1.6.2.Pyrolysis

Pyrolysis is a thermochemical decomposition of organic matterl at elevated temperatures in the absence of oxygen. In pyrolysis, large hydrocarbon molecules of biomass are broken down into smaller hydrocarbon molecules. Pyrolysis typically occurs under atmospheric or moderate pressure (0.1-0.5 MPa) at operating temperatures 350–700 ^oC. No oxygen is present, the material does not combust but the chemical compounds (i.e. cellulose, hemicellulose and lignin) that make up that material thermally decompose into combustible gases and charcoal (Figure 4). Great deal of the combustible gases, called tar, can be condensed at ambient temperatures into a combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gases (CO_­2, CO, H₂, light hydrocarbons). Thus pyrolysis of biomass produces three end products: liquid (bio-oil) solid (charcoal or bio-char), and gaseous (syngas). The proportion of these products depends on several factors including the composition of the feedstock and process parameters.

Figure 4. Pyrolysis of a biomass particle [4]

The pyrolysis process may be represented by a generic reaction

(1)

The progression of traditional slow pyrolysis of a hardwood in a retort that collects the liquids and gases from the process is the following [9]. Starting with 100% solid material at ambient temperature, by 250 ^oC the solid mass is at 88%, less than 10% liquid, and only a few percent of gas. Between 300 ^oC and 350 ^oC the char mass rapidly decreases to less than 60%, the liquids are 20% to 30%, and the gases are between 15% and 25%. The char at this stage contains some amount of both oxygen and hydrogen. Further heating of the char to 750 ^oC will decrease the mass of the char, and while its composition moves closer to pure carbon, the yield of gas increases and that of liquids decreases.

However, all things being equal, the yield of bio-oil is optimized when the pyrolysis temperature is around 500 C and the heating rate is high (1000 K/min) i.e. fast pyrolysis conditions. Under theses conditions bio-oil yields of 60-70 wt% of can be achieved from a typical biomass feedstock, with 15-25 wt% yields of bio-char. The remaining 10-15 wt% is syngas. Processes that use slower heating rates are called slow pyrolysis and bio-char is usually the major product of such processes. The pyrolysis process can be self-sustained, as combustion of the syngas and a portion of bio-oil or bio-char can provide all the necessary energy to drive the reaction.

Table 1. Typical product yields (dry wood basis) obtained by different modes of pyrolysis and gasification of wood [10]

Mode	Conditions	Liquid	Char	Gas
Fast	Moderate temperature, around 500 °C, short hot vapour residence time ~ 1 s	75%	12%	13%
Intermediate	Moderate temperature, around 500 °C, moderate hot vapour residence time ~ 10-20 s	50%	20%	30%
Slow (carbonisation)	Low temperature, around 400 °C, very long solids residence time	30%	35%	35%
Gasification	High temperature, around 800 °C, long vapour residence time	5%	10%	85%

The pyrolysis process consumes energy due to heat losses to the surrounding and with hot gaseous products leaving the reactor. Overall chemical reaction (1) can be exothermic or endothermic depending on reaction conditions [6]. Generally, the nearly linear relationship exists between the pyrolytic heat of reaction and the observed char yield. Pyrolysis is endothermic at char yield less than 16–18% and becomes more and more exothermic with the char yield further rise. Increasing pressure as well as decreasing flow rate of gas medium (e.g. nitrogen) cause the total heat of biomass pyrolysis to shift from values of around 20 to 100 J/g (endothermic) to values of -20 to -130 J/g (exothermic). If there is a high moisture content to begin with, the net energy yield of the pyrolysis process will be very low because the energy necessary for the pyrolysis and gasification processes comes mainly from combustion of one or more of the products of pyrolysis (e.g., char, oil/tar, or combustible gases) or raw feedstock [9] . Since most biomass is hygroscopic, the removal of water is even more endothermic because of the energy required to overcome the absorption energy. The behavior of solid biomass during heating is a complex interaction between the removal of water and the pyrolysis process. This is further compounded by the occurrence of reactions between the pyrolysis products and the char.

When the Biot number of a biomass particle is very small ( Bi < 10^-3) then the material conducts heat rapidly to provide a uniform temperature throughout. However for biomass samples such as wood the Biot number often has values much greater than 0.2, and consequently there are large temperature gradients within the solid wood material. Thus, at high external heat fluxes with large particles of > 2 cm thick, the surface rapidly reaches the external temperature, while the center of the particle is still cold. For a 1 cm³ cube of wood, a very slow heating rate of 0.01 °C/min (Bi = 10^-5) would result in an isothermal situation throughout the cube. In this case, the drying of the wood would take place independently of the pyrolysis process. Under the conditions that are often described as fast pyrolysis, the heating rates are on the order of 100 °C /min. For the same 1 cm³ cube Bi = 0.3 indicates a large thermal gradient.

The passage of the thermal wave from the outside of the particle to the center takes a relatively long time and is opposed by the diffusion of the products of drying (water vapor) and pyrolysis (organic molecules and permanent gases) migrating to the surface. Physically this separates the drying process from the pyrolysis process such that each small volume of wood polymer is totally dry when it pyrolyzes.

1.1 Slow pyrolysis

Based on heating rate, pyrolysis may be broadly classified as slow and fast. It is considered slow if the time, t_heating, required to heat the fuel to the pyrolysis temperature, is much longer than the characteristic pyrolysis reaction time, t_r, and vice versa. That is:

• Slow pyrolysis: t_heating >> t_r.

• Fast pyrolysis: t_heating << t_r.

These criteria may be expressed in terms of heating rate as well, assuming a simple linear heating rate (T_pyr/t_heating, K/s). The characteristic reaction time, t_r, for a single reaction is taken as the reciprocal of the rate constant, k, evaluated at the pyrolysis temperature [4].

Slow pyrolysis is the most efficient method of turning biomass into biochar and is therefore commonly cited in the literature as being one of the most promising technologies to produce biochar. In slow pyrolysis, the residence time of vapor in the pyrolysis zone (vapor residence time) is on the order of minutes or longer. This process is used primarily for char production and is broken down into two types: carbonization and conventional.

Slow pyrolysis requires low-to-medium temperatures between 350 and 700 °C at relatively long residence times typically taking hours or days (depending on kiln size) and generates three products: 35 to 40% biochar from the original weight of the biomass, water and syngas. The properties of the resulting biochar and syngas are heavily determined by feedstock material, temperature, and residence times. Typically the slow pyrolysis is conducted for hours to a maximum temperature of 400–500 ^oC.

The syngas stream from slow pyrolysis has an energy content of 8-10 MJ/kg with mainly consisting of 10-25% hydrogen (H₂), 15-25% carbon monoxide (CO), 8-15% methane (CH₄), and smaller amounts of ethane, propane, ethyl alcohol, and acetyle alcohol.

Slow pyrolysis is characterized by heating biomass in an environment of controlled oxygen, with the unifying feature that the temperature gradients are gradual enough that the local char properties are determined by the biomass as it rearranges and disproportionates into a thermally modified char and exiting vapours. As the temperature rises, additional char consolidation occurs, with more and different volatiles evaporating and leaving the transforming char behind. Under these conditions, the char properties are most strongly dictated by the highest temperature the biomass experiences for a long enough period for the biomass molecules to rearrange and the volatiles to form and leave the char mass as vapours.

The volatiles need to form and leave to complete the slow pyrolysis process. While the volatiles are formed within the biomass as it converts to char, many factors can influence the subsequent vaporization of the volatiles. It is this second step that differentiates char properties within Slow Pyrolysis chars produced at the same temperature. This is attributed to an independent char-creating process called “secondary char formation”. Many factors influence secondary char formation, including the biomass particle size, the pressure of the pyrolysis reactor, and the relative composition of the vapours within the reactor. As such, there can be a range of resulting char yields and biochar properties produced by Slow Pyrolysis technologies at exactly the same pyrolysis temperature, depending on the extent of secondary char formation. For this reason alone, all Slow Pyrolysis chars need to have the adsorption capacity measured to establish the actual biochar quality.

A typical slow pyrolysis, optimized for biochar production, might produce 25 kg of biochar and 75 kg of gaseous products from 100 kg of ash-free dry wood. Charcoal is manufactured from biomass by pyrolysis in large kilns or retorts. By-products are pyroligneous liquid and gases (volatiles). The yield of the different reaction products varies with biomass species and heating conditions. Lager particle sizes and slow heating favour the formation of charcoal by enhancing the contact time of the volatiles with the solid carbon product.