Different salts of potassium as KCl, CH3COOK, K2CO3 were also found to be effective in improving the gasification efficiency. Vamvuka et al. [30, 31] further explored the activity of Li, Rb, Ca Na, and K containing catalysts in gasification of waste and trend obtained as follows: Li2CO3 > K2CO3 > CaCO3 > Rb2CO3 > CaSO4 > Cs2CO3 > Na2CO3. Calcined dolomite due its abundance, inexpensive and catalytic activity towards reducing the tar formation attracts the researcher. Various studies have been reported using dolomite as catalyst under batch as well flow conditions such as fixed and fluidized bed reactor. Calcined Dolomite also suffers the drawback of melting point which makes it unstable at higher temperature and doesn’t achieve tar conversion beyond 90–95%. The catalytic activity of noble metal (Rh/CeO2) based catalyst lowered the gasification temperature to 600 C. RuO2 showed good catalytic activity towards the production of hydrogen under supercritical water under conditions of 44 MPa, 450 °C and a residence time of 120 min [32, 33].
3.2.3 Aqueous Phase Reforming Aqueous Phase Reforming
Aqueous phase reforming (APR) has certainly gained attention as most promising and suitable alternative for production of hydrogen as well as alkanes from lignocellulosic biomasses. The APR has several advantages over other methods as the reaction is wet or water-soluble feedstocks compatible, that can take place in both batch and continuous flow reactor, in a single step. As compared to conventional alkane steam reforming process, APR of carbohydrate is carried at relatively mild reaction conditions which facilitate water gas shift reaction leading to low CO production due to reduced decomposition rate of carbohydrates. However, in APR, methanation of CO2 and production of alkanes/alcohols competitively lowers the H2 yield [34–36]. Moreover, the type of starting raw material, design of reactor, reaction conditions and use of suitable catalyst cumulatively influence the APR reactions. APR was first popularized by Cortright et al. [37], who demonstrated mainly hydrogen production from oxygenated hydrocarbons extracted from renewable biomass and biomass waste streams. They carried the reaction at relatively low temperature and pressure (473–523 K, 15–50 bar) in single-reactor aqueous-phase reforming process using a Pt/Al2O3 catalyst and were able to generate hydrogen-rich fuel gas with high purity and yield. Since then, numerous substrates have been tested for H2 production.
Valenzuela et al. [38], for very first time used woody biomass as APR raw material for direct production of H2. They generated 18 vol% of H2 by treating southern pine sawdust with H2SO4 followed by soluble products reforming using Pt/Al2O3 catalyst in a one pottwo-step process. In subsequent years, use degreased cotton, filter paper and microcrystalline cellulose as a raw material in place lignocellulosic biomass was adopted as an attractive choice for H2/alkane production [39, 40]. As the chemical structure of feed molecule restrict the efficiency of hydrogen production, various studies have been undertaken to investigate ethylene glycol, glycerol, glucose, sucrose, sugars-derived polyols namely sorbitol and xylitol as starting material [36, 41–43]. It is suggested that xylitol and sorbitol are far more reliable raw material compared to glucose and cellulose giving fairly good H2 yield. Although, APR process of production hydrogen rich biofuel from renewable sources is a smart approach, its cost of feedstock limits its applications at industrial level. However, recent studies have shown the utilization of biomass-derived organic compounds obtained from industrial wastewater as feedstock for APR process. Oliveira et al. [44] showed brewery wastewater as an interesting way of valorization to H2-rich gas, thus, integrating wastewater treatment with waste-to-value added process. Zoppi et al. [45] recently proposed APR of water effluent derived from hydrothermal liquefaction to produce a gas mixture rich in hydrogen a well clean water effluent from its carbon content. Therefore, APR is an environmentally promising solution that can be used to subject different biomass derived raw material under suitable reaction conditions for selective production of H2/alkanes.
3.3 Types of Biomass
Biomass from decades has remained primary source of renewable energy for humanity. Jacobsson and Johnson [46] defined photosynthesis derived organic mass including land and water-based vegetation as biomass. Later, the concept of biomass was elaborated to all organic mass available through natural process or anthropogenic activities except plastics and fossil fuel [47]. Biomass is measured as dry weight mass per unit area (g m–2 or Mg ha–1). The processed biomass is called feedstock. The most prominent sources of biomass (illustrated in Figure 3.2) are agricultural and forestry residues, waste from aquatic ecosystem, energy crops, industrial solid waste, industrial effluents rich in organic loads, residues from livestock farms, municipal solid waste, sewage and anthropogenic generated waste streams [48] (Figure 3.1).
Based on origin, the biomass is classified into five different groups:
1 i) Wood and woody biomass
2 ii) Herbaceous biomass
3 iii) Aquatic biomass
4 iv) Animal and human waste biomass
5 v) Biomass mixtures and municipal biomass.
Figure 3.1 Different methods for biomass transformation to fuels and value-added chemicals.
Figure 3.2 Different sources of biomass.
3.3.1 Wood and Woody Biomass
The woody biomass mainly composed of carbohydrates and lignin is the most common renewable source in current world. It commonly consists of residual parts of trees, roots, bark and leaves of woody species [49]. Urban and agriculture waste, non-merchant timber, post-consumption wood wastes as well as production residues all comes under woody biomass. In short, it includes wood-based biomass obtained from forest and wood industry. The bark, wood blocks wood chips and logs obtained from forest as forest by-products, all fall under this category. Moreover, residues like saw dust, slabs and off-cut contributed by sawmill and timber mill industries are termed as industrial wood biomass.
3.3.2 Herbaceous Biomass
It is also called as non-woody biomass broadly includes agricultural residues, native plants, and non-wood plant fibers. Energy crops which come under nonwoody biomass, are specifically grown for their fuel value. These crops have good energy density, grow fast, require low nutrients and maintenance cost with maximum biomass yield. Usually, these are grown in barren lands with little irrigation and fertilizer investment, and therefore don’t interfere with the food plant cultivation. These include dry lignocellulosic woody and herbaceous energy crops (e.g. short rotation crop, miscanthus crop), oil energy crops and starch energy crops [50].
3.3.3 Aquatic Biomass
Microalgae, macroalgae, and emerging plants all together constitute the term aquatic biomass [51]. They efficiently convert light
