Pyrolysis can be defined as a process in which carbon-based matter is decomposed in the absence of oxygen and at high temperature into its constituent elements, such as bio-oil, syngas, and bio-char, as shown in Fig. 8.1. Generally, the higher heating value of bio-oil ranges between 15 and 38 MJ/kg while this value for solid char is about 17–36 MJ/kg [3]. On the contrary, the higher heating value of pyrolysis gas fraction is approximately 6.4–9.8 MJ/kg [4]. The energy of the char and gas products can be recovered and utilized for further heating the pyrolysis reactor.

Typically, pyrolysis processes can be classified as slow pyrolysis and fast pyrolysis depending on the time taken for the completion of thermal decomposition of the feed materials. On the other hand, a wide range of reactors are being used for the production of liquid bio-oil from different waste biomass [5]. The production rate and properties of this product will depend on the design of pyrolysis reactor, reaction temperature, heating rate, residence time, pressure, catalyst, type of biomass, and particle size, shape, and structure [6]. As, for example, a low temperature with slow rate pyrolysis normally produces higher amounts of bio-char, and the opposite condition produces higher amounts of bio-oil.

High sensible heat is required to increase the biomass temperature to thermal decomposition temperature to enhance the pyrolysis reaction. In the current pyrolysis system, the reactor is heated using conventional energy sources which associated energy expense and environmental pollution. This high expense and environmental threat are the major challenges to refrain pyrolysis process as a member of industrial level energy production option. Although, for maintaining the high-temperature condition for producing more bio-oil, the syngas that is produced in pyrolysis can be burned within the system, it is a challenge to design such kind of complex pyrolysis reactor. Therefore, a novel concept of solar heating is assimilated to the biomass heating system to mitigate the problems.

In this chapter, the overview of solar pyrolysis including history, design, challenges, and its solutions is presented. The chapter is organized as follows. Section 8.2 discusses the historical background and current state of biomass pyrolysis. Section 8.3 presents the major challenges of conventional pyrolysis technology highlighting the issues of reactor heating. Section 8.4 illustrates the possible means of heating the reactor for thermal decomposition of biomass. Section 8.5 depicts the history of solar heating approach and its classification. Additionally, Section 8.6 describes the incorporation of solar heating into pyrolysis technology including different heating system and updated design concepts. Moreover, present applications and feasibility of solar-assisted pyrolysis technology are presented in Sections 8.7 and 8.8, respectively. Finally, the possible challenges and future development scope of solar-integrated pyrolysis technology are demonstrated in Sections 8.9 and 8.10, respectively.

By having the aim to produce bio-char, which is a charcoal-like product and used to both enrich and stabilize the nutrient-poor rainforest soils, the pyrolysis process was used near about thousands of years ago in the Amazon rainforest. At that time, people were starting fires by covering the fuel with earth surface for making a low amount of oxygen environment so that in the absence of oxygen bio-char was produced rather than ash by the breaking down of fuel continuously.

During the two world wars, pyrolysis was done with wood waste in order to reduce transportation fuel when fossil fuels were limited. In the late 1950s, the modern development of pyrolysis mainly started its journey. Along with liquid yield expectation, pyrolytic gasification was first introduced in the United States at Bell Laboratories in 1958 [7].

Although pyrolysis gasification is one of the common thermochemical processes, further research is still required to improve the quality of intermediate and final products. As an example, while producing synthetic hydrocarbons by using pyrolytic gasification process, highly contaminant content gas is produced [8]. In addition, the oxygen content of the obtained product from pyrolytic gasification is too high [9]. In recent time, different laboratories has started working on it to improve the pyrolysis gasification quality.

The Bell Laboratories in the United States with some renowned universities and organizations of the world in 1958 had carried out research and development programs to determine the usefulness of pyrolysis. Production of gas by using waste materials is the purpose of the system. Then in the 1970s a pyrolysis plant producing 200 ton/day RDF (refuse-derived fuel) was built by the Occidental Research Corporation in San Diego, United States [10]. During late 1970s to early 1980s a flow pyrolysis process was developed by the Georgia Tech [11].

A more efficient batch systems gave way to continuous feed systems with a cone design that made the evacuation of the gases, which was developed during late 1970s to early 1980s. An indirect pyrolytic gasification system was designed by the Balboa Pacific Corporation in the United States in the mid-1980s. This indirect pyrolytic gasification system was patented in 1988. The work of Scotland with his group developed a flow pyrolysis system at the University of Waterloo, Canada, in the Georgia Tech, which was the next significant development in the field of pyrolysis. A great number of inspirable quasi-commercial scale pyrolysis systems were developed in the 1980s and 1990s [10]. Then during 1993–95, a two-year period, 50 ton/day pilot plant was refined and constructed, which was partially funded by Southern California Gas Company and private investors. For a period of 18 months, the plant operated in California, during which time, the system's ability to process, on a continuous feed basis, a wide variety of toxic and nontoxic, liquid or solid organic waste streams was exhaustively studied by different nation's environmental engineers such as Dames & Moore.

Besides all of this, pyrolysis was occurring with a lot of things, such as sugarcane, tire, plastic, wood, cellulose, as well as with different types of waste in different places of the world. For example, pyrolysis with sugarcane was published in 1980s. For a very long time truly from the ancient time wood pyrolysis was used by Mediterranean and northern civilizations after separating the charcoal from the furnace. During 18th and 19th centuries, a great number of experiments on wood pyrolysis show that the liquids obtained by it include several phases, such as water, wood spirit, light oils, pyrolytic acid, tars, and so on. [12].

A significant work on cellulose pyrolysis with their thermal behavior was published during 19th and 20th centuries, and sometime in 1918 this was reported by Pictet and Sarasin. They also discussed the thermal behavior of Amidon and ultimately points out on the industrial preparation of glucose and alcohol by using cellulose pyrolysis [13]. In 1920, Clément and Rivière recapitulated this issue in their book named “La cellulose.” Three articles during mid-1960s and 1970s report about the cellulose pyrolysis evidencing the phase changing phenomena in this pyrolysis process [12]. In the articles of Hawley and Stamm, in 1956, several references are found on thermal degradation of cellulose and wood pyrolysis [14].

M. J. Antal has been producing a lot of fluid fuels by using concentrated radiant energy [15]. In Copper Mountain's meeting, M. J. Antal described the solar furnace and bench-scale facilities which he performed at the Princeton University, in French. He demonstrated that the product of the concentrated radiant energy pyrolysis also known as flash pyrolysis can be either liquid syrups or permanent gases depending upon the condition of the reactor, which was made more clear by Dejenga and Antal in 1982 who showed that the majority of the gaseous product is LVG [16]. Then in 1983 to decipher the result acquired in a continuous solar reactor, Antal et al. used the BS model [15]. In 1984, Hopkins et al. interpreted about the cellulose flash pyrolysis which is carried out in a spouted bed settled at the focus of an arc image furnace (xenon lamp) [17].

In the same year, Mok and Antal calculated the requirement of heat in cellulose flash pyrolysis. After calculating the required heat, Mok and Antal proposed a new detailed model that can control the pressure and flow rate of the working fluid as per demand [18]. Few years later, in 1993, cellulose flash pyrolysis, which was proposed by Mok and Antal was scrutinized by Varhegyi et al. [19]. In 1993 inside a transport flow reactor under a maximum temperature of 1473K, Vladars-Usas studied the fast thermal decomposition of Avi-cel cellulose [20]. Then, in 2002 and 2004 fast pyrolysis in the heated reactor was carried out by Boutin et al. [21] and Luo et al. [22], respectively. The melting phenomena of cellulose were discussed by Schoeter and Felix in 2005 [23].

The pyrolysis technology with different method using different sources of energy was investigated by different people in several places of the world [24]. However, the development of advanced technologies for upgradation of bio-oil and to increase the percentage of bio-oil production are the next issues for the researchers, yet to be solved. Therefore, researchers have devoted themselves to investigate different technologies for upgrading the bio-oil [25]. Currently, the researchers have given emphasis to blend bio-oil with other oils to enhance their fuel properties [26]. Additionally, attention has been given to catalytic fast pyrolysis for production of hydrocarbons from biomass [27].

Since mid-2010s, mathematical modeling studies have been conducted to identify the nature of the chemical reaction during thermal decomposition and to understand the kinetics of biomass reactor. The analysis of hydrodynamic modeling and simulation of particles in pyrolysis, their interaction, and the effect on the performance of the reactors revealed that the characteristics of feedstock, physical and chemical properties of feedstock, and residence times are major factors for the desired performance of the pyrolysis process [28–31].

In the auto-thermal heating method, the heat flux required for the pyrolysis is supplied by the partial combustion of feed materials and yield bio-char. This method reduces the amount of char production as the heat indispensable for pyrolysis is produced from combustion of biochar itself thereby decreasing the fuel cost. On the other hand, in the case of the direct heating method, the hot carrier gas or solid heat carrier is introduced into the reactor and the feed materials absorb the heat for decomposition. The hot carrier gas produces CO₂ and H₂O during the combustion reaction which results in the reduction in the concentration of gas and heating value of oil. Besides, the use of solid heat carrier increases the oil yield, however, degrades the quality of oil due to the intake of a large amount of dust [41]. Contrarily from the direct heating method, in indirect heating, the reactor is heated from an external energy source where the heat is generated from the combustion of biomass or natural gas (NG) burning heater and electric heater. The heat is transferred through the reactor wall to raw materials by conduction and convection heat transfer principle. The oil yield and the efficiency of this heating system are affected by the thermal conductivity of reactor material, thermal diffusivity, and the temperature gradient between the reactor surface and center of the material inside the heating chamber. These constraints restrict the rapid heating of biomass and results in substantial input energy loss.

Therefore, the indirect heating system is less efficient; however, it produces less diluted gas compared to direct method. This reduces oil yield as well as increases char production that causes coking and fouling in the system [42]. Furthermore, the uneven nature of temperature profile causes an undesired secondary reaction in different regions of the pyrolysis reactor which produces toxic compounds [43,44]. On the contrary, this heating method has the advantages of producing retorting gas of high quality from a wide variety of particle size simultaneously. Since early 2000s, microwave (MW) heating pyrolysis is drawing the attention of researchers to overcome the limitations of conventional external heating of pyrolysis reactor. In 2001, this method was first implemented in pyrolysis of waste plastic [45]. The application of MW heating in pyrolysis reduces significant amount of energy input and time required for pyrolysis process due to direct and rapid heating. It also offers the advantages of uniform heating and less equipment dimension. In addition to this, the method improves the efficiency of the system as well as the quality of pyrolysis products. The MW heating enhances the high-grade product yield in reduced reaction time [46,47]. However, the foremost problems associated with this method are: different design approach considering the dielectric properties of biomass and the complication in heat and transfer mechanism and also in chemical reactions inside the different sections of heating chamber. Other electromagnetic heating, induction heating, is becoming popular in pyrolysis reactor heating due to its rapid heating rate and high efficiency, although the method is less flexible and requires high energy input and cost [48].

From the previous discussion, it is clear that a constant energy source is required to run the pyrolysis system, which involves consumption of conventional fuel. This means, a high-quality fuel is consumed to produce relatively less-quality fuel. Reactor heating with conventional fuel is not the economic approach of production of bio-oil. Therefore, reactor heating with another renewable source such as solar heating would be a great choice.

On the other hand, Morales et al. [52] examined the use of laboratory-scale real solar furnace heated with the parabolic-trough solar concentrator. Besides, Zeng et al. [77] designed a 1.5 kW vertical-axis solar furnace and investigated the heating effects for bio-char and bio-oil production. Haueter et al. [78] developed an improved design of a direct heating solar thermal chemical reactor (called ROCA reactor) for chemical decomposition of ZnO. The reactor has low thermal inertia and outstanding thermal shock resistance capability. However, the most promising design of rotating cavity solar reactor, namely ZIRRUS reactor, was invented by Müller et al. [79] to address the problem of inability to recuperate a large percentage of products from the reactor associated with the ROCA reactor.

Generally, a substantial amount of heat energy from biomass or electric heating sources is used for thermal decomposition of feed materials inside the reactor in pyrolysis. Therefore, complete or partial inclusion of solar energy in pyrolysis instead of these nonrenewable heating sources may reduce the amount of energy input as well as fuel cost significantly and thus increase the effectiveness of pyrolysis. In addition to these, the combustion of biomass is responsible for environmental pollution as it emits considerable amount of CO₂, CH₄, N₂O, NO_x, CO, and volatile carbon compounds, which is the major concern in the 21st century. Therefore, the heating of pyrolysis reactor with green and renewable solar energy will be an effective option to reduce environmental pollution.