a heating value nearly half that of conventional liquid fuels but has comparable flash and pour points.

Tar is a complex mixture of condensable hydrocarbons, including, among others, oxygen-containing, 1- to 5-ring aromatic, and complex polyaromatic hydrocarbons (Devi et al., 2003). Neeft et al. (1999) defined tar as “all organic contaminants with a molecular weight larger than 78, which is the molecular weight of benzene.” The International Energy Agency (IEA) Bioenergy Agree­ment, the U. S. Department of Energy (DOE), and the DGXVII of the European Commission agreed to identify all components of product gas having a molecu­lar weight higher than benzene as tar (Knoef, 2005, p. 278).

A common perception about tar is that it is a product of gasification and pyrolysis that can potentially condense in colder downstream sections of the unit. While this is a fairly good description, a more specific and scientific defi­nition may be needed for technical, scientific, and legal work. Presently, there is no universally accepted definition of tar. As many as 30 definitions are avail­able in the literature (Knoef, 2005, p. 279). Of these, that of the IEA’s gasifica­tion task force, as follows, appears most appropriate (Milne et al., 1998):

Biomass Gasification and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00004-0

Copyright © 2010 Prabir Basu. Published by Elsevier Inc. All rights reserved.

The organics, produced under thermal or partial-oxidation regimes (gasification) of any organic material, are called “tar" and are generally assumed to be largely aromatic.

Gasification reactions proceed at a finite speed; this process is divided into three steps: drying, devolatilization, and gasification. The time taken for drying and devolatilization of the fuel is much shorter than the time taken for gasification of the remaining char. Some models assume instantaneous drying and devola­tilization because the rate of reaction of the char, which is the slowest, largely governs the overall process.

The products of devolatilization are CO2, CO, H2O, H2, and CH4. The gases released during drying and devolatilization are not added instantaneously to the upflowing gas stream, but are added along the height of the gasifier in a predefined pattern. The total mass devolatilized, т0аШе, is therefore the sum of the carbon, hydrogen, and oxygen volatilized from the solid biomass.

mvolatile mchar + mhydrogen + moxygen (5.83)

Char gasification, the next critical step, may be assumed to move simultane­ously through reactions R1, R2, and R3 (Table 5.2). As these three reactions occur simultaneously on the char particle, reducing its mass, the overall rate is given as

mchar mBoudouard + msteam + mmethanation (5.84)

The conversion of the porous char particle may be modeled assuming that the process follows shrinking particle (diminishing size), shrinking core (dimin­ishing size of the unreacted core), or progressive conversion (diminishing density). The shift reaction is the most important homogenous reaction fol­lowed by steam reforming. The bed materials may catalyze the homogeneous reactions, but only in the emulsion phase, because the bubble phase is assumed to be free of solids.

The bed height (or depth) of a bubbling fluidized-bed gasifier is an important design parameter. Gas-solid gasification reactions are slower than combustion reactions, so a bubbling-bed gasifier is necessarily deeper than a bubbling-bed combustor, which is typically 1.0- to 1.5-m deep for units larger than 1 m in diameter. Besides pilot plant data or design experience, there is presently no simple means of deciding the bed depth. A deeper bed allows longer gas resi­dence time, but the depth should not be so great compared to its diameter as to cause slugging. The selection of bed height depends on economics. A higher bed height means a higher pressure drop and also a taller reactor. It also should provide a longer residence time for better carbon conversion.

The gasification agent, CO2 or H2O, entering the grid takes a finite time to react with char particles to produce the gas. The bulk of the gasifying agent travels up through the bubbles but very little reaction takes place in the bubble phase. Rather, the reaction takes place mostly in the emulsion phase. The extent to which oxygen or steam is converted into fuel gases thus depends on the gas exchange rate between the bubble and emulsion phases as well as on the char — gas reaction rate in the emulsion phase. This is best computed through a kinetic model of the gasifier as illustrated in Section 5.6.2. An alternative is to use an approach based on residence time, as described next.

Residence Time Design Approach A bubbling fluidized bed must be suffi­ciently deep to provide reactants the time to complete the gasification reactions. This is why residence time is an important consideration for determination of bed height. An approach based on residence time, developed primarily for coal gasification, can be used for biomass char gasification, which gives at least a first estimate of the bed height for a biomass-fueled bubbling fluidized-bed gasifier.

The residence time approach is based on the assumption that the conversion of char into gases is the slowest of all gasifier processes, so the reactor should provide adequate residence time for the char to complete its conversion to the desired level. Here is a simplified method.

Given the following assumption:

• The reactivity factor is fo, = 1 (which lies between 0 < fo < 1).

• The solid is in a perfectly mixed condition (i. e., continuous stirred-tank reactor).

Then, the volume of the fluidized bed, V, is calculated using the equation

V = (6.32)

Pb

where Wout is the char moving out; kg/s = (1-Х) Win; X is the fraction of the char in the converted feed; pb is the bed density, which can be estimated

theoretically from fluidization hydrodynamics and regime (kg/m3); and в is the residence time of the char in the bed, or reaction time (s).

The residence time approach assumes that the water-gas reaction, (C + H2O ^ CO + H2), as written in Eq. (6.33) is the main gasification reaction, where the char is consumed primarily by the steam gasification reaction for nth-order kinetics:

— ^ = к [ H2O Г (6.33)

m at

where m is the initial mass of the biomass and C is the total amount of carbon gasified in time, t. Taking a logarithm of this,

ln I — — | = ln (k) + n ln [ H2O ] (6.34)

V m dt )

experiments can be carried out taking a known weight of the biomass and measuring the change in carbon conversion at different time intervals for a given temperature, steam flow, and pressure. Using these data, graphs are

1 DC’

.mdt

give the value of k, and the slope will give the value of n. An example of such a plot is shown in Figure 6.23.

The experiment is carried out for different operating temperatures such as 700 °C, 800 °C, and 900 °C, so, for each temperature, one k value is obtained.

Now k can be expressed as

к = ко exp R Rt)

p

ln к = lnk0 — a (6.35)

0 rt

This shows that if we plot a graph between ln к and 1/T, the y-intercept will give the value of k0 and the slope will give the value of (-Ea /R).

The reaction rate for the steam gasification of biomass is given by

= k0 m exp V-R — Wof (6.36)

dt V RT j

This gives the generalized reaction rate that shows the dependence of the gas­ification rate on temperature, mass of carbon or char, and concentration of steam/air/oxygen.

From a knowledge of the reaction rate, the residence time, в, can be calcu­lated as

9 = Co X

r

where C0 is the initial carbon in the biomass particle, kg; X is the required carbon conversion (-); and r is the steam gasification reaction rate (kg/s). We can avoid such experiments if there is a suitable expression for the rate of steam gasification of the designed biomass char (Sun et al., 2007).

From knowledge of the required solid residence time, в, then, the bed volume, Vbed, is

where F[C] is the char feed rate into the gasifier and ps is the density of the bed solids. In a typical bubbling bed, the bed voidage is ~0.7. The bed generally contains 5 to 8% (by weight) of reacting char (xchar); the remaining solids are inert bed materials.

The bed height, Hbed, is known by dividing bed volumer by the bed area, Ab, which is known from chosen superficial velocity

Design charts for residence time, в, of test coals for different feed conver­sions and S/C or O/C ratios are given in the Coal Conversion Systems Technical Data Book (U. S. DOE, 1978). The residence time may be adjusted for the reactivity of the char in question and for the reactivity of its partial gasification before it enters the gasifier.

In an SCWG system, the product gas mixture is separated from water in two stages. In the first stage, initial separation takes place in a high-pressure but low-temperature separator. In the second stage, final separation occurs under low pressure and low temperature.

At low temperatures (25-100 °C), hydrogen or methane has very low solu­bility (0.001-0.006) in water, even at high pressure (Figure 7.13). So the bulk of the hydrogen is separated from the water when cooled. Figure 7.14 shows one such scheme where S1 is the hydrogen separator. Other gases like CO2 are also separated from the water but to a limited extent. As we can see from Figure 7.15, the solubility of carbon dioxide is an order of magnitude higher (0.01-0.03) that of hydrogen at this low temperature and high pressure.

This feature can be exploited to separate the hydrogen from the carbon dioxide, but the CO2’s equilibrium concentration may not be sufficient to dis­solve it entirely in the high-pressure water. Additional water may be necessary to dissolve all of these gases except hydrogen so that the hydrogen alone remains in the gas phase (S1, Figure 7.14). The equilibrium concentration of these gases in water can be calculated from the equation of state, such as Peng Robinson or SAFT.

The liquid mixture is next depressurized through a pressure regulator before it enters the second separator (S2, Figure 7.14). The solubility of most gases reduces with a decrease in pressure, so the second unit separates the rest of the CO2 from the gas.

Feng et al. (2004a, b) calculated the phase equilibrium of different gases in water for a plant using different relations. Values calculated using SAFT equi­librium showed the best agreement with experimental results. These results are shown in Table 7.6 to illustrate the process. It is apparent that at 25 °C the solu­bility of CO2 is orders of magnitude higher than that of methane and hydrogen. The solubility of methane and hydrogen is similar at nearly all pressures. For

Pressure (atm)

—— 298 К ——— 323 К ——- 348 К………. 373 K

FIGURE 7.15 Solubility of carbon dioxide in water. (Source: Adapted from Ji et al., 2006.)

their separation, then, it is necessary to use a system such as a pressure swing adsorber (S3), as shown in Figure 7.14.

An important consideration is the additional water required to keep the carbon dioxide dissolved while the hydrogen is being separated. The amount, which may be considerable, can be expressed as the ratio of water to gaseous product (R) on a weight basis. When pressure and R increase, the purification of hydrogen increases but the amount of hydrogen in the gas phase decreases. Therefore, we can recover more hydrogen with less purity or less hydrogen with more purity. This depends on an adjustment of the pressure and R. Example 7.1 illustrates the computation.

Example 7.1

Design a separator to produce 79% pure hydrogen from an SCWG operating at 250 bars of pressure. Assume the following overall gasification equation, which produces hydrogen, methane, carbon dioxide, and carbon monoxide.

C6H10O5 + 4.5H20 = 4.5C02 + 7.5H2 + CH4 + 0.5CO

Solution

We use the carbon dioxide solubility curve in Figure 7.15 to design the separator. Here, at 250 bars of pressure and 25 °C, we find the solubility of CO2 to be 0.028 mole fraction. This implies that 1 mol of water is needed to dissolve 0.028 mol of carbon dioxide.

To separate gaseous hydrogen from liquid water, we reduce the ambient temperature to 25 °C. From Figure 7.13 we find that the hydrogen solubility is only 0.0031 at 250 bars and 25 °C, so (1 — 0.0031) or 0.9969 fraction of hydrogen produced will be in the gas phase here. The gas may, however, contain other gases, so to ensure that the hydrogen is 79% pure, we need to add water to the separator. If we know the operating temperature, pressure, and weight ratio of the water to the gas mixture, the amount of product in the liquid and vapor phases can be calculated according to an equation of state. Here we use Figure 7.16 computed by the Peng-Robinson equation. For 250 bars of pressure and a mole

84

82

80

78

76

74

72

70

68 h

66

64

62

60

58

fraction of 79% in the gas phase, we get R = 80. Thus, the amount of water required is 80 x (the mass of product gas).

From the overall gas equation, the mass of product gas is 4.5 x 44 + 7.5 x 2 + 1 x 16 + 0.5 x 28 = 243 g/mol of biomass. The mass of water is 80 x 243 = 19,440 g = 19.4 kg.

From the property table of water, we get the density of water at 25 °C and 250 bars, which is 1008.5 kg/m[8]. The volume of water is 19.4kg/1008.5 kg/m3 = 0.0192 m3.

Volume of product gas = X(nRT/P)

= (4.5 + 7.5 +1 + 0.5) x 10-3 kmol x (8.314 kPa. m3.kmol-1.K-1 x 298 К)/(250 x 102 kPa)

= 0.00134 m3

Therefore, the total volume of biomass that is gasified is 0.0192 + 0.00134 = 0.0205 m3/mol.

• The irregular size and the low shape factor of biomass makes it difficult to flow.

• Pulverization is necessary for pumping the biomass, but it is very difficult to pulverize. Pretreatment of the feedstock is necessary.

• Fibrous by nature, biomass does not flow well through an augur or gear pump, and it is difficult to make a uniform slurry for pumping through impellers.

Most of the research work on SCWG generally used model water-soluble biomass such as glucose, digested sewage sludge, and wastewater (Blasi, 2007), which are easy to pump. For other types of biomass, Antal et al. (2000) used additives or emulsifiers such as corn starch gel, sodium carboxymethyl cellu­lose (CMC), and xanthan to make pumpable slurries. In an industrial applica­tion, large-scale use of emulsifiers is impractical.

A sludge pump was successfully used in a 100-kg/h pilot plant; however, the solids had to be ground to less than 1-mm particles and pretreated before pumping. Even then grass and fibrous materials clogged the membrane pump’s vents (Boukis et al., 2006). Cement pumps have been suggested but, to date, have not been tried for pumping biomass in an SCW gasifier (Knoef, 2005).

Another important problem is plugging of the feed line during the preheat­ing stage, in which the feed being heated can start breaking down. Char and other intermediate products can deposit on the tube walls, blocking the passage and thereby creating a dangerous situation.

Carbon buildup on the reactor wall has an adverse effect. It reduces the gas yield when the reactor is made of metals that have catalytic effects, although it is not associated with the feed system. Lu et al. (2006) showed that gas yields, gasification efficiency, and carbon efficiency are reduced by 3.25 mol/kg, 20.35%, and 17.39%, respectively, when carbon builds up on the reactor wall compared to when the reactor is clean. Similar results were found by Antal et al. (2000).

The primary purpose of storage is to retain the biomass in a good condition and in a position convenient for easy transfer to the next stage of operation,
such as drying or feeding into the gasifier. The stored biomass should be pro­tected from rain, snow, and infiltration of groundwater.

Once unloaded, the biomass is moved by belt conveyers to the storage yard, where it is stored in piles according to usage patterns. If the biomass is from several sources and is to be mixed before use, the piles are arranged in such a way that they can be mixed conveniently into the desired proportions. Because of the large volume of biomass, indoor storage may not be economical. Open — air storage is most common, though it can cause absorption of additional moisture from rain or snow and produce dust pollution. Storage can be of two types: above ground, for large-volume biomass, or enclosure in a silo or bunker.

Figure 8.2 showed the general arrangement of the solids-handling system in a typical biomass-fired plant. A truck-receiving station unloads into an under­ground hopper from which a belt conveyor takes the biomass to a screening station. After removal of foreign materials, the biomass is crushed and screened to the desired size range and then transported into silos for covered storage. From there, it is taken to the plant as required. Figure 8.4 is a photograph of receiving, size-screening, and above-ground outdoor storage.

Underground bunker storage is very convenient and cost effective from a fuel delivery standpoint, as it protects the biomass from rain and snow. However, because it needs good ventilation and drainage for safety and environmental protection, its capital cost is higher than that of above-ground storage. The hygroscopic nature of biomass is a major issue, as it causes the prepared biomass to absorb moisture even if stored indoors. Moreover, long-term storage can cause physical and chemical changes in the biomass that might adversely

affect its flow and gasification properties. For these reasons, it is desirable to occasionally turn the biomass. A simple and practical way of doing this is to draw it at a rate higher than that required and return the excess to the top of the pile.

Moving or retrieving the biomass from the storage piles to the gasifier plant requires careful design, because interruptions or delays can have a major effect on the operation of the plant. Generally, it is desirable to withdraw biomass from the bottom of the pile such that the first in-first out principle is followed to allow a relatively uniform shelf life.

The properties of the biomass determine the ease with which it is retrieved or handled. Oversized materials, frozen chunks (in cold countries), and compac­tion can lead to poor or interrupted fuel flow. If the fuel bin is not filled uni­formly, erratic operation can result, creating problems for hydraulic scrapers and bridging over the unloaders. Sticks, wires, and gloves, for example, can jam augers. Mobile loaders normally achieve uniformity in above-ground storage buildings or in live-bottom unloaders and augers in bins and silos. For large plants, a scraper connected to a conveyor, as shown in Figure 8.4, is more efficient for reclamation.

The following are some common methods for retrieval of biomass from storage:

• Simple gravity feed or chute

• Screw-type auger feed

• Conveyor belt

• Pneumatic blower

• Pumped flow

• Bucket conveyor

• Frontloader

• Bucket grab

Walking beams are sometimes used on the floors of large bunkers or storage buildings to facilitate the movement of biomass to the discharge end of the storage.

Above-Ground Outdoor Storage

In large-scale plants, above-ground outdoor storage is the only option (Figure 8.4). Indoor storage is usually too expensive. Biomass needs to be piled in patterns that allow maximum flexibility in retrieval as well as in delivery. Furthermore, it is necessary to ensure the first in-first out principle. In some cases, an emergency or strategic reserve is kept separate from the regular flow of biomass. This is a special consideration for long-term storage.

Good ventilation is important in storage design. Biomass absorbs moisture. Ventilation prevents condensation of moisture and the formation of moulds that can pose serious health hazards. It also prevents composting (formation of

methane), which not only reduces the energy content of the biomass, but also may run the risk of fire. Because tall storage piles are difficult to ventilate, the maximum height of a wood chip storage pile should not exceed 8 to 10 m (Biomass Energy Centre, 2009). For an indoor facility, water or moisture accu­mulation may occur inadvertently. Unless moved periodically, the biomass may form fungi and cause a health hazard. Drainage is an important issue, especially for outdoor storage.

Silos and Bins for Storage of Biomass

Improper storage not only makes retrieval difficult, it also can adversely affect the quality of the biomass. Retrieval or reclamation from storage is equally important, if not more so. It represents one of the most trouble-prone areas of biomass plant operation. The handling system and its individual components must be designed to ensure uninterrupted flow to the gasifier at a measured rate.

Bunkers, silos, and bins provide temporary storage in a protective environ­ment. Bunkers are a type of large-scale storage. Although the term bunker is generally associated with underground shelter, here it refers to the indoor storage of fuel in power or process plants that is not necessarily underground. Silos could be fairly large in diameter (4-10 m) and are very tall, which is good for storing grain-type biomass. For example, Figure 8.5 shows a tower silo for cattle feed. Bins are for smaller-capacity temporary storage.

FIGURE 8.5

Hopper Design

Hoppers or chutes facilitate withdrawal of biomass or other solids from tem­porary storage such as a bunker. Major issues in their design include (1) mode of solids flow, (2) slope angle of discharge, and (3) size of discharge end.

Funnel flow is characterized by an annular zone of stationary solids and a moving core of solids at the center. In this case, the solids flow primarily through the core of the hopper. Solids in the periphery either remain stationary (Figure 8.6c, left) or move very slowly (Figure 8.6c, right). Fine particles tend to move through the core while coarser particles stay preferentially in the annulus. The particles from the top surface can flow into the funnel, thus violat­ing the doctrine of first in-first out. If that does not happen, a stationary annulus is formed and the discharge stops, causing a rat hole to form through the hopper that becomes void and stops the flow. The rest of the solids in the hopper stay in the annulus (Figure 8.6a, right), which prevents the hopper from emptying completely. The only positive thing about a funnel-flow hopper is that it requires a lower height.

Mass flow (Figure 8.6b) is the preferred flow mode because the solids flow across the entire hopper cross-section. Though there may be some difference in velocity, this allows an uninterrupted and consistent flow with very little radial size segregation, which permits the hopper to effectively follow the first in-first out norm. However, because of the solids’ plug-flow behavior, there can be more wear on the hopper walls with abrasive solids. Therefore, the required height of a mass-flow hopper must be greater than that of a funnel-flow hopper. The steeper the cone angle of a hopper, the higher the probability of a mass flow of solids through it. Some common operating problems with hoppers are

• Rat holing

• Funnel flow

• Arching

• Flushing

• Insufficient flow and incomplete emptying

• Caking

Two of the most common problems experienced in an improperly designed silo or bin (hereafter referred to as silo) are no flow and erratic flow. No flow from a silo can be due to either arching or rat holing (Figure 8.6a).

Rat holing (Figure 8.6a, right) most often happens in the flow of biomass with particles that are cohesive and rough. This is a serious problem in hoppers. To facilitate solids flow, the rat hole must be collapsed by proper aeration in the hopper or by vibrations on the hopper wall.

Arching occurs when cohesive particles form an obstruction over the exit (Figure 8.6a, left), usually in the shape of an arch or a bridge above the hopper outlet that prevents further discharge. The arch can be interlocking, with the particles mechanically locking to form the obstruction, or it can be cohesive. Coarse particles can also form an arch while competing for an exit, as a traffic jam results from a large number of vehicles trying to pass through a narrow road in an unregulated manner. By making the outlet size at least 8 to 12 times the size of the largest particle, this type of arching can be avoided (Jacob, 2000).

Flushing results in the uncontrolled flow of fine solids—Geldart’s group A or group C particles (Basu, 2006, p. 443)—through the exit hole. It it is uncom­mon in relatively coarse biomass, but it can happen if the hopper is improperly aerated in an attempt to collapse a rat hole.

Another problem influenced by hopper design is inadequate emptying. This can happen if the sloped base of the hopper is improperly inclined, causing some solids to remain on the floor that cannot flow by themselves.

Erratic flow from an inappropriately designed hopper often results from alternating between an arch and a rat hole. A rat hole may collapse because of an external force, such as vibrations created by a plant pulverizer (mill), a passing train, or a flow-aid device such as an air cannon or vibrator. Some biomass discharges as the rat hole collapses, but the falling material can compact over the outlet and form an arch. The arch may break because of a similar external force, and the material flow will resume until the flow channel is emptied and a rat hole is once again formed (Hossfeld and Barnum, 2007).

Material discharge problems can also occur if the biomass stays in the bunker for a very long time, forming cakes because of humidity, pressure, and temperature. This easily results in arching or rat holes. To avoid this, renewal of solids in the hopper is necessary.

There are some special problems in fuel-handling systems. For example, spontaneous ignition of coal can occur if fine coal particles stay stagnant in a bunker for too long. Even in an operating silo, a stagnant region can be a problem for fuels like coal, which are prone to spontaneous combustion. Fine dust in the silo may lead to dust explosion. If the fuel flows through a channel

in the silo, the fuel outside of the channel remains stagnant for a long time. The residence time of such fuels in the silo should be limited by emptying the silo frequently or by using a first in-first out mass-flow pattern (Figure 8.6b), where all of the material is in motion whenever the fuel is discharged. Biomass is relatively free of this problem as most of it is not prone to spontaneous ignition.

The conventional means of producing ethanol from food sources like corn and sugarcane is, commercially, highly successful. In contrast, the production of ethanol from nonfood biomass (ligno-cellulose), although feasible in principle, is not widely used. More processing is required to make the sugar monomers in ligno-cellulose feedstock available to the microorganisms that produce ethanol by fermentation. However, production from food sources, even though it strains the food supply and is wasteful, is widespread.

Consider that only 50% of the dry kernel mass is transformed into ethanol, while the remaining kernel and the entire stock of the corn plant, regardless that it is grown using cultivation energy and incurs expenses, remains unuti­lized. It is difficult to ferment this part, which contains ligno-cellulose mass, so it is discarded as waste. Alternative methods are being developed to convert the cellulosic components of biomass into ethanol so that they can also be utilized for transport fuel. This option is discussed further in Section 9.5.4.

9.5.2 Gasoline

Petrogasoline is a mixture of hydrocarbons having a carbon number (i. e., the number of carbon-per-hydrocarbon molecules) primarily in the range of 5 to 11. These hydrocarbons belong to the following groups:

• Paraffins or alkanes

• Aromatics

• Olefins or alkenes

• Cycloalkanes or naphthenes

Gasifying agents react with solid carbon and heavier hydrocarbons to convert them into low-molecular-weight gases like CO and H2. The main gasifying agents used for gasification are

• Oxygen

• Steam

• Air

Oxygen is a popular gasifying agent, though it is primarily used for the combustion step. It may be supplied to a gasifier either in pure form or through air. The heating value and the composition of the gas produced in a gasifier are strong functions of the nature and amount of the gasifying agent used. A ternary diagram (Figure 5.1) of carbon, hydrogen, and oxygen (see Section 2.4.3) demonstrates the conversion paths of formation of different products in a gasifier.