In ultra-rapid pyrolysis, very high heating rates and temperatures of around 1,000°C with short vapor residence times gives predominantly a gaseous product. A rapid quenching of the primary product is done following pyrolysis. The heating is done using a heat carrier solid such as sand. A gas-solid separator separates the gas from the heat carrier solid.

Hydrouspyrolysis and Hydropyrolysis

Hydrouspyrolysis and hydropyrolysis involve thermal decomposition of biomass in the presence of water or hydrogen respectively, under high pressure conditions. The process usually takes place in two stages. The first stage involves treating biomass with water or hydrogen at 200-300°C under pressure; the second stage involves cracking of the hydrocarbon produced in the first stage into lighter hydrocarbons at a temperature of around 500°C. The bio-oil produced by this type of pyrolysis method has reduced oxygen content which is a desirable characteristic.

Vacuum pyrolysis

In vacuum pyrolysis, biomass is heated in vacuum in order to decrease the boiling point and avoid undesirable chemical reactions. Vacuum pyrolysis is carried out at temperatures of 400-500°C and at total pressure of 2-20 kPa. Under these con­ditions, the product of pyrolysis can be rapidly withdrawn from the hot reaction chamber enabling preservation of the primary fragments originating from the thermal decomposition of biomass. Heat transfer, which is a rate limiting factor in pyrolysis, is the major limitation in vacuum pyrolysis. In an actual pilot plant reactor developed by a company called Pyrovac, this has been effected by passing molten salts through hollow heating plates on which the biomass is placed inside the vacuum pyrolysis reactor. The biomass gets heated by conduction as well as radiation thus increasing the heat transfer efficiency. The details of vacuum pyrolysis, with particular reference to the theoretical aspects of heat transfer in vacuum pyrolysis, has been described at length by Roy et al. [15].

The pyrolysis technology is less developed than the combustion or gasification technologies. This is probably due to the fact that the bio-oil obtained from the pyrolysis process costs 10-100% more than fossil fuel and its availability is limited; it is unstable due to the presence of entrained fines of char particles; dedicated liquid handling such as modified pressure filtration is required for removal of these fines; the kinematic viscosity of the pyrolysis oils varies over a wide range depending on the nature of feedstock and temperature of pyrolysis among other factors; bio-oil is acidic in nature with a pH of around 2.5-3.0, making it corrosive to the commonly used construction materials such as carbon, steel, and aluminum. Some of the sealants used may also be affected; bio-oil has a water content of around 15-30% by weight of oil mass, which contributes to the low energy density of the oil—this cannot be removed by conventional methods like distillation. All the above properties of the bio-oil obtained from pyrolysis make it unsuitable for direct application as a transport fuel, as a precursor for generation of chemicals, etc. Upgrading of the bio-oil by methods such as catalytic upgrading, needs to be done before it can be used for the various applications. These are described in detail by Bridgwater [13]. However, fast pyrolysis and flash pyrolysis is advancing very rapidly with a number of commercial level plants being set up across the globe.

Gasification

Gasification of biomass is the thermochemical transformation of biomass at high temperature in the presence of restricted supply of oxygen, which may be supplied as such, or in form of air or steam. It is the latest biomass conversion technology among the thermochemical methods for biomass conversion. The product of gasification is a gaseous product which has applications in electric power gener­ation, manufacturing of liquid fuels, and production of chemicals from biomass. Gasification can be said to be an extension of pyrolysis, and has been optimized to give a maximum of the gas phase at the cost of char or liquid. The gas produced

from the gasification process is a mixture of carbon monoxide, hydrogen, and methane along with carbon dioxide and nitrogen also produced to some extent, and is called producer gas as it can be used as synthesis gas to produce ammonia or methanol, which, in turn, are used to produce synthetic fuel (synthetic petrol) or as a source of hydrogen. It can also be used as such as a heat source, or to generate electricity through gas turbines. Up to 50% efficiency with respect to electricity generation can be obtained if the gas turbine is integrated with a steam turbine in combined cycle gas turbine system (also called biomass integrated gasification combined cycle—BIGCC). In this system the waste gas from the gas turbine is recovered and used to generate steam in a steam turbine. Figure 1.11 shows a schematic flow sheet for such a system. With such a type of integration, biomass gasification plants can be as economical as coal-fired plants for electricity generation.

However, the power output is limited by economic supply of biomass and is generally limited to around 80 MW of electricity [16]. The medium used for gasification, i. e., oxygen, air, steam, etc. greatly affects the heating value of the product obtained. Gasification in the presence of steam has the highest heating value, followed by oxygen and air in that order. Although the product of gasifi­cation in the presence of steam has the highest heating value, higher operating temperatures are required for vaporization of water, increasing the cost of the process. Usually, a mixture of air and steam, with variable inlet ratio is employed. Commercial gasifiers are available in a wide range of sizes and a variety of types
which are capable of using a variety of biomass feedstock such as charcoal, wood, rice husk, and coconut shells. The newer gasification processes such as plasma gasification and hydrothermal gasification are also capable of processing muni­cipal solid waste (MSW). The following sections describe the mechanisms and chemistry of the biomass gasification process and the various types of gasifier designs developed for getting optimum product yields for a given biomass feedstock.

The gasification process increases the H/C ratio of the biomass by adding hydrogen to and removing carbon from the biomass.

Gasification consists of four main stages: preheating and drying, pyrolysis, char gasification, and combustion (also called flaming pyrolysis). These stages may take place either in specific regions or zones of the gasifier equipment (especially in moving bed gasifier designs), or these may take place at a microscopic level, within a particle (especially in the fluidized-bed gasifier designs). As in all thermochemical conversion methods, drying of the biomass is a very important step in the gasification process. Moisture contents in biomass vary over a very wide range (from 30 to 60%, and in some cases, even up to 90%). Every kilogram of moisture in the biomass can consume a minimum of 2,260 kJ of extra energy from the gasifier to vaporize the water, which is not recoverable [11]. Hence, pre-heating of biomass is done where the moisture content of the biomass is brought down to about 25%. The pre-heated biomass further dries in the gasifier when temperatures in the range of 100-200°C are encountered. The surface moisture as well as the inherent moisture present in the biomass is removed. The stage of drying is followed by pyrolysis as the tem­perature increases to 200-700°C. Pyrolysis is the first step in the gasification of biomass. In this stage, large molecules are broken down to smaller gas molecules (condensable as well as non-condensable), carbon char, and tars/oils. This stage is endothermic and does not involve reactions with oxygen or air or any medium. Following the initial pyrolysis of biomass, a number of secondary reactions occur where the products of pyrolysis react with each other and with the medium used for pyrolysis (oxygen, air, or steam), to give CO, CO2, H2, H2O and CH4. The carbon char is further gasified in the presence of restricted air, oxygen or steam to produce additional combustible gases, giving producer gas. The updraft gas­ifier and the downdraft gasifier designs (Fig. 1.12, 1.13) illustrate the different stages in a gasification process.

The overall reactions occurring after the initial pyrolysis of biomass in the gasification process are shown below (source Ref. [11]):

Carbon reactions:

C + CO2 $ 2CO C + H2O $ CO + H2 C + 2H2 $ CH4 C + 5O2 ! CO

A H° = +172kJ/mol ДЯ° = +131 kJ/mol A Я° = —74.8 kJ/mol A Я° = —111 kJ/mol

Oxidation reactions:

C + O2 ! CO2 AH° = -394 kJ/mol

CO + |O2 ! CO2 АH = -284 kJ/mol

CH4 + 2O2 $ CO2 + 2H2O AH°r = -803 kJ/mol H2 + 1O2 ! H2O АH = -242 kJ/mol

Water-gas shift reaction:

CO + H2O $ CO2 + H2 АH = -41.2kJ/mol

Methanation reactions:

2CO + 2H2 ! CH4 + CO2 AH° = -247 kJ/mol

CO + ЗН2 $ CH4 + H2O AH = -206 kJ/mol

CO2 + 4H2 ! CH4 + 2H2O AH° = -165 kJ/mol

Steam reforming reaction:

CH4 + H2O $ CO + ЗН2 AH° = +206 kJ/mol CH4 + 2O2 ! CO + 2H2 AH = — 36 kJ/mol

Fig. 1.13 Schematic of an updraft gasifier

particle depending on the surrounding temperature and the presence or absence of air/oxygen.

To summarize, gasification produces volatile gases and carbon char. The vol­atile gases are converted to CO, H2 and CH4, whereas the carbon char is com­busted to produce CO. In case of low temperatures and short residence times in the hot zone, medium-sized molecules may escape and condense as undesirable tars and oils. This tar, being viscous, creates problems of fouling in the gasifier, and needs to be removed. This can be done by catalytic cracking of the tar, which gives CO, H2, and H2O. The role of catalysis in cracking is discussed in detail by Bridgwater [13].

The performance efficiency of a gasifier process or a gasifier unit is described in terms of ‘‘% cold gas efficiency” which is defined as follows:

The extent of carbon conversion or fuel utilization can also be determined and related to the production efficiency of the gasifier process.

where

HHVgas = higher heating value of the producer gas, kJ/m3

HHVfuel = higher heating value of the biomass feedstock, kJ/m3

vgas = volumetric rate of producer gas, m3/h

mfuel = input mass rate of biomass fuel, kg/h

mash = mass rate of gas residue exiting the gasifier, kg/h

% Cash = weight percent of carbon in the ash residue, %

% Cfuel = weight percent of carbon in the biomass fuel, %

In general, operating the gasifier with 100% carbon utilization, with simulta­neous maximization of cold gas efficiency, is not possible. The carbon efficiency has to be always sacrificed in order to achieve producer gas of the desired spec­ifications [17].