This section describes some important properties of significance for the product composition after gasification.

6.3.1 Fuel types and properties

The performance and the selection of gasification technology are dependent on the characteristics of the fuel used. Therefore is a proper understanding of the characteristics of biomass and black liquor fuels important for a reliable selection and design of a gasifier. This chapter will briefly describe the different fuels and fuel properties of importance for gasification.

6.3.1.1 Biomass

Biomass is a diversified source and refers to any organic materials that are derived from plants or animals (Loppinet-Serani, 2008). Its diversity may be classified in a variety of ways depending on the origin and structure. One example is the division into two groups as proposed by Basu (2010):

• Virgin biomass, including ligno-cellulosic biomass in the form of wood, plants, and leaves; carbohydrate-based biomass such as crops and vegetables.

• Waste, including solid and liquid municipal solid waste (MSW), sewage, animal and human waste, gases derived from landfilling, as well as agricultural wastes.

Ligno-cellulosic biomass is the major source of biomass and will be briefly described below. This form of biomass can be divided into two types. Herbaceous plants are plants that have leaves and stems and die at the end of the growing season. Examples are wheat, rice, grasses, and oats. Non-herbaceous plants, which are non-seasonal remaining alive during the dormant season, including woody, plants, such as trees, shrubs and vines. Figure 6.3 shows the main components of woody biomass, consisting of extractives, cell wall components and ash. The cell wall compo­nents can in turn be divided into cellulose, lignin and hemi-cellulose. Extractives can basically be removed from the biomass by extraction with neutral solvents, including compounds, such as gums, fats, resins, sugars, oils, starches, tannins and alkaloids. The cell wall components are the polysaccharides, cellulose, linkage of glucose molecules in long chains form the structural framework of plant cell walls, and different types of hemi-celluloses, also chain-like and com­posed of several kinds of monosaccharides, and the complex aromatic polymer lignin. Ash is the inorganic part of the plant with the main components Si, K, Ca, S and Cl. The proportions of the main constituents are, as exemplified for wood, as follows (in percent of dry biomass): cellulose 40-45% (about the same in so-called softwoods and hardwoods); lignin 25-35% in softwoods and 17-25% in hardwoods; hemicelluloses 20% in softwoods and 15-35% in hardwoods, while the amount of extractives varies from 1 to more than 10% and ash 1-3%.

6.3.1.2 Black liquor

Black liquor is the residue extracted from sulfite or kraft processes after the cook in a batch or continuous digester. It contains a mixture of organics, inorganics and water. The inorganics consist of Na2CO3/K2CO3, NaCl/KCl, Na K2SO4/K2SO4, and some residual NaOH/KOH, NaHS/KHS and non-process elements like silica, phosphate and metal ions. The organics consist mostly of lignins, but also some hemicellulose and a little cellulose. Due to the composition of the organics

some liquors are swelling rapidly when heated, while others do not. Because of this, different liquors can behave quite differently when gasified. The water content can vary from 50% moisture down to close to 0% moisture if dried in a last step of processing, but the moisture content (MC) typically is 20-30%.