**5. Biochar characterization**

A summary of the characterization of raw biomass samples and their biochars produced at different temperatures and other pyrolysis reaction conditions along with their values for proximate and ultimate analysis is shown in **Table 1**. The carbon and ash contents of biochar increase on increasing pyrolysis temperature while the contents of volatiles decrease with temperature [1, 37]. Pyrolysis temperature influences the structure of biochar due to the release of volatiles, thus increasing the pyrolysis temperature leads to a decreased content of volatile matter. This was observed because the increasing temperature resulted in further cracking of the volatile fractions into low molecular weight liquids and gases instead of biochar [1, 31, 34, 37, 40]. The fixed carbon and elemental carbon content of biochar increase with increasing the pyrolysis temperature, as depicted in **Table 1**. Lee et al. [27] studied characteristics of biochar produced from slow pyrolysis of Geodae-Uksae and showed the increment of the carbon content of biochar at higher temperatures. The increase in elemental carbon content of biochar at higher pyrolysis temperature implies that the biochar became increasingly carbonaceous at high temperatures, releasing hydrogen and oxygen contents. A similar trend, an increase of carbon content with pyrolysis temperature, is obtained for different raw materials, date palm waste [31], rice husk [33], rice straw [34], beechwood feedstock [28], hinoki cypress [37], corrugated cardboard [39], chicken manure, Coffee husk, and sugarcane bagasse [38], pinewood, wheat straw, green waste and dry algae [40]. As one of the purposes of biochar production is to improve carbon contents in soil, thus, the high carbon content of biochar is beneficial in terms of maximizing the amount of carbon storage. Higher temperature pyrolysis is preferred for biochar production if the biochar application is to improve soil fertility. Several studies indicate that the yield of biochar is highly dependent on the pyrolysis conditions such as temperature, heating rate and heating time and is also greatly influenced by chemical, physical and biological properties of the biomass. **Table 1** shows that the temperature of pyrolysis plays an important role in the yields of the characteristic properties of biochar. The physicochemical properties of biochars depend not only on the nature of the starting biomass but also, to a very large extent, on the condition of preparation. Pyrolysis at lower temperatures would result in a large amount of biochar, indicating at high temperature large part of the biomass is lost as volatile matters.

The proximate analysis of biochar produced at different temperatures shows the fixed carbon and ash contents increase on increasing pyrolysis temperature, while the volatile contents decrease with temperature. The proximate analysis of date palm waste driven biochar [31] shows that the fixed carbon and ash contents increase from 45.49% to 74.7% and 14.42% to 21.39%, respectively while volatile contents decrease from 40.08% to 3.91% on increasing pyrolysis reaction temperatures from 300–800°C. Park et al. [34] reported the proximate analysis of biochar produced from rice straw at different temperatures ranging from 300C to 700. The volatile contents decrease from 34.54% to 5.88% upon increasing the aforementioned temperature range, while the fixed carbon and ash contents increase from 28.06% to 39.52% and 37.4% to 54.6%, respectively. A similar trend is also observed by different authors [27, 32, 33, 37, 40] using different raw material and different pyrolysis temperatures.

The ultimate analysis indicates that pyrolysis temperature is the most influential parameter to determine the elemental composition of biochar samples as shown in **Table 1**. It is observed that carbon content of date palm waste driven biochar [31] increases from 57.99% to 74.63% on increasing pyrolysis temperature from 300– 800°C. On the other hand Oxygen and Hydrogen contents decrease from 20.8% to 2.27% and 4.08% to 0.86%, respectively for the same pyrolysis temperature increase. Similarly, the ultimate analysis for metallic contents of Ca, Mg, K, and Na increase from 2.53% to 8.08%, from 0.68% to 2.02%, from 1.32% to 2.71% and from 0.28% to 0.58%, respectively for the same increment of pyrolysis temperature. Vieira et. el. [33] also reported the trend of increasing the carbon contents from 47.15–56%, from 46.14% to 58.4% and from 46.16% to 57.35% on increasing temperature from 300–500°C for pyrolysis reaction times of 60 min, 90 min and 120 min respectively, for biochar produced from rice husk. Moreover, a decrease of oxygen, hydrogen, and nitrogen contents of the biochar is observed on increasing the pyrolysis temperature. Lee et al. [27] showed the increment of the carbon content from 66.19% to 85.93% on increasing temperature from 300–700°C, for biochars produced from Geodae-Uksae 1. Several other researchers also reported the increment of carbon content and decrement of hydrogen, oxygen, and nitrogen contents with temperature for biochars produced from rice straw [34], beech wood [28], and hinoki cypress [37].

The molar ratios of Hydrogen, Oxygen, nitrogen, and sulfur to carbon are observed to decrease with temperature, as more volatile components are removed at higher temperatures making the biochar rich in carbon [31, 38].

pH of biochar is a guiding parameter to define the application of biochar as fuel or as soil fertility enhancing chemical and is correlated with the formation of carbonates and the contents of inorganic alkalis. Biochar is used in the soil as an acidity-correcting agent [47], so it is recommended that the pH conditions of the biochar should be basic because it can replace CaO due to such features. Soil acidity neutralization provides the most favorable conditions for microorganism proliferation and soil fertilization [9, 48]. Thus, the pH of biochar has been associated with having a liming effect on soil acidity, thus increasing the soil pH following the addition of biochar. Biochar can also be used as fuel, the use of acid biochar as fuel can lead to corrosion in the combustion equipment. Biochar having basic pH can

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

cause fouling due to its mineral composition and, consequently, higher ash content than the raw biomass feed. Moreover, the pH of the biochar directly impacts the adsorption process when the carbon is used in filtration process. Therefore, a neutral pH is generally preferred [49]. Most of biochar products have alkaline pH. Some studies have indicated that ash content of feedstock in conjunction with pyrolysis severity could influence the final pH of biochar samples suggested that a large proportion of the ash in high-ash feedstock contains carbonates which could cause a liming effect [15]. The pH values of biochars produced from rice husk [33] is observed to increase with temperature, ranging from 5.3 to 8.8 for temperature range from 300–500°C with a reaction time of 60 min, and from 4.2 to 8.3 for the same temperature range with a reaction time of 120 min. Yu et al. [37] reported the pH of biochars produced from hinoki cypress at temperatures ranging from 350–600°C and their pH increases from 7.95 to 9.66. Similarly, Domingues et al. [38] reported the pH of biochars produced from chicken manure at temperatures ranging from 350–750°C and their pH increases from 9.70 to 11.7. The pH of biochar produced from different feedstock is observed to increase with temperature [40].

Thus, biochar with desirable properties can be deduced from both its proximate and ultimate analysis. The lower the O/C and H/C ratios, the higher is the loss of oxygen and hydrogen during the combustion process producing a product richer in higher elemental carbon. The International Biochar Initiative (IBI) recommends a maximum value of 0.7 for the molar H/C ratio [17] to distinguish biochar from biomass that has not been or only somewhat thermochemically altered. Thus suitable working conditions and technologies must be selected in order to produce biochar of high quality. The pH of biochar from different biomass is around 10 and the microscopic surface structure of biochars ranges from around 3 m2 g for rice husk biochar to around 500 m2 <sup>g</sup> for biochar from wood [20]. Biochar produced from different feedstock showed in increasing surface areas on increasing pyrolysis temperature [27, 34, 39].
