**5. Products of biomass pyrolysis**

Thermochemical decomposition of biomass yields mixture of gases, liquid and solid bio-char as major products as stated earlier. The yield of any of the pyrolysis products is largely depends on pyrolysis temperature and biomass' moisture content among others like design of pyrolyzer, vapor retention time and heating rate [22].

### **5.1 Gas**

Mixture of gases formed from primary (non condensable gases) and secondary degradation (tar and volatile organic compounds called condensable vapor) of biomass in the pyrolyzer during pyrolysis after moisture dehydration are called syn-gas. Components of syn-gas vary from one biomass specie to another. Often formed gases from primary degradation of biomass are carbon(ii) oxide, carbon(iv) oxide, hydrogen gas, gaseous water, nitrogen and hydrocarbons of lower carbon content (methane, ethane, ethylene). Additional gases are produced through secondary cracking of condensable vapor at high temperature [35]. Very high pyrolysis temperature enhances yield of these mixture of gases and reduces yield of bio-char. The increase in yield of syn-gas might be as a result of thermal decomposition of tar and hydrocarbons (condensable vapor) to produce more oxides of carbon and other gases.

Lower carbon content hydrocarbons (eg methane, ethane, ethane and butane) are often used as domestic cooking fuels. The heating value of syn-gas obtained from secondary degradation of biomass (condensable vapor) is about 82% higher than those from biomass' primary degradation. Hydrogen can be blended with oil to upgrade it to transportation fuel [36]. Hydrogen gas and carbon(ii) oxide are mainly used as bio-fuel inform of water gas [11].

Liquefied carbon(iv) oxide is used as major component in production of fire extinguisher. It is also used as cooling agent and preservative by bottling company. Recent research work by Patel [37] claimed possibility of converting CO2 to starch in the laboratory about nine times more efficient as obtainable from corn plant using combination of chemical catalysts and enzymes. During the conversion process, useful industrial chemical like methanol was obtainable. This will serve as sustainable way of feeding the ever growing world population without need of land, seed, pesticide, fertilizer and water. The approach can equally be said to be a good form of food security and reducing CO2 emissions [37].

Nitrogen is utilized in production of ammonia from Haber process for preparation of fertilizer by agrochemical industry in order to enhance food production for feeding human population and livestock [38].

### **5.2 Liquid**

Major liquid components of biomass themochemical degradation are bio-oil (bio-crude) and black tar, others are organic solvents. Bio-oil contains mixture of heavy molecular weight hydrocarbons. High pyrolysis temperature coupled with quick vapor condensation result in high bio-oil yield (>70%) [39]. Condensed pyrolysis vapor contains more than three hundred mixture of condensable gaseous *Pyrolysis: A Convenient Route for Production of Eco-Friendly Fuels and Precursors for Chemical… DOI: http://dx.doi.org/10.5772/intechopen.101068*

compounds. Higher yield of bio-oil is obtainable from agricultural residues when compared with one obtainable from woody biomass. Quality of pyrolysis bio-oil is mostly affected by storage time and temperature. The higher the storage time, the more viscous and poorer quality of the bio-oil due to escape of volatile components. Decline in quality of bio-oil due to aging can be minimized through cool storage temperature. Bio-oil is mainly used as bio-fuel and chemicals for industrial productions. Bio-oil heating value can be maintained by monitoring its metal and water content. It is used as transportation fuel, when mixed in certain ratio with fossil fuel it minimizes greenhouse gas emission that causes major environment pollution, especially in the city. Bio-oil can serve as precursor for production of soap through saponification process in soap and detergent industry. Cosmetic industry uses biooil in production of shoe polish, hair and body cream [40]. Organic solvents, such as benzene is a very good precursor in production of important materials, like dyes, pigments, synthetic tyres and textile materials. Others are production of plastics, resins, rubber lubricants, detergents, drugs and pesticides. Benzene is also a good solvent used in chemical, biochemical and biological laboratories, as well as aviation bio-fuel [41]. Other organic solvents like hyroxyketones, hydroxyaldehydes, carboxylic acids, phenolic compounds, sugar and dehydrosugars are important precursors for chemical and allied industry [11].

### **5.3 Bio-char**

The solid component of pyrolysis products is known as bio-char. It is a black amorphous carbonaceous solid matter [42]. Properties and yield of bio-char are dependents of pyrolysis temperature, heating rate, vapor retention time, inert gas flow rate and nature of biomass [43]. A coarse and high yield bio-char is obtained from woody biomass, if pyrolysis temperature between 500 and 550°C, heating rate less than 1°C/s and greater than 450 s vapor retention time are used for its thermochemical process under inert environment. In contrary, if a fine bio-char is a desired material, agro-residue would be required as pyrolysis feedstock using degradation temperature greater than 600°C, high heating rate between 10 and 300°C/s and vapor retention time less than 20 s [11]. Bio-char with acceptable properties contains carbon as major component; other fractions are oxygen, hydrogen, nitrogen, sulfur and ash (inorganic metals). Bio-char with a long half life has its carbon to oxygen ratio (C:O) usually greater than 3:2, but one with 1:5 (C:O) has short half life. Inorganic ash component of bio-oil is very small in comparison with fossil fuel and it has low heating value of about 32 MJ/kg, which is about 60% higher than that of lower heating value of the parent material (biomass). Bio-char has many areas of application, ranging from soil enrichment in agro-chemistry, adsorbent in wastewater treatment to energy in electrochemical capacitor (supercapacitor) and bio-fuel [10].

### *5.3.1 Bio-char for soil amendment*

Incessant ability of agricultural activities depends largely on biological, physical and chemical properties of soil. A little change in soil organic carbon has significant impact on plant growth. Farmers mostly use synthetic fertilizers for achieving this goal, but ironically the soil fertility decreases. It is only bio-char that provides soil with organic carbon that boots soil nutrient [22]. Environmental awareness has increased interest of global community for using bio-char as soil amendment agent. Tilling soil with bio-char stabilizes soil pH, enhances water retention capacity and available nutrients for plant growth [44]. Enhancement of soil nutrient by bio-char might be as a result of presence of potassium, magnesium, calcium, iron, zinc, phosphorous, sulfur and nitrogen in the bio-char. Equally, amendment of soil by

bio-char reduces leaching out of soil nutrient and aids slow nutrients release to the plant, where by making nutrients available for long period of time. This promotes plant growth and mitigates climate change through consumption of carbon(iv) oxide (greenhouse gas) by growing plants during photosynthesis (smart carbon cycle). Therefore, application of bio-char in soil amendment makes our environment greener and increase production of food for geometrically increasing human population [45].
