Pyrolyzers have been used since ancient times to produce charcoal (Figure 3.2). Early pyrolyzers operated in batch mode using a very slow rate of heating and for long periods of reaction to maximize the production of char. If the objective of pyrolysis was to produce the maximum amount of liquid or gas, then the rate of heating, the peak pyrolysis temperature, and the duration of pyrolysis had to be chosen accordingly. These choices also decided what kind of reactor was to be used. Table 3.5 shows how the yield is maximized for different choices of heating rate, temperature, and gas residence time.

Modern pyrolyzers are more concerned with gas and liquid products, and require a continuous process. A number of different types of pyrolysis reactor have been developed. Based on the gas-solid contacting mode, they can be broadly classified as fixed bed, fluidized bed, and entrained bed, and then further subdivided depending on design configuration. The following are some of the major pyrolyzer designs in use:

• Fixed or moving bed

• Bubbling fluidized bed

• Circulating fluidized bed

• Ultra-rapid

• Rotating cone

• Ablative

• Vacuum

Except for the moving bed other gasifier types are illustrated in Figure 3.9.

The following sections describe some important thermophysical properties of biomass that are relevant to gasification.

1.4.1 Physical Properties

Some of the physical properties of biomass affect its pyrolysis and gasification behavior. For example, permeability is an important factor in pyrolysis. High permeability allows pyrolysis gases to be trapped in the pores, increasing their residence time in the reaction zone. Thus, it increases the potential for second­ary cracking to produce char. The pores in a wood are generally oriented longitudinally. As a result, the thermal conductivity and diffusivity in the longitudinal direction are different from those in the lateral direction. This anisotropic behavior of wood can affect its thermochemical conversion. A densification process such as torrefaction (Chapter 3) can reduce the anisotropic behavior and therefore change the permeability of a biomass.

Densities

For a granular biomass, we can define four characteristic densities: true, appar­ent, bulk, and biomass (growth).

True Density

True density is the weight per unit volume occupied by the solid constituent of biomass. Total weight is divided by actual volume of the solid content to give its true density.

Total mass of biomass Solid volume in biomass

The cell walls constitute the major solid content of a biomass. For common wood, the density of the cell wall is typically 1530 kg/m3, and it is constant for most wood cells (Desch and Dinwoodie, 1981). The measurement of true density of a biomass is as difficult as the measurement of true solid volume. It can be measured with a pycnometer, or it may be estimated using ultimate analysis and the true density of its constituent elements. True densities of some elements are given in Table 2.4.

Apparent Density

Apparent density is based on the apparent or external volume of the biomass. This includes its pore volume (or that of its cell cavities). For a regularly shaped biomass, mechanical means such as micrometers can be used to measure dif­ferent sides of a particle to obtain its apparent volume. An alternative is the use of volume displacement in water. The apparent density considers the internal

pores of a biomass particle but not the interstitial volume between biomass particles packed together.

The pore volume of a biomass expressed as a fraction of its total volume is known as its porosity, ep. This is an important characteristic of the biomass.

Apparent density is most commonly used for design calculations because it is the easiest to measure and it gives the actual volume occupied by a particle in a system.

Bulk Density

Bulk density is based on the overall space occupied by an amount or a group of biomass particles.

Bulk volume includes interstitial volume between the particles, and as such it depends on how the biomass is packed. For example, after pouring the biomass particles into a vessel, if the vessel is tapped, the volume occupied by the particles settles to a lower value. The interstitial volume expressed as func­tion of the total packed volume is known as bulk porosity, eb.

To determine the biomass bulk density, we can use standards like the American Society for Testing of Materials (ASTM) E-873-06. This process involves pouring the biomass into a standard-size box (305 mm x 305 mm x 305 mm) from a height of 610 mm. The box is then dropped from a height of 150 mm three times for settlement and refilling. The final weight of the biomass in the box divided by the box volume gives its bulk density.

The total mass of the biomass may contain the green moisture of a living plant, external moisture collected in storage, and moisture inherent in the biomass. Once the biomass is dried in a standard oven, its mass reduces. Thus,
the density can be either green or oven-dry depending on if its weight includes surface moisture. The external moisture depends on the degree of wetness of the received biomass. To avoid this issue, we can completely saturate the biomass in deionized water, measure its maximum moisture density, and specify its bulk density accordingly.

Three of the preceding densities of biomass are related as follows:

papparent ptrue (1 -£p)

pbulk papparent(1 &b)

where Ep is the void fraction (voidage) in a biomass particle, and eb is the voidage of particle packing.

Biomass (Growth) Density

The term biomass (growth) density is used in bioresource industries to express how much biomass is available per unit area of land. It is defined as the total amount of above-ground living organic matter in trees expressed as oven-dry tons per unit area (e. g., tonnes per hectare) and includes all organic materials: leaves, twigs, branches, main bole, bark, and trees.

A major attraction of gasification is that it can convert waste or low-priced

fuels, such as biomass, coal, and petcoke, into high-value chemicals like metha­nol. Biomass holds great appeal for industries and businesses, especially in the

energy sector. For example:

• Downstream flue-gas cleaning in a gasification plant is less expensive than that in a coal-fired plant with flue-gas desulphurization, selective catalytic reducers (SCRs), and electrostatic precipitators.

• Polygeneration is a unique feature of a gasifier plant. It can deliver steam for process, electricity for grid, and gas for synthesis, thereby providing a good product mix. Additionally, for high-sulfur fuel a gasifier plant pro­duces elemental sulfur as a by-product; for high-ash fuel, slag or fly ash is obtained, which can be used for cement manufacture.

• For power generation, an IGCC plant can achieve a higher overall efficiency (38-41%) than can a combustion plant with a steam turbine alone. Gasifica­tion therefore offers lower power production costs.

• Carbon dioxide capture and sequestration (CCS) may become mandatory for power plants. An IGCC plant can capture and store CO2 at one-half of what it costs a traditional PC plant (www. gasification. org). Other applications of gasification that produce transport fuel or chemicals may have even lower CCS costs. Established technologies are available to capture carbon dioxide from a gasification plant, but that is not so for a combustion plant.

• A process plant that uses natural gas as feedstock can use locally available biomass or organic waste instead, and thereby reduce dependence on imported natural gas, which is not only rising sharply in price but is also experiencing supply volatility.

• Gasification provides significant environmental benefits, as described in Section 1.4.2.

• Total water consumption in a gasification-based power plant is much less than that in a conventional power plant (Table 1.5). Furthermore, a plant can be designed to recycle its process water. For this reason, all zero — emission plants use gasification technology.

• Gasification plants produce significantly lower quantities of major air pol­lutants like SO2, NOx, and particulates. Figure 1.8 compares the emission from a coal-based IGCC plant with that from a combustion-based coal-fired

□ NOX

—

“SOX

■ PM

steam power plant and a natural-gas-fired plant. It shows emissions from the gasification plant are similar to those from a natural-gas-fired plant.

• An IGCC plant produces lower CO2 per MWh than a combustion-based steam power plant.

Carbonization is a slow pyrolysis process, in which the production of charcoal or char is the primary goal. It is the oldest form of pyrolysis, in use for thousands of years. The biomass is heated slowly in the absence of oxygen to a relatively low temperature (~400 °C) over an extended period of time, which in ancient times ran for several days to maximize the char formation. Figure 3.2 is a sketch of a typical beehive oven in which large logs were stacked and covered by a clay wall. A small fire at the bottom provided the required heat, which essentially stayed in the well-insulated closed chamber. Carboniza­tion allows adequate time for the condensable vapor to be converted into char and noncondensable gases.

Conventional pyrolysis involves all three types of pyrolysis product (gas, liquid, and char). As such, it heats the biomass at a moderate rate to a moderate temperature (~600 °C). The product residence time is on the order of minutes.

Fast Pyrolysis

The primary goal of fast pyrolysis is to maximize the production of liquid or bio-oil. The biomass is heated so rapidly that it reaches the peak (pyrolysis)

temperature before it decomposes. The heating rate can be as high as 1000 to 10,000 °C/s, but the peak temperature should be below 650 °C if bio-oil is the product of interest. However, the peak temperature can be up to 1000 °C if the production of gas is of primary interest. Fluidized beds similar to the one shown in Figures 3.5 and 3.9(a) and (b) (see p. 82), may be used for fast pyrolysis.

Four important features of the fast pyrolysis process that help increase the liquid yield are: (1) very high heating rate, (2) reaction temperature within the range of 425 to 600 °C, (3) short residence time (< 3 s) of vapor in the reactor, and (4) rapid quenching of the product gas.

Flash Pyrolysis

In flash pyrolysis biomass is heated rapidly in the absence of oxygen to a rela­tively modest temperature range of 450 to 600 °C. The product, containing condensable and noncondensable gas, leaves the pyrolyzer within a short resi­dence time of 30 to 1500 ms (Bridgwater, 1999). Upon cooling, the condens­able vapor is then condensed into a liquid fuel known as bio-oil. Such an operation increases the liquid yield while reducing the char production. A typical yield of bio-oil in flash pyrolysis is 70 to 75% of the total pyrolysis product.