LAS BRECHAS DE DESIGUALDAD LATINOAMERICANAS

Thermochemical conversion technologies such as pyrolysis, gasification and combustion can convert biomass to energy. Pyrolysis is described as the thermal decomposition/degradation of biomass or a solid fuel in the absence of oxygen to produce solid char, liquid and gases. Biomass gasification is the partial oxidation of a combustible material, usually a solid fuel (biomass), which converts biomass into a gaseous component where the process is starved of oxygen. Pyrolysis and gasification are the most studied conversion processes for advanced thermal conversion. Combustion is the oldest and most common biomass conversion technique and has been practised for centuries. It consists of direct burning of biomass to convert the chemical energy into heat, mechanical power or electricity using stoves furnaces, boilers or steam turbines. Combustion processes are used mostly today for heat generation. The heat energy generated must be used immediately, as the heat cannot be stored or transported like the liquid and gaseous products from pyrolysis or gasification. This chapter will begin with a description and overall definition of biomass and its constituents, then move on to discuss both advanced thermochemical conversion techniques as well as give an overview of catalytic upgrading options to use with these advanced techniques.

2.2

Biomass

Biomass is a generic term that is used for any organic matter of recent origin including crops, wood and wood wastes, agricultural residues, animal wastes and both municipal and industrial wastes. Biomass has stored solar energy in chemical bonds via the photosynthesis process.

Biomass contains the elements carbon (45-55 wt.%), hydrogen (5-7 wt.%), oxygen (40-50 wt.%) and small amounts of sulphur (0-0.05%) and nitrogen (0-10 wt.%). Carbon and hydrogen are the main combustible components of the biomass. [14]

The main building blocks of biomass are water, lignin, cellulose, hemicelluloses, organic extractives and inorganic matter, as is illustrated in Figure 4 below. The following sub- sections describe each of the major biomass components in more detail.

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Figure 4 Biomass Composition [15]

2.2.1 Water

The amount of water present in biomass can vary depending on the type of biomass. Crops and woody biomass can contain high levels of water approximately 50%; this is dependent on weather effects and conditions when planted. Agricultural or industrial waste can contain much higher quantities of water up to 80%; this is dependent on storage as-well as how wet the residue material is when it exits the industrial process. For most thermo-chemical processes, biomass must be pre-treated to reduce the moisture content to 10-15%. For pyrolysis much of the water will end up in the bio-oil product, ultimately for both pyrolysis and gasification processing higher moisture contents will reduce the thermal efficiency of the process as energy will be used to evaporate the unwanted water. The moisture content can be determined using proximate analysis which will be discussed in chapter 4.

2.2.2 Cellulose

The cellulose component is the same in all types of biomass, except for the degree of polymerisation which can vary slightly in the most uniform sample. Cellulose is a glucan polymer consisting of linear chains of β (1, 4)-D-glucanpyranose units. The aggregation of these linear chains within the micro fibrils provides a crystalline structure, highly inert and inaccessible to chemical reagents. The cellulose content for most deciduous and coniferous trees varies between 40% and 45%, but can reach 55% for some. [16] An illustration of the structure is provided in Figure 5.

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Figure 5 Structure of Cellulose [17]

2.2.3 Hemicellulose

Hemicellulose is a mixture of polysaccharides composed almost entirely from glucose, mannose, galactose, xylose, arabinose, 4-O-Methylglucuronic acid and galacturonic acid residues. It is generally much lower in molecular weight than cellulose. In contrast to cellulose, the hemicelluloses are amorphous. For deciduous trees the hemicelluloses (xylans or pentosans) represent 20-35% of the total mass. For coniferous trees, there are 20-40% hemicelluloses (mannans and xylans). [9] An illustration of the structure of hemicellulose is given in Figure 6.

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Figure 6 Structure of Hemi cellulose [18]

2.2.4 Lignin

Lignin is a randomly linked, amorphous, high molecular weight phenol compound. It is more abundant and has a higher degree of polymerisation in softwoods than in hardwoods. Its composition in these two types of wood also present some differences. The lignin content is 24% to 30% for coniferous trees and 17 % to 24% for deciduous trees. [14] The structure of lignin is illustrated in Figure 7.

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Figure 7 Structure of Lignin [19]

2.2.5 Organic extractives

Biomass contains a small fraction of organic extractives that are low in molecular weight. The fraction can vary quite significantly. Examples of biomass extractives are fats, waxes, alkaloids, proteins, phenolics, simple sugars, pectins, mucilages, gums, resins, terpenes, starches, glycosides, saponins[20]. These can be extracted from biomass by using various solvents. The extractive contents can be determined by chemical compositional analysis using neutral and acidic detergents and will be discussed in Chapter 4.

2.2.6 Inorganic materials

The inorganic elements present in biomass, namely chlorine (Cl), calcium (Ca), Iron (Fe), potassium (K), magnesium (Mg), sodium (Na), silicon (Si), sulphur (S) and phosphorous (P) are collectively known as ash. These ash materials vary in concentration depending on the biomass.[21] The ash content can be determined by proximate analysis which will be discussed in Chapter 4.

2.3

Pyrolysis

Pyrolysis is the thermal decomposition/degradation in the absence of oxygen. As well as a conversion method in its own right, it is also the first step in combustion and gasification, where it is then followed by total or partial oxidation of the primary products. [22] An example of all three is taken place in a flaming match, illustrated in Figure 8.

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Figure 8 Pyrolysis, Gasification and Combustion [23]

Pyrolysis is considered to be an attractive technology as reactions take place under controlled conditions with a wide range of products suitable for different applications.[24, 25] The products of the pyrolysis process are char (a solid), bio-oil (a liquid formed from condensable pyrolysis vapours) and permanent gases. There are several processes in which the pyrolysis of biomass has been applied for heat and power applications, or combined with gasification as a pre-conditioning step for hydrogen production or sequential catalysis to produce methanol or synthetic fuels. [26]

Table 1 below provides the distribution of products from different modes of pyrolysis process. Low process temperatures and long solids residence time favor the production of charcoal. High temperature and long solids residence time increase the biomass conversion to gas. Moderate temperature, short solid and vapour residence times and high heating rates favor production of liquids. [26] The distribution of the products can be controlled to some extent by controlling these main reaction parameters.

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Table 1: Modes of Pyrolysis

Mode Conditions Liquid Char Gas Fast Moderate temperature, around 500°C,

short vapour and solids residence times ~ 1 sec

75% 12% 13%

Intermediate Moderate temperature, around 450°C, moderate solids residence time ~ 1-30 mins

50% 20% 30%

Slow Low temperature, around 400°C, very long solids residence time ~ hours/days

30% 35% 35%

2.3.1 Modes of Pyrolysis

2.3.1.1 Slow Pyrolysis

The classical ‘slow’ approach leads to charcoal (with a woody feedstock). Conventional slow pyrolysis is the irreversible thermal degradation of organic components in biomass, (usually lignocellulosic) in the absence of oxygen. Slow pyrolysis is also known as carbonisation and is used to maximise solid charcoal production. This method of pyrolysis has been practiced for centuries and requires relatively slow reaction at low temperatures to maximise solid char yield [27-29].

2.3.1.2 Fast Pyrolysis

Fast pyrolysis occurs with solids and vapour residence times of few seconds or less and very high heating rates. It is used primarily to maximise liquid products (up to 75 wt.%.). After cooling and condensation of the pyrolysis vapours, a dark brown mobile liquid is formed (“bio-oil”) which has a heating value about half that of conventional fuel oil. While it is related to the traditional pyrolysis processes for making charcoal, fast pyrolysis is an advanced process, with carefully controlled parameters to give high yields of liquid.

The essential features of fast pyrolysis process for producing liquids are:

very high heating and heat transfer rates at the reaction interface, which usually requires a finely ground biomass feed,

a carefully controlled pyrolysis reaction temperature of around 500 ºC and, short vapour residence times of typically less than 2 seconds and,

rapid cooling of the pyrolysis vapours to give the bio-oil product.[22]

Most successful work with fast pyrolysis has been carried out with woody, low-ash, highly homogeneous feedstock’s, and the process is often not successful with more “difficult” feedstock’s which can produce highly reactive liquids rich in high-MW tars leading to storage and processing issues. [26, 30-32].

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2.3.1.3 Intermediate Pyrolysis

Intermediate pyrolysis takes place at moderate temperatures of 350-450ºC with moderate solids residence times of 1-30 minutes. It can process a diverse range of feedstocks such as waste wood, food wastes, sewage sludge, grass and algae, and is relatively insensitive to feedstock ash and to some extent moisture content. The distribution of the product phases and the composition of the liquid phase depend strongly on the feedstock and to a lesser extent on process conditions.

The ability of intermediate pyrolysis to deal with “difficult” high-ash feedstocks with relatively high moisture contents is a significant advantage over fast pyrolysis for feedstocks such as BSG. In particular, the liquids produced from non-woody biomass are very low in high molecular weight tars and can be suitable for direct application in engines.

For the reasons given in the previous section the present work will focus on the intermediate pyrolysis route for the experimental pyrolysis of BSG.