**5. Thermochemical conversion**

There are many types of thermochemical conversion processes through which biomass is converted to solid, liquid and gaseous products. Thermochemical conversion processes use high temperatures to breakdown the bonds of organic matter. Thermochemical conversion routes can be classified according to the oxygen content used in the process, including combustion (complete oxidation), gasification (partial oxidation) and pyrolysis (thermal degradation in the absence of oxygen). Torrefaction is also performed in the absence of oxygen. Hydrothermal processing is an alternative route to process wet biomass using heat and pressure in the presence of water, which can also be considered a thermal degradation in the absence of oxygen. The typical products of thermochemical conversion of biomass are carbon-rich solid residue (biochar), condensable vapors (bio-oil or tar) and non-condensable gases. The distribution of products (biochar, bio-oil/tar and gases) depends primarily on the conversion process [21]. A brief description of biochar, bio-oil and gases are given below.

*Biochar -* Biochar is a porous carbonaceous material with a high degree of aromatization and strong antidecomposition ability. The physical, chemical and mechanical properties of biochar will depend on the feedstock material characteristics and pyrolysis conditions used for the production of biochar. It has a wide range of potential applications in various agronomic and industrial sectors. Biochar is used in agriculture to upgrade the soil quality, in waste treatment to remove organic contaminants, heavy metals and different types of dyes and pigments from textile industries and in power generation as a fuel. The most successful approach for highyield biochar production is via slow pyrolysis [21].

*Bio-oil -* Bio-oil is a dark brown, free-flowing organic liquid mixture. It generally comprises of 15–35 wt% water (resulting from both the original moisture and as a pyrolysis product) and a mixture of organic compounds, such as acids, alcohols, ketones, aldehydes, phenols, ethers, esters, sugars, furans, alkenes, nitrogen compounds, miscellaneous oxygenates and solid particles. The final water content of bio-oils depends on the initial moisture content of biomass feedstock and water formation during pyrolysis. Water cannot be removed from bio-oil by conventional methods like distillation. Bio-oil has a low (15–20 MJ/kg) HHV in comparison to conventional petroleum fuel HHV of 42–45 MJ/kg due to the increased oxygen content (35–40 wt% on a dry basis). Bio-oil density is approximately 1200 kg/m3 ; the viscosity ranges from 25 to 1000 cP (depending on the composition). It is acidic in nature (pH value of 2–4) due to the presence of organic acids such as formic and acetic acid and, hence, corrosive. Bio-oil has a large amount of oxygenated compounds and organic material; it is highly polar. As a result, bio-oil is hydrophilic. A distinct aqueous phase is only observed with bio-oil having water content in the range 30–45 wt% [23, 55]. Bio-oil will not mix with hydrocarbon liquids. Bio-oil has a complex mixture of oxygenated compounds that provide the potential and challenge for its utilization. It has a range of uses in energy applications, can be used in boilers for heat and power generation, cofired with natural gas/coal in power plants or blended with other fuels such as ethanol or gasoline. Bio-oil can be converted into fuels (ethanol and diesel) and chemicals through hydrocracking/ hydroprocessing [56].

*Non-condensable gases –* Gases produced in biomass pyrolysis may consist of carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), methane (CH4), ethane (C2H6) and ethylene (C2H4), and small amounts of other gases, such as propane (C3H8), ammonia (NH3), nitrogen oxides (NOX), sulfur oxides (SOX) and alcohols of low carbon numbers. The composition of the non-condensable gases will be determined by the pyrolysis temperature and the vapors condensing temperature. Lower pyrolysis temperatures (such as torrefaction) result in higher amounts of CO and CO2, while higher pyrolysis temperatures result in increased content of CH4 and H2.

### **5.1 Combustion**

Combustion is simply the burning of biomass in air. Chemically it is hightemperature exothermic oxidation of biomass in the presence of oxygen. Complete combustion of biomass involves the production of heat due to the oxidation of carbon and hydrogen of biomass to CO2 and H2O, respectively. The process consists of consecutive heterogeneous and homogeneous reactions. Biomass combustion basically depends on the properties of the feedstock and particle size, temperature and combustion atmosphere. Char (contains some organic carbon) and ash (typically includes inorganic oxides and carbonates) are the solid byproducts of combustion. Combustion temperatures are usually in the range of 700–1400°C [52, 57].

Energy stored in biomass can be converted into heat and power via combustion. The chemical composition and the combustion properties of biomass vary considerably depending on the biomass type. A wide range of biomass sources can be considered for combustion. Seasonal, regional variances and parts of the plant (bark, branches and leaves etc.) of the woody biomass (wood chips, wood pellets and waste woods etc.) can result in differences in the chemical composition of the feedstock. Straw is also considered as having potential as an alternative feedstock. Straw is essentially a waste product from agricultural crop production. This feedstock does not compete with agricultural products for the limited land resources. Besides wood and straw, a wide variety of waste products such as rice husks, wheat bran, peanut shells, coffee grounds, bagasse, etc., can be used as feedstock. These can be used as an inexpensive fuel to produce heat or electricity needed for industrial processes. The best quality fuels contain high amounts of carbon and hydrogen and low amounts of other elements (oxygen, nitrogen, sulfur and trace elements). Biomass usually contains higher levels of oxygen than fossil fuels. Impurities such as sulfur and nitrogen are associated with the emission of SOX and NOX. Trace elements such as potassium and sodium can cause fouling; chlorine leads to corrosion and silica causes excessive wear to milling equipment [52, 57].

Fresh woodchips can contain 50% moisture and leaves can be over 90% moisture. Most furnaces and boilers recommended biomass with less than 20% moisture. It is extremely difficult to maintain combustion with a moisture content of more than 55%. Higher levels of moisture affect combustion efficiency and increase the amount of smoke emitted. The water content also increases the combustion time of a biomass particle and, thus, extends the required residence time in a boiler/ furnace. Torrefaction upgrades biomass by removing lighter volatiles and moisture. It improves the heating value of biomass, increases the hydrophobicity and stability and, thus, can be stored under the open sky [52, 57].

Combustion can be split into four stages: drying, pyrolysis (devolatilization), volatiles combustion and char combustion. When biomass particles enter a hot environment, moisture in the particles starts to evaporate. On further heating, volatile gases and tars are released from biomass particles, followed by the combustion of volatiles. The remaining char will essentially retain its original shape. In the

char combustion stage, char reacts with oxygen to form mainly CO2 (and CO due to incomplete combustion) and ash remains after combustion is completed. Detailed chemical reactions kinetics that takes place during biomass combustion are complex [52, 57, 58].

The initial combustion stage requires heat to evaporate moisture; hence, it is necessary to have biomass with minimal moisture content. Biomass has a significantly higher volatile matter content compared to coal and the fixed carbon to volatile matter (FC/VM) ratio is significantly low. Lower values of the FC/VM ratio leads to high ignition behavior. Biomass releases VM at a lower temperature and more rapidly than coal, thus, reducing ignition temperature compared to coal. Proper design of the air supply is important due to the faster release of VM in order not to delay combustion. Combustion of VM is fast compared to combustion of solid charcoal and a low ratio of FC/VM decreases the residence time in the boiler/ furnace [52, 57, 58].

Incomplete combustion results in the formation of intermediates, including pollutants such as CO, CH4 and particulate matter (PM). Ash handling, high emissions of NOX, SOX, CO2 and particulate matter make combustion environmentally challenging. Biomass is more corrosive, tends to foul heating surfaces, ash from biomass tends to agglomerate. The boilers have to be redesigned to burn biomass properly. Depending on the condition and combustion properties of the biomass to be burned, different furnace designs and combustion parameters can be selected to ensure optimum efficiency. Direct combustion is currently the principal method of generating electricity around the world [52, 57, 58].
