High heating rate and short residence time in the pyrolysis zone are two key requirements of high liquid yield. The ultra-rapid pyrolyzer, shown in Figure 3.9(c), developed by the University of Western Ontario provides extremely short mixing (10-20 ms), reactor residence (70-200 ms), and quench (~20 ms) times. Because the reactor temperature is also low (~650 °C), one can achieve a liquid yield as high as 90% (Hulet et al., 2005). The inert gas nitrogen is heated 100 °C above the reactor temperature and injected at very high velocity into the reactor to bombard a stream of biomass injected in the reactor. The reactor can also use a heat-carrier solid like sand that is heated externally and bombarded on a biomass stream through multiple jets. Such a high-velocity impact in the reactor results in an exceptionally high heating rate. The biomass is thus heated to the pyrolysis temperature in a few milliseconds. The pyrolysis product leaves the reactor from the bottom and is immediately cooled to sup­press a secondary reaction or cracking of the oil vapor. This process is therefore able to maximize the liquid yield during pyrolysis.

Three types of primary fuel are produced from biomass:

• Liquid (ethanol, biodiesel, methanol, vegetable oil, and pyrolysis oil)

• Gaseous (biogas (CH4, CO2), producer gas (CO, H2, CH4, CO2 , H2), syngas (CO, H2), substitute natural gas (CH4))

• Solid (charcoal, torrefied biomass)

From these come four major categories of product:

• Chemicals such as methanol, fertilizer, and synthetic fiber

• Energy such as heat

• Electricity

• Transportation fuel such as gasoline and diesel

The use of ethanol and biodiesel as transport fuels reduces the emission of CO2 per unit of energy production. It also lessens dependence on fossil fuel. Thus, biomass-based energy not only is renewable but is also clean from a greenhouse gas (GHG) emission standpoint, and so it can take the center stage on the global energy scene. This move is not new. Civilization began its energy use by burning biomass. Fossil fuels came much later, around 1600 A. D. Before the twentieth century, wood (a biomass) was the primary source of the world’s energy supply. Its large-scale use during the early Industrial Revolution caused so much deforestation in England that it affected industrial growth. As a result, from 1620 to 1720 iron production decreased from 180,000 to 80,000 tons per year (Higman and van der Burgt, 2008, p. 2). This situation was rectified by the discovery of coal, which began displacing wood for energy as well as for metallurgy.

Chemicals

Most chemicals produced from petroleum or natural gas can be produced from biomass. The two principal platforms for chemical production are sugar based and syngas based. The former involves sugars like glucose, fructose, xylose, arabinose, lactose, sucrose, and starch.

The syngas platform synthesizes the hydrogen and carbon monoxide con­stituent of syngas into chemical building blocks. Intermediate building blocks for different chemicals are numerous in this route. They include hydrogen, methanol, glycerol (C3), fumaric acid (C4), xylitol (C5), glucaric acid (C6), and gallic acid (Ar), to name a few (Werpy and Petersen, 2004). These inter­mediates are synthesized to produce large numbers of chemicals for industries involving transportation, textiles, food, the environment, communications, health, housing, and recreation. Werpy and Petersen (2004) identified 12 inter­mediate chemical building blocks having the highest potential for commercial products.

Energy

Biomass was probably the first on-demand source of energy that humans exploited. However, less than 22% of our primary energy demand is currently met by biomass or biomass-derived fuels. The position of biomass as a primary source of energy varies widely depending on the geographical and socio­economic conditions. For example, it constitutes 90% of the primary energy source in Nepal but only 0.1% in the Middle East. Cooking, although highly

FIGURE 1.3 Cooking stove using fire logs.

inefficient, is one of the most extensive uses of biomass in less-developed countries. Figure 1.3 shows a cooking stove still employed by millions in the rural areas using twigs or logs as fuel. A more efficient modern commercial use of biomass is in the production of steam for process heat and electricity generation like the facility shown in Figure 1.4.

Heat and electricity are two forms of primary energy derived from biomass. The use of biomass for efficient energy production is presently on the rise

in developed countries because of its carbon-neutral feature while its use for cooking is declining because of a shortage of biomass in less-developed countries.

Transport Fuel

Diesel and gasoline from crude petro-oil are widely used in modern transporta­tion industries. Biomass can help substitute these petro-derived transport fuels. Ethanol, produced generally from sugarcane and corn, is used in gasoline (spark-ignition) engines, while biodiesel, produced from vegetable oils such as rape seed, is used in diesel (compression-ignition) engines.

Pyrolysis, fermentation, and mechanical extraction are three major ways to produce transport fuel from biomass. Of these, commercially the most widely used method is fermentation, where sugar (sugarcane, etc.) or starch (corn, etc.) produces ethanol. It involves a relatively simple process where yeast helps ferment sugar or starch into ethanol and carbon dioxide. The production and refining of marketable ethanol takes a large amount of energy.

Extraction of vegetable oil from seeds, like rape seed, through mechanical means has been practiced for thousands of years. Presently, oils like canola oil are refined with alcohol (transesterification) to produce methyl ester or biodiesel.

Pyrolysis involves heating biomass in the absence of air to produce gas, char, and liquid. The liquid is a precursor of bio-oil, which may be hydro­treated to produce “green diesel” or “green gasoline.” At this time, ethanol and biodiesel dominate the world’s biofuels market.

Gasification and anaerobic digestion can produce methane gas from biomass. The methane gas can then be used directly in some spark-ignition engines for transportation, or converted into gasoline through methanol.

Proximate analysis gives the composition of the biomass in terms of gross components such as moisture (M), volatile matter (VM), ash (ASH), and fixed carbon (FC). It is a relatively simple and inexpensive process. For wood fuels, we can use standard E-870-06. Separate ASTM standards are applicable for determination of the individual components of biomass:

• Volatile matter: E-872 for wood fuels

• Ash: D-1102 for wood fuels

• Moisture: E-871 for wood fuels

• Fixed carbon: determined by difference

The moisture and ash determined in proximate analysis refer to the same moisture and ash determined in ultimate analysis. However, the fixed carbon in proximate analysis is different from the carbon in ultimate analysis: In

( Л

TABLE 2.8 Comparison of Ultimate Analysis (Dry Basis) of Some Biomass and Other Fossil Fuels

proximate analysis it does not include the carbon in the volatile matter and is often referred to as the char yield after devolatilization.

Volatile Matter

The volatile matter of a fuel is the condensable and noncondensable vapor released when the fuel is heated. Its amount depends on the rate of heating and the temperature to which it is heated. For the determination of volatile matter, the fuel is heated to a standard temperature and at a standard rate in a controlled

environment. The applicable ASTM standard for determination of volatile matter is E-872 for wood fuels and D-3175-07 for coal and coke.

Standard E-872 specifies that 50 g of test sample be taken out of no less than a 10-kg representative sample of biomass using the ASTM D-346 protocol. This sample is ground to less than 1 mm in size, and 1 g is taken from it, dried, and put in a covered crucible so as to avoid contact with air during devolatiliza­tion. The covered crucible is placed in a furnace at 950 °C and heated for seven minutes. The volatiles released are detected by luminous flame observed from the outside. After seven minutes, the crucible is taken out, cooled in a desicca­tor, and weighed to determine the weight loss due to devolatilization.

Standard D-3175-07, when used for nonsparking coal or coke, follows a similar process except that it requires a 1-g sample ground to 250 |am. The sample is heated in a furnace at 950 °C for seven minutes. For sparking coal or coke, the heating process deviates slightly from that specified in E-872: D-3175-07 specifies that the sample be gradually heated to 600 °C within six minutes and then put in a 950 °C furnace for six minutes. After this, the crucible containing the sample is removed and cooled for 15 minutes before it is weighed. Heating rates faster than this may yield higher volatile matter content, but that is not considered the volatile matter of the fuel’s proximate analysis.

Ash

Ash is the inorganic solid residue left after the fuel is completely burned. Its primary ingredients are silica, aluminum, iron, and calcium; small amounts of magnesium, titanium, sodium, and potassium may also be present. Ash content is determined by ASTM test protocol D-1102 for wood, E-1755-01 for other biomass, and D-3174 for coal.

Standard D-1102 specifies a 2-g sample of wood (sized below 475 micron) dried in a standard condition and placed in a muffle furnace with the lid of the crucible removed. Temperature of the furnace is raised slowly to 580 to 600 °C to avoid flaming. When all the carbon is burnt, the sample is cooled and weighed. Standard E-1755-01 specifies 1 g of biomass dried, initially heated to 250 °C at 10 °C/min, and held there for 30 minutes. Following this, the temperature is increased to 575 °C and kept there until all the carbon is burnt. After that the sample is cooled and weighed.

For coal or coke, standard D-3174-04 may be used. Here a 1-g sample (pulverized below 250 micron) is dried under standard conditions and heated to 450 to 500 °C for the first one hour and then to 700 to 750 °C (950 °C for coke) for the second hour. The sample is heated for two hours or longer at that temperature to ensure that the carbon is completely burnt. It is then removed from the furnace, cooled, and weighed.

Strictly speaking, this ash does not represent the original inorganic mineral matter in the fuel, as some of the ash constituents can undergo oxidation during

burning. For exact analysis, correction may be needed. The ash content of biomass is generally very small, but may play a significant role in biomass utilization especially if it contains alkali metals such as potassium or halides such as chlorine. Straw, grasses, and demolition wood are particularly suscep­tible to this problem. These components can lead to serious agglomeration, fouling, and corrosion in boilers or gasifiers (Mettanant et al., 2009).

The ash obtained from biomass conversion does not necessarily come entirely from the biomass itself. During collection, biomass is often scraped off the forest floor and then undergoes multiple handlings, during which it can pick up a considerable amount of dirt, rock, and other impurities. In many plants, these impurities constitute the major inorganic component of the biomass feedstock.

Moisture

High moisture is a major characteristic of biomass. The root of a plant biomass absorbs moisture from the ground and pushes it into the sapwood. The moisture travels to the leaves through the capillary passages. Photosynthesis reactions in the leaves use some of it, and the rest is released to the atmosphere through transpiration. For this reason there is more moisture in the leaves than in the tree trunk.

The total moisture content of some biomass can be as high as 90% (dry basis), as seen in Table 2.9. Moisture drains much of the deliverable energy from a gasification plant, as the energy used in evaporation is not recovered. This important input design parameter must be known for assessment of the cost of or energy penalty in drying the biomass. The moisture in biomass can remain in two forms: (1) free, or external; and (2) inherent, or equilibrium.

Free moisture is that above the equilibrium moisture content. It generally resides outside the cell walls. Inherent moisture, on the other hand, is absorbed within the cell walls. When the walls are completely saturated the biomass is said to have reached the fiber saturation point, or equilibrium moisture. Equi­librium moisture is a strong function of the relative humidity and weak function of air temperature. For example, the equilibrium moisture of wood increases

Source: Compiled from Kitani and Hall, 1 989, p. 863.

v___________________________ I____________________________ J

from 3 to 27% when the relative humidity increases from 10 to 80% (Jenkins, 1996, p. 864).

Moisture content (M) is determined by the test protocol given in ASTM standards D-871-82 for wood, D-1348-94 for cellulose, D-1762-84 for wood charcoal, and E-949-88 for RDF (total moisture). For equilibrium moisture in coal one could used D-1412-07. In these protocols, a weighed sample of the fuel is heated in an air oven at 103 °C and weighed after cooling. To ensure complete drying of the sample, the process is repeated until its weight remains unchanged. The difference in weight between a dry and a fresh sample gives the moisture content in the fuel.

Standard E-871-82, for example, specifies that a 50-g wood sample be dried at 103 °C for 30 minutes. It is left in the oven at that temperature for 16 hours before it is removed and weighed. The weight loss gives the moisture (M) of the proximate analysis.

Standard E-1358-06 provides an alternative means of measurement using microwave. However, this alternative represents only the physically bound moisture; moisture released through chemical reactions during pyrolysis con­stitutes volatile matter. The moisture content of some biomass fuels is given in Table 2.10.

Moisture Basis

Biomass moisture is often expressed on a dry basis. For example, if Wwet kg of wet biomass becomes Wdry after drying, its dry basis (Mdry) is expressed as

This can give a moisture percentage greater than 100% for very wet biomass, which might be confusing. For that reason, the basis of moisture should always be specified.

The wet-basis moisture is

The wet basis (Mwet) and the dry basis (Mdry) are related as

Fixed Carbon

Fixed carbon (FC) in a fuel is determined from the following equation, where M, VM, and ASH stand for moisture, volatile matter, and ash, respectively.

FC = 1 — M — vm — ash

This represents the solid carbon in the biomass that remains in the char in the pyrolysis process after devolatilization. With coal, FC includes elemental carbon in the original fuel, plus any carbonaceous residue formed while heating, in the determination of VM (standard D-3175).

Biomass carbon comes from photosynthetic fixation of CO2 and thus all of it is organic. During the determination of VM, a part of the organic carbon is transformed into a carbonaceous material called pyrolytic carbon. Since FC depends on the amount of VM, it is not determined directly. VM also varies with the rate of heating. In a real sense, then, fixed carbon is not a fixed quantity, but its value, measured under standard conditions, gives a useful evaluation parameter of the fuel. For gasification analysis, FC is an important parameter because in most gasifiers the conversion of fixed carbon into gases determines the rate of gasification and its yield. This conversion reaction, being the slowest, is used to determine the size of the gasifier.

Char

Char, though a carbon residue of pyrolysis or devolatilization, is not pure carbon; it is not the fixed carbon of the biomass. Known as pyrolytic char, it contains some volatiles and ash in addition to fixed carbon. Biomass char is very reactive. It is highly porous and does not cake. This noncaking property makes it easy to handle.

1.4.2 Thermogravimetric Analysis

Because of the time and expense involved in proximate analysis by ASTM D-3172, Klass (1998) proposed an alternative using thermogravimetry (TG) or differential thermogravimetry (DTG). In these techniques, a small sample of the fuel is heated in a specified atmosphere at the desired rate in an electronic microbalance. This gives a continuous record of the weight change of the fuel sample in a TG apparatus. The DTG apparatus gives the rate of change in the

weight of the fuel sample continuously. Thus, from the measured weight loss — versus-time graphs, we can determine the fuel’s moisture, volatile mater, and ash content. The fixed carbon can be found from Eq. (2.23). This method, though not an industry standard, can quickly provide information regarding the thermochemical conversion of a fuel. Table 2.10 compares results of proximate analysis (dry basis) of some biomass from the ASTM and TG methods.

TG analysis provides additional information on reaction mechanisms, kinetic parameters, thermal stability, and heat of reaction. A detailed database of thermal analysis is given in Gaur and Reed (1995).

Instead of burning it entirely, we can gasify the carbon by restricting the oxygen supply. The carbon then produces 72% less heat than that in combustion, but the partial gasification reaction shown here produces a combustible gas, CO.

C +1/2 O2 ^ CO -110,530 kJ/kmol (1.5)

When the gasification product, CO, subsequently burns in adequate oxygen, it produces the remaining 72% (283 MJ) of the heat. Thus, the CO retains only 72% of the energy of the carbon, but in complete gasification the energy recov­ery is 75 to 88% owing to the presence of hydrogen and other hydrocarbons.

The producer gas reaction is an endothermic gasification reaction, which produces hydrogen and carbon monoxide from carbon. This product gas mixture is also known as synthesis gas, or syngas.

Producer gas reaction: C + H2O ^ CO + H2 +131,000 kJ/kmol (1.6)

Production of heavy oil residue in oil refineries is an important application of gasification. Low-hydrogen residues are gasified into hydrogen.

Heavy oil gasification: CnHm + (n/2) O2 ^ nCO + (m/2) H2 (1.7)

This hydrogen can be used for hydrocracking of other heavy oil fractions into lighter oils.

The reaction between steam and carbon monoxide is also used for maximi­zation of hydrogen production in the gasification process at the expense of CO.

Shift reaction: CO + H2O ^ H2 + CO2 — 41,000 kJ/kmol (1.8)

During pyrolysis, a fuel particle is heated at a defined rate from the ambient to a maximum temperature, known as the pyrolysis temperature. The fuel is held there until completion of the process. The pyrolysis temperature affects both composition and yield of the product. Figure 3.6 is an example of how, during the pyrolysis of a biomass, the release of various product gases changes with different temperatures. We can see that the release rates vary widely for differ­ent gaseous constituents.

The amount of char produced also depends on the pyrolysis temperature. Low temperatures result in more char; high temperatures result in less. Figure 3.7 shows how the amount of char produced from the pyrolysis of a birch wood particle decreases with increasing temperature.

3.3.1 Effect of Heating Rate

The rate of heating of the biomass particles has an important influence on the yield and composition of the product. Rapid heating to a moderate temperature (400-600 °C) yields higher volatiles and hence more liquid, while slower heating to that temperature produces more char. For example, Debdoubi et al. (2006) observed that, when the heating rate increased from 5 to 250 °C/min to 400 to 500 °C/min, the liquid yield from Esparto increased from 45 to 68.5%.

The heating rate alone, however, does not define the product. The residence time of the product in the reactor is also important. During slow heating, a slow or gradual removal of volatiles from the reactor permits a secondary reaction to occur between char particles and volatiles, leading to a secondary char formation.

The operating parameters of a pyrolyzer are adjusted to meet the require­ment of the final product of interest. Tentative design norms for heating in a pyrolyzer include the following:

• To maximize char production, use a slow heating rate (<0.01-2.0 °C/s), a low final temperature, and a long gas residence time.

• To maximize liquid yield, use a high heating rate, a moderate final tempera­ture (450-600 °C), and a short gas residence time.

• To maximize gas production, use a slow heating rate, a high final tempera­ture (700-900 °C), and a long gas residence time.

Production of charcoal through carbonization uses the first norm. Fast pyrolysis uses the second to maximize liquid yield. The third norm is used when gas production is to be maximized.

3.4 PYROLYSIS KINETICS

A study of pyrolysis kinetics provides important information for the engineer­ing design of a pyrolyzer or a gasifier. It also helps explain how the different processes in a pyrolyzer affect product yields and composition. Three major processes that influence the pyrolysis rate are chemical kinetics, heat transfer, and mass transfer. This section describes the physical and chemical aspects that govern the process.

3.4.1 Physical Aspects

From a thermal standpoint, we may divide the pyrolysis process into four stages. Although divided by temperature, the boundaries between them are not sharp; there is always some overlap:

Drying (~100 °C). During the initial phase of biomass heating at low tem­perature, the free moisture and some loosely bound water is released. The free moisture evaporates, and the heat is then conducted into the biomass interior (Figure 3.4). If the humidity is high, the bound water aids the melting of the lignitic fraction, which solidifies on subsequent cooling. This phenomenon is used in steam bending of wood, which is a popular practice for shaping it for furniture (Diebold and Bridgwater, 1997).

Initial Stage (100-300 °C). In this stage, exothermic dehydration of the biomass takes place with the release of water and low-molecular-weight gases like CO and CO2.

Intermediate Stage (>200 °C). This is primary pyrolysis, and it takes place in the temperature range of 200 to 600 °C. Most of the vapor or precursor to bio-oil is produced at this stage. Large molecules of biomass particles decompose into char (primary char), condensable gases (vapors and precur­sors of the liquid yield), and noncondensable gases.

Final stage (~300-900 °C). The final stage of pyrolysis involves secondary cracking of volatiles into char and noncondensable gases. If they reside in the biomass long enough, relatively large-molecular-weight condensable gases can crack, yielding additional char (called secondary char) and gases. This stage typically occurs above 300 °C (Reed, 2002, p. III-6). The con­densable gases, if removed quickly from the reaction site, condense outside in the downstream reactor as tar or bio-oil. It is apparent from Figure 3.6 that a higher pyrolysis temperature favors production of hydrogen, which increases quickly above 600 °C. An additional contribution of the shift reaction (Eq. 1.8) further increases the hydrogen yield above 900 °C.

Temperature has a major influence on the product of pyrolysis. The carbon dioxide yield is high at lower temperatures and decreases at higher tempera­tures. The release of hydrocarbon gases peaks at around 450 °C and then starts decreasing above 500 °C, boosting the generation of hydrogen.

Hot char particles can catalyze the primary cracking of the vapor released within the biomass particle and the secondary cracking occurring outside the particle but inside the reactor. To avoid cracking of condensable gases and thereby increase the liquid yield, rapid removal of the condensable vapor is very important. The shorter the residence time of the condensable gas in the reactor, the less the secondary cracking and hence the higher the liquid yield.

Some overlap of the stages in the pyrolysis process is natural. For example, owing to its low thermal conductivity (0.1-0.05 W/m. K), a large log of wood may be burning outside while the interior may still be in the drying phase, and water may be squeezed out from the ends. During a forest fire this phenomenon is often observed. The observed intense flame comes primarily from the com­bustion of the pyrolysis products released from the wood interior rather than from the burning of the exterior surface.

Wood is typically made of hollow, elongated, spindle-shaped cells arranged parallel to each other. Figure 2.4 is a photograph of the cross-section of a tree trunk showing the overall structure of a mature tree wood.

Bark is the outermost layer of a tree trunk or branch. It comprises an outer dead portion and an inner live portion. The inner live layer carries food from the leaves to the growing parts of the tree. It is made up of another layer known as sapwood, which carries sap from the roots to the leaves. Beyond this layer lies the inactive heartwood. In any cut wood we easily note a large number of radial marks. These radial cells (wood rays) carry food across the wood layers.

Wood cells that carry fluids are also known as fibers or tracheids. They are hollow and contain extractives and air. The cells vary in shape but are generally short and pointed. The length of an average tracheid is about 1000 microns (|am) for hardwood and typically 3000 to 8000 |am for softwood (Miller, R. B., 1999).

Tracheids are narrow. For example, the average diameter of the tracheid of softwood is 33 |am. These cells are the main conduits for the movement of sap along the length of the tree trunk. They are mostly aligned longitudinally, but there are some radial tracheids (G) that carry sap across layers. Lateral chan­nels, called pith, transport water between adjacent cells across the cell layers. Softwood can have cells or channels for carrying resins. A hardwood, on the other hand, contains large numbers of pores or open vessels.

The tracheids or cells typically form an outer primary and an inner second­ary wall. A layer called the middle lamella, joins or glues together adjacent cells. The middle lamella is predominantly made of lignin. The secondary wall (inside the primary layer) is made up of three layers: S1, S2, and S3 (Figure 2.5). The thickest layer, S2, is made of macrofibrils, which consist of long cel­lulose molecules with embedded hemicellulose. The construction of cell walls in wood is similar to that of steel-reinforced concrete, with the cellulose fibers

LCompound middle lamella

Distance

FIGURE 2.6 Distribution of cellulose, hemicellulose, and lignin in cell walls and their layers.

acting as the reinforcing steel rods and hemicellulose surrounding the cellulose microfibrils acting as the cement-concrete. The S2 layer has the highest concentration of cellulose. The highest concentration of hemicellulose is in layer S3. The distribution of these components in the cell wall is shown in Figure 2.6.

This process, shown in Figure 3.9(d), involves creation of high pressure between a biomass particle and a hot reactor wall. This allows uninhibited heat transfer from the wall to the biomass, causing the liquid product to melt out of the biomass the way frozen butter melts when pressed against a hot pan. The biomass sliding against the wall leaves behind a liquid film that evaporates and leaves the pyrolysis zone, which is the interface between biomass and wall. As a result of high heat transfer and short gas residence time, a liquid yield as high as 80% is reported (Diebold and Power, 1988). The pressure between biomass and wall is created either by mechanical means or by centrifugal force. In a mechanical system a large piece of biomass is pressed against a rotating hot plate.

The bulky and inconvenient form of biomass is a major barrier to a rapid shift from fossil to biomass fuels. Unlike gas or liquid, biomass cannot be handled, stored, or transported easily, especially in its use for transportation. This provides a major motivation for the conversion of solid biomass into liquid and gaseous fuels, which can be achieved through one of two major paths (Figure 1.5): (1) biochemical (fermentation) and (2) thermochemical (pyrolysis, gasification).

Biochemical conversion is perhaps the most ancient means of biomass gasification. India and China produced methane gas for local energy needs by anaerobic microbial digestion of animal wastes. In modern times, most of the ethanol for automotive fuels is produced from corn using fermentation. Ther­mochemical conversion of biomass into gases came much later. Large-scale use of small biomass gasifiers began during the Second World War, when more than a million units were in use. Figure 1.5 shows that the two broad routes of

conversion are subdivided into several categories. A brief description of these follows.

The composition of a fuel is often expressed on different bases depending on the situation. The following four bases of analysis are commonly used:

• As received

• Air dry

• Total dry

• Dry and ash-free

A comparison of these is shown in Figure 2.14.

As-Received Basis

When using the as-received basis, the results of ultimate and proximate analy­ses may be written as follows:

Ultimate: С + H + О + N + S + ASH + M = 100% (2.24)

Proximate: VM + FC + M + ASH = 100% (2.25)

where VM, FC, M, and ASH represent the weight percentages of volatile matter, fixed carbon, moisture, and ash, respectively, measured by proximate analysis; and C, H, O, N, and S represent the weight percentages of carbon, hydrogen,

-As-received basis­—Air-dry basis

-Total-dry basis

-Dry and ash-free basis-

-Volatile-

A ash O oxygen Mi inherent moisture H hydrogen N nitrogen Ms surface moisture C carbon S sulfur

FIGURE 2.14 Bases for expressing fuel composition.

oxygen, nitrogen, and sulfur, respectively, as measured by ultimate analysis. The ash and moisture content of the fuel is the same in both analyses. As received can be converted into other bases.

Air-Dry Basis

When the fuel is dried in air its surface moisture is removed while its inherent moisture is retained. So, to express the constituent on an air-dry basis, the amount is divided by the total mass less the surface moisture. For example, the carbon percentage on the air-dry basis is calculated as

cad = %

100 — ma

where Ma is the mass of surface moisture removed from 100 kg of moist fuel after drying in air. Other constituents of the fuel can be expressed similarly.

Total-Dry Basis

Fuel composition on the air-dry basis is a practical parameter and is easy to measure, but to express it on a totally moisture-free basis we must make allow­ance for surface as well as inherent moisture. This gives the carbon on a total — dry basis, Ctd:

where M is the total moisture (surface + inherent) in the fuel: M = Ma + Mi.

Dry Ash-Free Basis

Ash is another component that at times is eliminated along with moisture. This gives the fuel composition on a dry ash-free (DAF) basis. Following the afore­mentioned examples, the carbon percentage on a dry ash-free basis, Cdaf, can be found.

where (100 — M — ASH) is the mass of biomass without moisture and ash. The percentages of all constituents on any basis totals 100. For example:

Cdaf + Hdaf + 0daf + Ndaf + Sdaf = 100%

Syngas is also produced from natural gas (>80% CH4), using a steam-methane­reforming reaction, instead of from solid carbonaceous fuel alone. The reform­ing reaction is, however, not gasification but a molecular rearrangement.

Steam reforming reaction: CH4 + H2O (catalyst) ^ CO + 3H2 ^ „)

+ 206,000 kJ/kmol (1.)

Partial oxidation of natural gas or methane is an alternative route for produc­tion of syngas. In contrast to the reforming reaction, partial oxidation is exo­thermic. Partial oxidation of fuel oil also produces syngas.

Partial oxidation reforming: CH4 +1/2 O2 ^ CO + 2H2 (i-m)

— 36,000 kJ/kmol (L10)

The hydrogen may be used as fuel in fuel cells or in production of chemical feedstocks like methanol and ammonia.