A typical gasifier design starts with a desired composition of the product gas. Equilibrium and other calculations are carried out to check how closely that targeted composition is made through a choice of design parameters.

The product of combustion reactions is predominantly made up of carbon dioxide and steam, the percentages of which can be estimated with a fair degree of accuracy from simple stoichiometric calculations. For gasification reactions, this calculation is not straightforward; the fraction of the fuel gasified and the composition of the product gas needs to be estimated carefully. Unlike combus­tion reactions, gasification reactions do not always reach equilibrium, so only a rough estimate is possible through an equilibrium calculation. Still, this can be a reasonable start for the design until detailed kinetic modeling is carried out in the design optimization stage.

The reactors used so far for SCWG research have been either batch or continu­ous (flow). Depending on their type of mixing, they can be further divided as follows:

• Autoclave

• Tubular steel

• Stirred tank

• Quartz capillary tube

• Fluidized bed

A batch reactor is simple, does not require a high-pressure pump and can be used for almost all biomass feedstock. However, its reaction processes are not isothermal and it needs time to heat up and cool down. During heat-up many reactions occur that cause transformation of the feedstock; this does not happen in a continuous-flow reactor.

Reactor type has an important effect on the influence of feed concentration. The drop in gasification efficiency with feed concentration, noted in tubular reactors, was not found in the stirred-tank reactor studied by Matsumura et al. (2005). However, the reactor used was exceptionally small (1.0 mm in diam­eter), so validation of this finding in a reasonably large reactor (Matsumura and Minowa, 2004) is necessary. The process development of SCW gasifiers is lagging laboratory research because of engineering difficulties and the high cost of pilot plant construction.

7.4 APPLICATION OF BIOMASS CONVERSION IN SCWG

Three major areas of application for biomass SCWG are: (1) energy conversion, (2) waste remediation, and (3) chemical production.

7.5.1 Energy Conversion

All three of the followng important feedstocks for the energy industry can be produced by biomass conversion in supercritical water:

•
Bio-oil: Potential use in the transport sector

• Methanol: Though a chemical feedstock, may be used for combustion

• Hydrogen: Potential use in fuel cells

The overall efficiency of an energy conversion system depends on the technology route, on the wetness of the biomass, and on many other factors. Yoshida et al. (2003) compared the effect of moisture content on the net effi­ciency of seven options for electricity generation, including an SCWG com­bined cycle. Interestingly, the SCWG-based system shows a total efficiency independent of moisture content, while for all other systems, total efficiency decreases with increasing moisture. Total electricity generation efficiency is even higher than that for conventional combustion-based systems. Integrated gasification combined cycle (IGCC) efficiency is higher than that of SCWG for biomass containing less than 40% moisture. Above 40%, its efficiency drops below that of SCWG (Figure 7.9).

Yoshida et al. (2003) also compared the total heat utilization efficiency of seven energy conversion processes:

• Direct combustion of biomass

•

Combustion of biomass-oil produced by liquefaction or pyrolysis

FIGURE 7.9 Dependence of net electricity generation efficiency of different biomass-based processes on biomass moisture content. (Source: Adapted from Yoshida et al., 2003.)

• Combustion of methanol produced by thermal gasification

• Combustion of methanol produced by SCWG

• Combustion of biogas produced by thermal gasification

• Combustion of methanol produced by supercritical water gasification

• Anaerobic digestion

Supercritical water gasification has the distinction of easily separating CO2 from the product gas. This makes it an optimal technology for generation of electricity and heat from biomass when CO2 emission limits become binding.

Fuel cells have the highest energy conversion efficiency for electricity generation, but they need hydrogen as their fuel. For hydrogen production, from very wet biomass, SCW gasification could be an attractive route. However, the capital costs of a fuel cell and that of a gasification plant have an important bearing on the economic viability of this generation option.

In an underbed feed system (Figure 8.20b), fuel particles are crushed into sizes smaller than 8 to 10 mm. Introduced in Section 8.4.2 as pneumatic feeding, this system is relatively expensive, complicated, and less reliable than the overbed

( ; л

TABLE 8.1 Feed Points for Some Commercial Bubbling Fluidized-Bed Boilers

system (especially with moist fuels), but it does achieve high char conversion efficiency.

Fuel entering at a feed point disperses over a much smaller area than it does in overbed feeding, so the feed points are more numerous and more closely spaced. Spacing greatly affects gasification. Because a deeper bed allows wider dispersion of the fuel and hence works with wider feed-point spacing, increased spacing with no sacrifice of char conversion efficiency can be achieved, but only if it is compensated by a corresponding increase in bed height. A decrease in bed height must be matched by increased feed-point spacing; otherwise, the conversion efficiency can drop. Coarser particles take longer to gasify and are less prone to entrainment. Therefore, wider spacing is preferred for them; finer particles require closer spacing.

The freeboard can provide room for further reaction of particles entrained from the bed. Freeboard design is important, especially when wide feed-point spacing is used.

As we can see in Table 4.3, tar is a mixture of various hydrocarbons. It may also contain oxygen-containing compounds, derivatives of phenol, guaiacol, veratrol, syringol, free fatty acids, and esters of fatty acids (Razvigorova et al., 1994). The yield and composition of tar depends on the reaction temperature, the type of reactor, and the feedstock. Table 4.3 shows that benzene is the largest component of a typical tar.

Tar may be classified into four major product groups: primary, secondary, alkyl tertiary, and condensed tertiary (Evans and Milne, 1997). Short descrip­tions of these follow.

Primary Tar

Primary tar is produced during primary pyrolysis. It comprises oxygenated, primary organic, condensable molecules. Primary products come directly from the breakdown of the cellulose, hemicellulose, and lignin components of biomass. Milne et al. (1998) listed a large number of compounds of acids, sugars, alcohols, ketones, aldehydes, phenols, guaiacols, syringols, furans, and mixed oxygenates in this group.

Secondary Tar

As the gasifier’s temperature rises above 500 °C, primary tar begins to rearrange, forming more noncondensable gases and heavier molecules called secondary tar, of which phenols and olefins are important constituents.

Tertiary Tar Products

The alkyl tertiary product includes methyl derivatives of aromatics, such as methyl acenaphthylene, methylnaphthalene, toluene, and indene (Evans and Milne, 1997).

Condensed tertiary aromatics make up a polynuclear aromatic hydrocarbon (PAH) series without substituents (atoms or a group of atoms substituted for hydrogen in the parent chain of hydrocarbon). This series contains benzene, naphthalene, acenaphthylene, anthracene/phenanthrene, and pyrene.

The secondary and tertiary tar products come from the primary tar. The primary products are destroyed before the tertiary products appear (Milne et al., 1998).

Figure 4.2 shows that with increasing temperature the primary tar product decreases but the tertiary product increases. Above 500 °C the secondary tar increases at the expense of the primary tar. Once the primary tar is nearly destroyed, tertiary tar starts appearing with increasing temperature. At this stage the secondary tar begins to decrease. Thus, high temperatures destroy the primary tar but not the tertiary tar products.

6.1 INTRODUCTION

The design of a gasification plant includes the gasifier reactor as well as its auxiliary or support equipment. A typical biomass gasification plant design comprises the following systems:

• Gasifier reactor

• Biomass-handling system

• Biomass-feeding system

• Gas-cleanup system

• Ash or solid residue-removal system

This chapter deals with the design of the gasifier reactor alone. Chapter 8 dis­cusses the design of the handling and feeding systems. Gas-cleaning systems are briefly discussed in Chapters 4 and 9.

As with most process plant equipment, the design of a gasifier may be divided into three major phases:

Phase 1. Process design and preliminary sizing Phase 2. Optimization of design Phase 3. Detailed mechanical design

For cost estimation and/or for submission of initial bids, most manufacturers use the first step of sizing the gasifier. The second step is considered only for a confirmed project—that is, when an order is placed and the manufacturer is ready for the final stage of detailed mechanical or manufacturing design.

This chapter mainly concerns the first phase and, briefly, the second phase (design optimization). To set the ground for design methodologies, a short description of different gasifier types is presented, followed by a discussion of design considerations and design methodologies.

Most commercial entrained-flow gasifiers operate under pressure and therefore are compact in size. Table 6.8 gives data on some of these operating in the United States and China.

A typical downflow entrained-flow gasifier is a cylindrical pressure vessel with an opening at the top for feed and another at the bottom for discharge of ash and product gas. The walls are generally lined with refractory and insulating materials, which serve three purposes: (1) they reduce heat loss through the wall, (2) they act as thermal storage to help ignition of fresh feed, and (3) they prevent the metal enclosure from corrosion.

The thickness of the refractory and insulation used is to be chosen with care. For example, biomass ash melts at a lower temperature and is more corrosive than most coal ash, so special care needs to be taken in designing the gasifier vessel for biomass feedstock.

The construction of a side-fed gasifier is more complex than that of a top-fed gasifier, as the reactor vessel is not entirely cylindrical and requires numerous openings. The bottom opening is for the ash drain, the top opening is for the product gas, and the side ports are for the feed. Additional openings may also be required depending on the design. Because of the complexity in the design of a pressure vessel operating at 30 to 70 bars and temperatures exceeding 1000 °C, any additional openings or added complexity in the reactor configura­tion must be weighed carefully against perceived benefits and manufacturing difficulties.

According to Marrone and Hong (2008), corrosion prevention in a supercritical water unit is broadly classified in these four ways: (1) contact avoidance, (2) corrosion-resistant barriers, (3) process adjustments, and (4) corrosion- resistant materials.

Contact Avoidance

The following are some innovative options that may be used to reduce contact between corroding species and the reactor wall:

• A transpiring wall on which water constantly washes down, preventing any corroding material’s contact with the wall surface.

• A centrifugal motion created in the reactor to keep lighter reacting fluids away from the wall.

• In a fluidized bed, neutralizing or retaining of the corrosive species by the fluidized particles.

Several options for the production of bio-oil are available. They are either thermochemical or biochemical.

• Gasification of biomass and the synthesis of the product gases into liquid (thermochemical)

• Production of biocrude using fast pyrolysis of biomass (thermochemical)

• Production of bio-diesel (fatty acid methyl ester, or FAME) from vegetable oil or fats through transesterification (biochemical)

• Production of ethanol from grains and cellulosic materials (biochemical)

The important steps in the production of bio-oil from biomass are as follows:

1. Receipt at the plant and storage

2. Drying and sizing

3. Reaction (pyrolysis, gasification, fermentation, hydrolysis, etc.)

4. Separation of products into solids, vapor (liquid), and gases

5. Collection of the vapor and its condensation into liquid

6. Upgrading of the liquid to transport fuel or extraction of chemicals from it

In pyrolysis no external agent is added. In a slow pyrolysis process, the solid product moves toward the carbon corner of the ternary diagram, and more char is formed. In fast pyrolysis, the process moves toward the C-H axis opposite the oxygen corner (Figure 5.1). The oxygen is largely diminished, and thus we expect more liquid hydrocarbon.

Pyrolysis, which precedes gasification, involves the thermal breakdown of larger hydrocarbon molecules of biomass into smaller gas molecules (condens­able and noncondensable) with no major chemical reaction with air, gas, or any other gasifying medium. For a detailed description of this process, see Chapter 3.

One important product of pyrolysis is tar formed through condensation of the condensable vapor produced in the process. Being a sticky liquid, tar creates a great deal of difficulty in industrial use of the gasification product. A discussion of tar formation and ways of cracking or reforming it into useful noncondensable gases is presented in Chapter 4.

Chemical looping is a relatively new concept. Its primary motivation is produc­tion of two separate streams of gases—a product gas rich in hydrogen and a gas stream rich in carbon dioxide—such that the CO2 can be sequestrated while the hydrogen can be used for applications that require hydrogen-rich gas. The system uses calcium oxide as a carrier of carbon dioxide between two reactors: a gasifier (bubbling fluidized bed) and a regenerator (circulating fluidized bed). The CO2 produced during gasification is captured by the CaO and released in a second reactor during sorbent regeneration.

Figure 6.14 is a schematic of the chemical looping process. Biomass is fed into the gasifier that receives calcium oxide from the regenerator and super­heated steam from an external source. During gasification, the carbon dioxide produced is captured by the calcium oxide that makes up the bubbling fluidized bed (Acharya et al., 2009), as follows:

CO2-

Cyclone

CFB/transport regenerator

Heat exchanger

Water-

Fuel

Product gas for application CO2 for " sequestration

Steam

External heating of regenerator <_

Gasification reaction: CnHhO0 +(2n — p) H2O + nCaO о NCaCO3

+ ^ 2 + 2n — H2

CO + H2O о CO2 + H2
CO2 removal reaction: CaO + CO2 ^ CaCO3

Immediate removal of the reaction product, CO2, from the system increases the rate of forward reaction (Eq. 6.2), enhancing the water-gas shift reaction, therefore yielding more hydrogen in the product gas. The calcium carbonate formed in the gasifier (Eq. 6.3) is transferred to a circulating/transport regenera­tor, where it is calcined into calcium oxide and carbon dioxide.

Regeneration: CaCO3 ^ CaO + CO2 +178.3 kJ/mol (6.4)

The carbon dioxide and the product gas leave the regenerator and gasifier, respectively, at a high temperature. The hot product can be used for generation of steam needed for gasification.