The product gas of gasification is generally a mixture of several gases, including moisture or steam. Its composition may be expressed in any of the following ways:

• Mass fraction, mi

• Mole fraction, n

• Volume fraction, Vi

• Partial pressure, Pi

It may also be expressed on a dry or a wet basis. The wet basis is the composi­tion gas expressed on the basis of total mass of the gas mixture including any moisture in it. The dry basis is the composition with the moisture entirely removed.

The following example illustrates the relationship between different ways of expressing the product gas composition.

Example 2.2

The gasification of a biomass yields M kg/s product gas, with the production of its individual constituents as follows:

Hydrogen—MH, kg/s Carbon monoxide—MCO, kg/s Carbon dioxide—MCO2, kg/s Methane—MCH4, kg/s

Other hydrocarbon (e. g., C3H8)—MHC, kg/s Nitrogen—MN/ kg/s Moisture—MH2O, kg/s

Find the composition of the product gas in mass fraction, mole fraction, and other fractions.

Solution

Since the total gas production rate, M, is

M = Mh + Mco + Mco2 + Mch4 + Mh + MN + Mh2o Kg/s (i)

the mass fraction of each species is found by dividing the individual production rate by the total. For example, the mass fraction of hydrogen is mH = MH/M.

The mole of an individual species is found by dividing its mass by its molecular weight:

Moles of hydrogen, nH = mass/molecular weight of H2 = mH/2 (ii)

The total number of moles of all gases is found by adding the moles of i species of gases, n = X (ni)) moles. So the mole fraction of hydrogen is xH = nH/n. Similarly for any gas, the mole fraction is

Xi = n;/n (iii)

where the subscript refers to the ith species.

The volume fraction of a gas can be found by noting that the volume that 1 kmol of any gas occupies at NTP (at 0 °C and 1 atm) is 22.4 m3. So, taking the example of hydrogen, the volume of 1 kmol of hydrogen in the gas mixture is 22.4 nm3 at NTP.

The total volume of the gas mixture is V = summation of volumes of all con­stituting gases in the mixture = X[number of moles (n,) x 22.4] nm3 = 22.4 n. The volume fraction of hydrogen in the mixture is volume of hydrogen/total volume of the mixture:

VH = 22 AnHj(22.4 Ъц) = nH/n = xH

Thus, we note that

Volume fraction = mole fraction

The partial pressure of a gas is the pressure it exerts if it occupies the entire mixture volume, V. Ideal gas law gives the partial pressure of a gas component, , as

P = njRT/V Pa

The total pressure, P, of the gas mixture containing total moles, n, is

P = nRT/V Pa

So we can write

x,. = n = P = n.

n P V

Partial pressure as fraction of total pressure = mole fraction = volume fraction

The partial pressure of hydrogen is PH = xH P.

The molecular weight of the mixture gas, MWm, is known from the mass frac­tion and the molecular weight of individual gas species.

MWm = ‘L[XjMWj ] (vi)

where MWi is the molecular weight of gas component i with mole fraction xi.

Symbols and Nomenclature

ASH = weight percentage of ash (%)

C = weight percentage of carbon (%)

Cp = specific heat of biomass (J/g. K)

Cpg = specific heat of biomass at temperature в C (J/g. C)

Cw = specific heat of water (J/g. K)

dpore = pore diameter (m)

erad = emissivity in the pores (-)

FC = weight percentage of fixed carbon (%)

G(x), F(x), H(x) = functions of the cell structure and its dimensionless length of the biomass in Eq. (2.10) (-)

HR = heat of combustion or heat of reaction (kJ/mol)

HF = heat of formation (kJ/mol)

H = weight percentage of hydrogen (%)

HHV = high heating value of fuel (kJ/kg) hg = latent heat of vaporization (kJ/kg)

Kf = effective thermal conductivity of biomass (W/m. K)

Ks = thermal conductivity of the solid in dry wood (W/m. K)

Kw = thermal conductivity of the moisture in dry wood (W/m. K)

Kg = thermal conductivity of the gas in dry wood (W/m. K)

Krad = radiative contribution to the conductivity of wood (W/m. K)

LHV = low heating value of fuel (kJ/kg)

Mwet = biomass moisture expressed in wet basis (-)

Mdry = biomass moisture expressed in dry basis (-) md = moisture percentage (by weight, %)

M = weight percentage of moisture (%)

Ma = mass of surface moisture in biomass (kg)

Mi = mass of inherent moisture in biomass (kg)

Mf = mass of fuel (kg)

Mw = mass of moisture in the fuel (kg)

Mash = mass of ash in the fuel (kg) mt = mass fraction of the ith gas (-)

MW = molecular weight of gas mixture (-) n = number of moles (-)

Пі = mole fraction of the ith gas (-)

N = weight percentage of nitrogen (%)

O = weight percentage of oxygen (%)

Pi = partial pressure of the ith gas (Pa)

P = total pressure of the gas (Pa)

Q = heat content of fuel (kJ)

R = universal gas constant (8.314 J/mol. K) sp. gr = specific gravity (-)

S = weight percentage of sulfur (%)

T = temperature (K)

Wwet = weight of wet biomass (kg)

Wdry = weight of dry biomass (kg)

VM = weight percentage of volatile matter (%)

V = volume of gas (m3)

V = volume fraction of the ith gas (-)

&Hcomb = heat of combustion or reaction, kJ/mol ptme = true density of biomass (kg/m3) papparent = apparent density of biomass (kg/m3)

Ptuik = bulk density of biomass (kg/m3)

£b = bulk porosity of biomass (-)

Ep = porosity of biomass (-)

a = Steven-Boltzmann’s constant (5.67 x 10 8 W/ m2K4) в = temperature (°C)

Subscripts

ad, ar, db = subscripts representing air dry, as-received basis, and dry basis daf = dry ash-free basis td = total-dry basis i = ith component m = mixture

Methane (CH4), an important ingredient in the chemical and petrochemical industries, can come from natural gas as well as from solid hydrocarbons like

biomass or coal. For the latter source, the hydrocarbon is hydrogenated to produce synthetic gas, or substitute natural gas (SNG). The overall reaction for SNG production may be expressed as

C + 2H2 ^ CH4 — 74,800 kJ/kmol (1.14)

More details on these reactions are given in Chapters 5 and 9.

Symbols and Nomenclature

LHV = lower heating value of gas (kJ/mol)

Hc = combustion efficiency (-)

Hs = steam turbine efficiency (-)

Ha = efficiency of combustion and steam turbine (-)

Hg = gas turbine efficiency (-)

Hcc = combined efficiency of steam turbine and gas turbine (-)

Hb = overall efficiency, including gasification efficiency (-)

T1 = inlet steam temperature in the steam turbine (K)

T2 = exhaust steam temperature in the steam turbine (K)

Tgj = inlet temperature in the gas turbine (K)

Tg2 = exhaust temperature in the gas turbine (K)

The preceding discussions assume that the heat or mass transport rate is too high to offer any resistance to the overall rate of pyrolysis. This is true in the temperature range of 300 to 400 °C (Thurner and Mann, 1981), but at higher temperatures heat and mass transport influence the overall rate and so cannot be neglected. This section deals with heat transport during pyrolysis.

During pyrolysis, heat is transported to the particle’s outer surface by radia­tion and convection. Thereafter, it is transferred to the interior of the particle by conduction and pore convection (Figure 3.4). The following modes of heat transfer are involved in this process (Babu and Chaurasia, 2004b).

• Conduction inside the particle

• Convection inside the particle pores

• Convection and radiation from the particle surface

In a commercial pyrolyzer or gasifier, the system heats up a heat-transfer medium first; that, in turn, transfers the heat to the biomass. The heat-transfer medium can be one or a combination of the following:

• Reactor wall (for vacuum reactor)

• Gas (for entrained-bed or entrained-flow reactor)

• Heat-carrier solids (for fluidized bed)

Bubbling fluidized beds use mostly solid-solid heat transfer. Circulating fluidized beds and transport reactors make use of gas-solid heat transfer in addition to solid-solid heat transfer.

Since heat transfer to the interior of the biomass particle is mostly by thermal conduction, the low thermal conductivity of biomass (~0.1 W/m. K) is a major deterrent to the rapid heating of its interior. For this reason, even when the heating rate of the particle’s exterior is as fast as 10,000 °C/s, the interior can be heated at a considerably slower rate for a coarse particle. Because of the associated slow heating of the interior, the secondary reactions within the particles become increasingly important as the particle size increases, and as a result the liquid yield reduces (Scott and Piskorz, 1984). For example, Shen et al. (2009) noted that oil yield decreased with particle size within the range of 0.3 to 1.5 mm, but no effect was noted when the size was increased to

3.5 mm. Experimental results (Seebauer et al., 1997), however, do not show much effect of particle size on the biomass.

Classification based on the atomic ratio helps us to understand the heating value of a fuel, among other things. For example, the higher heating value (HHV) of a biomass correlates well with the oxygen-to-carbon (O/C) ratio, reducing from 38 to about 15 MJ/kg while the O/C ratio increases from 0.1 to 0.7. When the hydrogen-to-carbon (H/C) ratio increases, the effective heating value of the fuel reduces.

The atomic ratio is based on the hydrogen, oxygen, and carbon content of the fuel. Figure 2.10 plots the atomic ratios (H/C) against (O/C) on a dry ash­free basis for all fuels, from carbon-rich anthracite to carbon-deficient woody biomass. This plot, known as van Krevelen diagram, shows that biomass has much higher ratios of H/C and O/C than fossil fuel. For a large range of biomass, the H/C ratio might be expressed as a linear function of the (O/C) ratio (Jones et al., 2006).

(H/C) = 1.4125 (O/C) + 0.5004 (2.3)

Fresh plant biomass like leaves has very low heating values because of its high H/C and O/C ratios. The atomic ratio of a fuel decreases as its geological age increases, which means that the older the fuel, the higher its energy content. Anthracite, for example, a fossil fuel geologically formed over many thousands of years, has a very high heating value. Its lower H/C ratio gives higher heat, but the carbon intensity or the CO2 emission from its combustion is high.

Among all hydrocarbon fuels biomass is highest in oxygen content. Oxygen, unfortunately, does not make any useful contribution to heating value and

makes it difficult to transform the biomass into liquid fuels. The high oxygen and hydrogen content of biomass results in high volatile and liquid yields, respectively. High oxygen consumes a part of the hydrogen in the biomass, producing less beneficial water, and thus the high H/C content does not translate into high gas yield.

This section discusses pyrolyzer design considerations in the production of liquid fuel and charcoal through pyrolysis.

3.7.1 Production of Liquid through Pyrolysis

Pyrolysis is one of several means of production of liquid fuel from biomass. The maximum yield of organic liquid (pyrolytic oil or bio-oil) from thermal decomposition may be increased to as high as 70% (dry weight) if the biomass is rapidly heated to an intermediate temperature and if a short residence time in the pyrolysis zone is allowed to reduce secondary reactions. Table 3.2 earlier in the chapter shows how heating rate, pyrolysis temperature, and residence time affect the nature of the pyrolysis product. These findings may be sum­marized as follows:

• A slower heating rate, a lower temperature, and a longer residence time maximize the yield of solid char.

• A higher heating rate, a higher temperature, and a shorter residence time maximize the gas yield.

• A higher heating rate, an intermediate temperature, and a shorter residence time maximize the liquid yield.

There is an optimum pyrolysis temperature for maximum liquid yield. The yield is highest at 500 °C and drops sharply above and below this temperature (Boukis et al., 2007). The residence time is generally in the range of 0.1 to 2.0 seconds. These values depend on several factors, including the type of biomass (Klass, 1998). We can use a kinetic model for a reasonable yield assessment. The one proposed by Liden et al. (1988) is successful in predicting pyrolysis liquid yields over a wide range of conditions.

Heat transfer is a major consideration in the design of a pyrolyzer. The heat balance for a typical pyrolyzer may be written as

[Heat released by char combustion] + [Heat in incoming stream] (3 ^)

= [Heat required for pyrolysis] + [Heat loss]

Assessing heat loss accurately is difficult before the unit is designed. So, for preliminary assessment, we can take this to be 10% of the heat in the incoming stream (Boukis et al., 2007, p. 1377).

Fast, or flash, pyrolysis is especially suitable for pyrolytic liquefaction of biomass. The product is a mixture of several hydrocarbons, which allows pro­duction of fuel and chemicals through appropriate refining methods. The heating value of the liquid produced is in the same range (15-19 MJ/kg) as that of the parent biomass. The pyrolytic liquid contains several water-soluble sugars and polysaccharide-derivative compounds and water-insoluble pyrolytic lignin.

Pyrolytic liquid contains a much higher amount of oxygen (~50%) than does most fuel oil. It is also heavier (specific gravity ~1.3) and more viscous. Unlike fuel oil, pyrolytic oil increases in viscosity with time because of polymerization. This oil is not self-igniting like fuel oil, and as such it cannot be blended with diesel for operating a diesel engine.

Pyrolytic oil is, however, a good source of some useful chemicals, like natural food flavoring, that can be extracted, leaving the remaining product for burning. Alternately, we can subject the pyrolytic oil to hydrocracking to produce gasoline and diesel.

Gasification is almost as ancient as combustion, but it is less developed because commercial interest in it has not been as strong as in combustion. However, there has been a recent surge of interest in conversion of biomass into gas or liquid due to three motivating factors:

• Renewability benefits

• Environmental benefits

• Sociopolitical benefits

1.4.1 Renewability

Fossil fuels like coal, oil, and gas are good and convenient sources of energy, and they meet the energy demands of society very effectively. However, there is one major problem: Fossil fuel resources are finite and not renewable. Biomass, on the other hand, grows and is renewable. A crop cut this year will grow again next year; a tree cut today may grow up within a decade. Unlike fossil fuels, then, biomass is not likely to be depleted with consumption. For this reason, its use, especially for energy production, is rising fast.

We may argue against cutting trees for energy because they serve as a CO2 sink. This is true, but a tree stops absorbing CO2 after it dies. On the other hand, if left alone in the forest it can release CO2 in a forest fire or release more harmful CH4 when it decomposes in water. The use of a tree as fuel after its life provides carbon-neutral energy as well as avoids greenhouse gas release from deadwood. The best option is new planting following cutting, as is done by some pulp industries. Fast-growing plants like switch grass and Miscanthus are being considered as fuel for new energy projects. These plants have very short growing periods that can be counted in months.

3.1 INTRODUCTION

Pyrolysis is a thermochemical decomposition of biomass into a range of useful products, either in the total absence of oxidizing agents or with a limited supply that does not permit gasification to an appreciable extent. It is one of several reaction steps or zones observed in a gasifier. During pyrolysis, large complex hydrocarbon molecules of biomass break down into relatively smaller and simpler molecules of gas, liquid, and char (Figure 3.1).

Pyrolysis has similarity to and some overlap with processes like cracking, devolatilization, carbonization, dry distillation, destructive distillation, and thermolysis, but it has no similarity with the gasification process, which involves chemical reactions with an external agent known as gasification medium. Pyrol­ysis of biomass is typically carried out in a relatively low temperature range of 300 to 650 °C compared to 800 to 1000 °C for gasification.

Torrefaction is a relatively new process that heats the biomass in the absence of air to improve its usefulness as a fuel. Interest in torrefaction is rising on account of its several advantages.

This chapter explains the basics of pyrolysis and torrefaction. A brief dis­cussion of the design implications of the two is also presented.

2.1 INTRODUCTION

The characteristics of biomass greatly influence the performance of a biomass gasifier. A proper understanding of the physical and the chemical properties of biomass feedstock is essential for the design of a biomass gasifier to be reliable. This chapter discusses some important properties of biomass that are relevant to gasification and related processes.

2.2 WHAT IS BIOMASS?

Biomass refers to any organic materials that are derived from plants or animals (Loppinet-Serani et al., 2008). A generally accepted definition is difficult to find. However, the one used by the United Nations Framework Convention on Climate Change (UNFCCC, 2005) is relevant here:

[A] non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms. This shall also include products, by-products, residues and waste from agriculture, forestry and related industries as well as the non-fossilized and bio­degradable organic fractions of industrial and municipal wastes.

Biomass also includes gases and liquids recovered from the decomposition of nonfossilized and biodegradable organic materials. In the United States, there has been much debate on a legal definition. Appendix A gives a recent legal interpretation of renewable biomass.

As a sustainable and renewable energy resource, biomass is constantly being formed by the interaction of CO2, air, water, soil, and sunlight with plants and animals. After an organism dies, microorganisms break down biomass into elementary constituent parts like H2O, CO2, and its potential energy. Because the carbon dioxide, a biomass releases through the action of microorganisms or combustion was absorbed by it in the recent past, biomass combustion does not increase the total CO2 inventory of the Earth. It is thus called greenhouse gas neutral or GHG neutral.

Biomass Gasification and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00002-7

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

Biomass includes only living and recently dead biological species that can be used as fuel or in chemical production. It does not include organic materials that over many millions of years have been transformed by geological processes into substances such as coal or petroleum. Biomass comes from botanical (plant species) or biological (animal waste or carcass) sources, or from a combination of these. Common sources of biomass are:

• Agricultural: food grain, bagasse (crushed sugarcane), corn stalks, straw, seed hulls, nutshells, and manure from cattle, poultry, and hogs

• Forest: trees, wood waste, wood or bark, sawdust, timber slash, and mill scrap

• Municipal: sewage sludge, refuse-derived fuel (RDF), food waste, waste paper, and yard clippings

• Energy: poplars, willows, switchgrass, alfalfa, prairie bluestem, corn, and soybean, canola, and other plant oils

• Biological: animal waste, aquatic species, biological waste

Mass transfer can influence the pyrolysis product. For example, a sweep of gas over the fuel quickly removes the products from the pyrolysis environment.

Thus, secondary reactions such as thermal cracking, repolymerization, and recondensation are minimized (Sensoz and Angin, 2008).

A biomass can also be classified on the basis of its relative proportion of cel­lulose, hemicellulose, and lignin. For example, we can predict the behavior of a biomass during pyrolysis from knowledge of these components (Jones et al., 2006). Figure 2.11 plots the ratio of hemicellulose to lignin against the ratio of cellulose to lignin. In spite of some scatter, a certain proportionality can be detected between the two. Biomass falling within these clusters behaves simi­larly irrespective of its type. For a typical biomass, the cellulose-lignin ratio increases from ~0.5 to ~2.7, while the hemicellulose-lignin ratio increases from 0.5 to 2.0.