**4. Biochar quality**

The quality of biochar varies with feedstock used and production conditions. Some of the commonly measured quality parameters of biochar include pH, volatile compound content, ash content, bulk density, organic carbon content, nutrient content, elemental composition, surface area, porosity, surface functional groups,

cation exchange capacity, iodine number, C stability, water holding capacity (WHC), moisture content, heavy metals, electrical conductivity, polycyclic aromatic hydrocarbons (PAH) and sorption properties. The European Biochar Certificate [42] defines biochar as a "heterogeneous substance rich in aromatic carbon and minerals. It is produced by pyrolysis of sustainably-obtained biomass under controlled conditions with clean technology and is used for any purpose that does not involve its rapid mineralisation to carbon dioxide and may eventually become a soil amendment." This definition differentiates biochar from other forms of carbonaceous materials such as char and charcoal considering the sources of biomass for the production of biochar need to be renewable and sustainable [3]. The International Biochar Initiative (IBI) [4] considers several parameters relevant for assessing and comparing different biochars. These include proximate analysis, elemental composition analysis, pH, porosity, and BET surface area. In this study, proximate analysis and ultimate analysis of biochars produced from different feedstock under different pyrolysis reaction conditions are presented along with other biochar characteristics and component molar ratios of its constituent elements.

The major constituent components of lignocellulosic biomass [43], cellulose, hemicellulose, and lignin play important roles for most of the physical and chemical property modifications during the pyrolysis process. The mechanisms of pyrolysis of these polymers are chemically different from biomass species to species due to the differences in their compositions. The thermal decompositions of cellulose and hemicellulose polymers occur over a narrower temperature range whereas lignin degrades over a wider temperature range compared to those of cellulose and hemicellulose. The lignocellulosic biomass may contain some minor components other than the aforementioned polymers including some inorganic compounds and organic extractives together with substantial quantities of free and bound water [3, 31, 37, 44]. The inorganic compounds of lignocellulosic biomass which constitute less than 10% by weight of biomass, form ash in the pyrolysis process. The organic extractives of biomass refer to the nonstructural components that can be extracted by polar or nonpolar solvents including fats, waxes, proteins, terpenes, simple sugars, gums, resins, starches, alkaloids, phenolics, pectins, glycosides, mucilages, saponins, and essential oils [23, 31, 37, 44].

The biochar properties that could affect its final application will depend on the type of feedstock material characteristics and pyrolysis reaction conditions used for its production [3]. Pyrolysis temperature is the main process parameter that determines the degree of devolatilisation of the biomass. Water content of the biomass, both free and bound, is the first constituent removed in heating of biomass to temperatures up to 160°C. Thermal decomposition of biomass begins with devolatilisation or decomposition of extractives at temperatures about ≤220°C. Hemicellulose is the least stable polymer and break down first at temperatures of 220–315°C. Cellulose has a high degree of polymerization and exhibits higher thermal stability and it decomposes in the temperature range 315–400°C. Lignin is the most difficult component to pyrolyse which decomposes in a wide temperature range from 160–900°C [45].

Biochar characterization methods are always independent of production feedstocks, methods, conditions, and properties of the final product [27]. Several chemical characterizations of biochar ranging from biochar surface analysis to elemental composition and physical properties such as the surface area, pore size, and pore volume are commonly analyzed. The quality of biochar varies with the feedstock type and pyrolysis process conditions. Pyrolysis parameters such as heating rate, residence time, and final temperature greatly influence biochar quality [11]. Pyrolysis temperature has a critical role in biochar properties such as

### *Recent Perspectives in Biochar Production, Characterization and Applications DOI: http://dx.doi.org/10.5772/intechopen.99788*

elemental composition, particle size, specific surface area, pore size distribution, thermal capacity, and electrical conductivity.

Certain biochar quality parameters are more important than others depending on the expected final use or application of the biochar. For agricultural application in crop production, the important quality parameters of biochar include pH, volatile compound content, ash content, water holding capacity, bulk density, pore volume, and specific surface area [24]. Carbon stability of biochar is a critical quality parameter in carbon sequestration and soil fertility enhancement. The other important quality parameters in soil fertility enhancement include surface area and nutrient content. The molar ratio of hydrogen to carbon (H/C) is another important characterization parameter of biochar which is an indicator of the degree of carbonization and the stability of biochar. The higher values of molar H/C ratio greater than 0.7 indicate the lower biochar quality and pyrolysis deficiencies. Molar oxygen to carbon (O/C) ratio is also relevant for characterizing biochar and differentiating it from other carbonization products [42] with values greater than 0.4 indicating lower biochar stability. Literatures show that the molar H/C and O/C ratios of lignocellulosic biomass are approximately 1.5 and 0.7, respectively. During pyrolysis, the biomass undergoes devolatilization and the solid portion gets enriched in carbon. The H and O are preferably removed over C and the H/C and O/C ratios tend to decrease as biomass undergoes its transformation into biochar. The H/C and O/C ratios are used to assess the degree of aromaticity and maturation [46]. The characterization of biochar produced from different feedstock and their feedstock are discussed in the next section.
