Bioenergy Production

Abstract

In this chapter, an overview of bioenergy importance toward energy systems with low (zero or negative) greenhouse gas emissions and general conversion technologies to produce different types of bioenergy products from various biomass feedstock is presented. The bioenergy products from biomass cover all physical phases including solid (biochar), liquid (bio-oil and bio-crude oil), and gases phase (bio syngas) which make them an interesting field in terms of both academic types of research and industrial scale. A discussion on the available technologies for thermochemical, biochemical, and extraction processes is presented, which is followed by some important parameters on each separate process that cause the optimum production rate and desired products. In addition, in the final part, an overview of the technology readiness level for the processes is reported.

1. Introduction

Energy is an indispensable and prominent factor to accelerate economic and social development all over the world, undoubtedly. Therefore, due to the rise of the global population and incline to growth in both developed and developing countries, the energy request has been growing [1, 2, 3]. On one side, strong concerns over the depletion of fossil fuel reserve/resource and their accessibility in the next decades for long-term planning, and on another side, serious warns related to greenhouse gas emission due to fossil fuel consumption and destructive predictions of climate change consequences at the global level necessitate a huge scale transition toward new energy arrangements with reduced or even negative greenhouse gases [4, 5, 6]. In addition, The Paris Agreement on climate change calls on members to preserve the global temperature rise below 1.5 to 2 degrees Celsius (°C) above pre-industrial levels [7]. One of the most significant measures toward energy systems with low (zero or negative) greenhouse gas emissions to mitigate global temperature rise in the long-term and is the application of renewable energies and their share into global energy consumption instead of fossil fuel [8, 9, 10]. Biomass is clean renewable energy that accumulates and transfers sun energy in the form of chemical energy during the growth of plants and trees through the photosynthesis process [11]. Therefore, biomass has been recognized as one of the renewable energy sources, with carbon capture capability and carbon neutrality [12, 13, 14]. In this context as it is shown in Figure 1, biomass also demonstrates the capability of transformation of the accumulated energy into multiple general forms of final energy carries such as solid, liquid, and gaseous which are compatible for various sectors comprising heat, power, and transport fuel [15]. In order to convert biomass energy to carrier energy products, some approaches such as thermochemical, biochemical, and coupled hybrid bio-refinery platforms or processes have been developed to ease access to green energetic biofuels with high value-added and clean energy chemicals [16].

Figure 1.

General processes of bioenergy production from biomass feedstock.

2. Thermochemical conversion

Thermochemical conversion is defined as the degradation of organic matters due to heat exposition of biomass and chemical reactions. The process is mainly categorized in some processes named combustion, torrefaction, gasification, pyrolysis, and hydrothermal [17, 18]. In the thermochemical conversion of biomass, heat and catalysts are applied to transform biopolymers of biomass into biofuels and other valuable chemical components [19]. Based on the process the outputs mainly are biochar (carbon-rich solid residue,) liquid biofuel including bio-oil, bio-crude oil and tar (condensable vapors), and syngas (non-condensable gases) [20].

2.1 Combustion

Combustion is defined as high-temperature exothermic oxidation of biomass in the presence of oxygen and the presence of consecutive heterogeneous and homogeneous reactions which resulted in the production of heat as the main product. Combustion is divided into four stages: drying, pyrolysis (de-volatilization), volatiles combustion, and char combustion. As soon as biomass particles enter the burning environment, the particles moisture evaporate, on further heating, volatile gases and tars are released which follow by their combustion. The remaining char will essentially retain its original shape. The process outcomes mostly depend on the properties of the feedstock, particle size, temperature, and combustion atmosphere that can have char and ash (typically includes inorganic oxides and carbonates) as the solid byproducts of combustion [21, 22]. Carbon dioxide (CO2) and water vapor (H2O) are also produced during the complete combustion of biomass, however, it is not achieved under any conditions which cause the production of carbon monoxide (CO), methane (CH4), non-methane hydrocarbons (NMHCs), particulate matter (PM) and nitrogen and sulfur species mainly NO_x and SO_x during the incomplete combustion the biomass material [23]. The drawbacks are mainly controlled through modification of combustion processes via flue gas recirculation, boiler modification, and re-burning technology which often mitigate such emissions economically [24].

2.2 Hydrothermal conversion

The hydrothermal conversion process is a suitable technology especially for wet biomass into bio-fuel which is defined as a thermochemical transformation of biomass in high temperatures (100–700°C) and high pressures (5–40 MPa) in a liquid media or hot supercritical water [25]. In hydrothermal liquefaction (HTL) as an important hydrothermal process, raised temperatures (200–350°C) and high pressures (5–20 MPa) in the presence of solvent (sub−/super-critical water) applied to boost biomass decomposition and reformation to produce bio-crude (as the main output) bio-char, water-soluble organic polar fractions and gaseous [26, 27, 28]. During the HTL process, several complex mechanisms such as hydrolysis activate which degrade biomass macromolecules and then decompose them into smaller components to reactive fragments by bond cleavage and several reactions such as dehydration, dehydrogenation, deoxygenation, and decarboxylation while some complex chemicals such as bio-crude produce through depolymerization [29, 30, 31]. Derived bio-crude oil through HTL shows a higher heating value between 36 to 40 MJ/kg which is close to petroleum-derived oil characteristics [32, 33, 34]. HTL technology which is currently at the pilot/demonstration scale has several positive points in comparison to another thermochemical process including the ability to use high moisture content biomass inputs, lower operating temperature, higher throughput, and removal of oxygen from the bio-crude [35, 36]. In addition to biomass feedstock elemental composition, various operational parameters such as temperature, reaction time, pressure, presence of a catalyst (catalyst type and amount), solvent/biomass ratio, and reaction medium influence the process in terms of quantity and quality of produced bio-crude [37, 38, 39, 40]. HTL in comparison with other processes reveals various advantages including application feedstock with high moisture content without drying requirement, exploitation of the properties of superheated fluids to reduce mass transfer resistances, and penetration of the solvent to biomass structure to enable the fragmentation of biomass molecules due to high pressure which result to obtain high-quality bio-crude oil [41].

Hydrothermal Carbonization (HTC) is the second hydrothermal conversion which is performed in a temperature range of 180 to 350°C during which the biomass is submerged in water and heated under pressure (2 to 6 MPa) for 5 to 240 min while the main product of HTC is hydro-char [42]. Hydrothermal gasification (Supercritical water gasification) is a thermochemical conversion process in which, wet biomass was directly converted into combustible gases under 400 to 500°C (processed till 700°C) and 24 to 36 MPa pressure with/without catalyst aid. Further, supercritical water is (<374°C, 22.1 MPa pressure) is acting as a reactant and solvent that splits organic compounds. During gasification, decomposition of biomass causes dissolution of reactive species that promote the yield of gaseous products by impeding the biochar production at supercritical [39, 43]. The hydrothermal gasification technologies) have considerable economic, environmental, and technical advantages over other high-demand energy conversion technologies. These processes are compatible with wet feedstock (not suitable candidates for another thermochemical process). Also due to the reactions taking place at lower temperatures (less energy consumption) and the use of a wide range of feedstock processing [44]. Due to the unique dissolution properties of water during hydrothermal gasification, less coke and tars are produced while pressurized produced syngas is typically free from gaseous that do not usually require further processing and can lower compression costs [45].

2.3 Pyrolysis

Pyrolysis defines as thermal decomposition in the absence of oxygen to break biomass chemical bonds in high temperatures to produce biofuels [46]. Depending on process requirement and desired product the process temperatures vary between 280 to 1000°C [47, 48]. During the process, generally three-step mechanisms including de-hydrogenation, de-polymerization and fragmentation occur to transfer biomass to biofuel [49]. The percentage of main products including bio-oil and bio-char and bio-syngas as the byproduct differ depending on heating rate, solid residence time, and temperature as the main operational parameters in the process [50]. Lower pyrolysis temperatures and longer residence times (Slow pyrolysis) tend to produce more bio-char while high temperatures and longer residence times increase the production of gas. Moderate or high temperatures and short residence times (Fast and Flash pyrolysis) resulted in more bio-oil [51]. Several technologies and reactors with the semi-continuous or continuous process have been developed on a laboratory scale and considered as suitable reactors for commercialization of pyrolysis including Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB), Circulating Spouted Bed (CSB), Rotary Cone (RC), Ablative reactor and Screw/auger reactor [52]. In addition, plasma pyrolysis reactor configuration, Vacuum pyrolysis, Microwave-assisted, and solar-assisted pyrolysis have been extensively investigated as the state-of-the-art technologies related to biomass pyrolysis which demonstrates their advantages over conventional electrical-heating-assisted biomass pyrolysis [53, 54].

The higher heating value (HHV) of the bio-oils normally ranges between 15 and 20 MJ/kg which is only 40–50% of the conventional petroleum fuels with HHV between 42 to 45 MJ/kg. The HHV of the bio-oils can be approximately calculated through some empirical equations formulated by elemental analysis of the bio-oil (CHNOS analysis plus ash content) as represented in (Eq. (1)) [49].

$$\begin{array}{l}\mathrm{H}\mathrm{H}\mathrm{V}\left(\frac{\mathrm{M}\mathrm{J}}{\mathrm{kg}}\right)=0.3491\times \mathrm{C}+1.1783\times \mathrm{H}+0.1005\\ \times \mathrm{S}‐0.1034\times \mathrm{O}‐0.0151\times \mathrm{N}‐0.0211\times \mathrm{Ash}\end{array}$$E1

Since the liquid bio-fuel which contains oxygenated compounds such as acids, alcohols, phenols, ketones, and esters is commonly considered as poor quality, thus, it requires upgrading into a higher value-added product through promising methods such as catalytic steam reforming. The process of bio-oil quality upgrading and the water gas shift (WGS) reaction is presented in (Eq. (2)) and (Eq. (3)) respectively [55].

$${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{k}}+\left(\mathrm{n}‐\mathrm{k}\right){\mathrm{H}}_2\mathrm{O}\circledR \mathrm{n}\mathrm{C}\mathrm{O}+\left(\mathrm{n}+\left(\frac{\mathrm{m}}{2}\right)‐\mathrm{k}\right){\mathrm{H}}_2$$E2

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$E3

2.4 Gasification

In the condition in which production of biogas fuel is required, the gasification process under a reduced oxygen atmosphere applies to convert solid biomass to a gaseous fuel known as synthesis gas [56]. The biomass gasification process is conducted in four main stages including drying of the biomass particles followed by pyrolysis of the dried biomass particles(de-volatilization), in the next step partial oxidation of the pyrolysis gases and/or char occurred and finally char gasification happened (reduction). In contrast to pyrolysis, the feed is brought into contact with a gasifying agent (air) to ease the reaction between oxygen and biomass content in higher temperatures between 600°C and 1500°C. The produced gas contains various percentages of CO, H₂, CH₄, CO₂, H2_O, N₂, and eleven other gases depending on the quality of the biomass used and the way gasification is conducted [57, 58]. Fixed bed, fluidized bed, entrained flow, rotary kiln reactor, and plasma reactor can be utilized based on the operational conditions in gasification [59]. Briefly, biomass feedstock type and composition, particle size, moisture and ash content (higher ash content cause ash agglomeration during the process especially in high temperature), operational temperature, pressure and residence time, gasifying media, equivalence ratio (actual air-to-biomass ratio), steam-to-biomass ratio (S/B) and finally catalyst type and amount are the most prominent factors during the gasification process [60].

According to the Figure 2 Biomass pass their steps of drying, pyrolysis, and partial oxidation before reach to the gasification point. Each stage is accrued in a specific range of temperature [61]. After the drying step, biomass is decomposed to solid char and pyrolysis which will be faced with the second decomposition stage and conversion into decomposes gases (non-condensable) and volatile hydrocarbons. Then, these products react with the oxidizing agent to produce syngas and smaller amounts of lower hydrocarbon gases (C1–C4) [62]. The global reaction inside the gasifier (except for unconverted solid carbon) can be described as (Eq. (4)) while for simplicity only the amount of hydrogen, carbon, nitrogen, oxygen, and sulfur of the biomass are considered in the model [58]:

Figure 2.

Gasification steps and the temperature zones.

$$\begin{array}{l}C{H}_{\mathit{a}}{O}_{\mathit{b}}{N}_{\mathit{c}}{S}_{\mathit{d}}+w{H}_{\mathbf{2}}{O}_{\left(\mathit{l}\right)}+s{H}_{\mathbf{2}}{O}_{\left(\mathit{g}\right)}+e{O}_{\mathbf{2}}+\mathrm{\rho }e{N}_{\mathbf{2}}\leftrightarrow \left(1-\alpha \right)C_{\mathit{s}}\\ +n_{\mathit{c}\mathit{o}}CO+n_{\mathit{C}{\mathit{O}}_{\mathbf{2}}}C{O}_{\mathbf{2}}+n_{{\mathit{O}}_{\mathbf{2}}}{O}_{\mathbf{2}}+n_{{\mathit{H}}_{\mathbf{2}}}{H}_{\mathbf{2}}+n_{{\mathit{H}}_{\mathbf{2}}\mathit{O}}{H}_{\mathbf{2}}O+n_{\mathit{C}{\mathit{O}}_{\mathbf{2}}}C{O}_{\mathbf{2}}\\ +n_{{\mathit{N}}_{\mathbf{2}}}{N}_{\mathbf{2}}+n_{\mathit{No}}\mathit{NO}+n_{\mathit{N}{\mathit{O}}_{\mathbf{2}}}N{O}_{\mathbf{2}}+n_{\mathit{N}{\mathit{H}}_{\mathbf{3}}}N{H}_{\mathbf{3}}+n_{\mathit{H}\mathit{C}\mathit{N}}HCN\\ +n_{{\mathit{H}}_{\mathbf{2}}\mathit{s}}{H}_{\mathbf{2}}S+n_{\mathit{S}{\mathit{O}}_{\mathbf{2}}}S{O}_{\mathbf{2}}+n_{\mathit{S}{\mathit{O}}_{\mathbf{3}}}S{O}_{\mathbf{3}}+n_{cos\mathit{C}\mathit{O}\mathit{s}}\end{array}$$E4

One of the important issues during gasification is the removal of tar which is formed during the pyrolysis stage (as a transition step toward the gasification). Various tars components are released which can condense and form sticky deposits by quenching downstream when they contact cold points of the gasification system [63]. Tar roots severe damage to gas engines or turbines through fouling and coking in the system. Therefore, it is very important to reduce the tar content and particulate matter, in the syngas below the level of 100 mg/m3 and 50 mg/m3 respectively to apply for gas engine consumption [64]. Therefore, even though gasification is a relatively well-known technology, the share of gasification in overall energy demand is insignificant due to barriers concerning biomass harvesting and storage, biomass pre-treatment (drying, grinding, and densification), gas cleaning (physical, thermal or catalytic), process efficiency and syngas quality issues [65].

3. Biochemical conversion

3.1 Anaerobic digestion

In addition to thermochemical operation, bio-chemical processes such as anaerobic digestion (AD) and fermentation are promising technology as a renewable source of energy products [66, 67]. Regarding human health, environment, economy, and energy conservation issues, AD systems have attracted remarkable attention by the production of bio-methane gas (renewable energy source) through bio-chemical conversion of biodegradable wastes [68]. AD process has occurred in an insufficient O2 atmosphere which prepares suitable conditions for activation of the microorganism to degrade organic matter into biogas [69]. To convert the feedstock to bio-methane, a series of bi-metabolism steps including hydrolysis/acidogenesis, acetogenesis, and methanogenesis occurred in the AD systems reactors [70]. During the first stage, the high molecular weight complex insoluble organic matter is degraded into simple soluble molecules by the extracellular enzymes [69]. During the hydrolysis phase, the organic components of carbohydrate, protein, and lipid polymers are hydrolyzed into simple sugar, amino acid, and long-chain fatty acid respectively [71]. Meanwhile, monosaccharides are produced through hydrolysis of the insoluble compounds of cellulose and hemicellulose by enzymatic microorganisms (Streptococcus and Enterobacterium) [72]. However, at this step, rigid lignin structure which is resistant to the penetration of microorganisms requires delignification as a pretreatment process to undergo biodegradation [73]. In the next step acidogenic bacterizes such as Clostridium, Peptococcus Anaerobus, Lactobacillus, Psychrobacter, Anaerococcus, Bacteroides, Acetivibrio, Butyrivibrio, Halocella, and Actinomyces (highly active fermenter and the most abundant bacterizes in AD) applied to dissolve and bounded oxygen in the solution and carbon [74, 75]. At the final steps of the process, acetotrophic, hydrogenotrophic and methylotrophic pathways occurred which are the main route of methane production [76]. In the methanogenesis phase, acetic acid and hydrogen that formed in the acetogenesis phase are transformed to biomethane via methanogenic microorganisms while the pH in the system will increase to neutral values within the range of 6.8–8 [71]. The methanogenesis phase effectiveness is very reliant on the balanced relationship between bio-kinetics of microorganisms (Crenarchaeota, Euryarchaeota, etc.) with its growth environment (food supply and accessibility) [77, 78].

Working conditions in AD generally influence the formation of the produced biogas. The degradation process is affected by several factors including operation temperature, carbon to nitrogen (C: N) ratio, pH level, organic loading rate (OLR), Hydraulic retention time (HRT), and stirring [76]. Defining an optimum temperature that causes the stability of the enzymes and co-enzymes activity can have a significant influence on AD and bio-methane production while the efficient AD process is dependent on the optimum temperature [79, 80]. The optimum temperature for digestion process operation of anaerobic microorganisms could be range in psychrophilic (10–30°C), mesophilic (30–40°C), or thermophilic (50–60°C) conditions [81].

Alkalinity or acidity of the substrate is categorized by the important parameter of pH value. The stability of acidogenic activity and methanogenic bacteria is directly influenced by the changes in [82, 83]. Ideally, the optimum pH for acidogenesis and methanogenic stages place in a range from pH 5.5 to 6.5 and from 6.5 to 8.2, respectively [84]. Neutralization is essential in cases of excessively high or low pH during the anaerobic digestion feedstock especially before the plant is fed. The pH is chemically improved by adding the base, such as lime, to the reactor if negligible acidification happens during the AD process [85]. The next effective parameter in the AD process is the ratio of carbon to nitrogen in organic material [86]. A high C: N ratio indicated the low nitrogen sources that are needed to sustain the material supply for digestion. Meanwhile, the low C: N ratio signified the potential of NH4+ inhibition in the digestion process. Ideally, the optimum C: N ratio for the AD process is within the range of 20–35 [87]. The HRT which is defined as the retention period of the substrates inside the digester can vary based on the feedstock composition and digester temperature [88]. High HRT will result in improvement in biogas yields while the lower HTR is interested since decreasing cost of production and enhancement of process efficiency [89, 90]. The OLR also can affect The AD process negatively or positively [91]. Minor OLRs may cause malnutrition of microorganisms and adversely affects the AD while in contrast, a great ORL ratio may cause insufficient resources to support the development of microbial organisms [92, 93]. Temperature condition, characteristics of the substrates, and HRT of the AD operation impacts the OLR behavior and amount [76].

In terms of technological requirements, several types of reactors have been developed that generally can classify into wet or dry reactors based on their total solid contents [94]. In the design and operation of the anaerobic reactor, two parameters of reactor volume to daily flow and OLR have the most important value. The dry types (serve the feedstock with a solid concentration of more than 15%) itself could be categorized into three different types including horizontal plug-flow, vertical plug-flow, and non-flow (batch type) [95, 96]. In contrast, the wet digesters are defined to serve the feedstock having a total solid less than 15%value [97].

3.2 Fermentation

Fermentation is considered as another biochemical technology that can be applied to get energy from biomass. Fermentation defines as a process of central metabolism in which alcohol (for instance ethanol) or acid is produced by an organism through the conversion of carbohydrates, such as starch or sugar. Wines, beers, and ciders are traditionally carried out with fermentation process by using Saccharomyces cerevisiae strains, the most common and commercially available yeast [98, 99]. The utilization of feedstock such as wheat, corn, sugarcane, etc. for biofuel production (first generation biofuel) causes the problem of food security. The use of biomass feedstock (second generation) in bioethanol production solves this matter in many aspects [100]. Depending on fermentation conditions such as temperature, pH, aeration, and nutrient supplementation microorganisms are susceptible to lignocellulosic hydrolysate to produce bio ethanol [101]. Nevertheless, the production of biofuel through fermentation of promising sources (rice straw, wheat straw, corn straw, and sugarcane bagasse) is quite interesting but still meets some technical issues to release the fermentable sugars from lignocellulosic. The problem necessitates a pretreatment process including Physical (mechanical, extrusion, Irradiation), chemical (organosolv, ozonolysis, ionic liquid, acid, and alkali washing) physicochemical, and biological [102].

Several fermentation technologies such as batch and continues and fed-batch modes have been utilized. The complete subtract and highest conversion rate but lower volumetric production can be done through batch mode rather than continuous mode which led to high productivity (due to high dilution ratio and long duration process) and steady residual concentration [103]. Overly, batch fermentation could be applied for high viscous feedstock, while continuous fermentation methods can offer better plant capacity utilization [104]. During the batch and continuum operation, the addition of Indigenous Consortium Streptococcus sp. or enzyme glucoamylase has been reported that helps to fermentation process [105, 106].

4. Extraction

In addition to the thermochemical and biochemical process, the extraction method indeed is applied to oil from oil seeds or nuts materials such as hazelnut, almond nut, sesame seed, sunflower seed, or rapeseed. Traditionally, the oil can be extracted through cold pressing, hot pressing, or solvent extraction methods where the pressing is a mechanical method while solvent extraction is a chemical method [107, 108]. The pressing technique (solvent-free) is traditionally applied to extract edible oil from various sources such as nut or seed samples. Before the extraction sample preparation through various pretreatments on the sample is required before extraction in order to enhance the extraction efficiency [109]. The objectives of the pretreatment process are to destroy or soften the cellular structure of the sample and reduce the moisture content, which can increase the efficiency of the later extraction stage by destroying or softening the cellular structure of the sample and reduction of the seed moisture content [110]. The process is normally continued with solvent extraction (use solvent polar or non-polar) process due to the significant amount of oil remaining in the press cake, which is around 20–30%100. However new techniques such as microwave-assisted extraction [111], supercritical fluid extraction [112], ultrasound-assisted extraction can be applied in order to extract separate desired oil liquid from a solid–liquid sample [113].

5. Technology readiness level

It must be noticed that each mentioned process of bioenergy production is placed in a certain level of technical maturity as briefly demonstrated in the Figure 3 [114, 115, 116]. The maturity level of each technic is represented by a term which is called technical readiness level (TRL) and it is divided from lab scale (1–3), pilot-scale (4–6) to the highest level of maturity which is proven, tested, and qualified all parameters with a full commercial plant and industrialized scale (6–9) to produce products for public usage [114].

Figure 3.

General technical readiness level of each conversion process of biomass to bioenergy.

Acknowledgments

I would like to acknowledge and give my warmest thanks to my supporters at the University of Tehran who made this work possible.

Conflict of interest

The authors declare no conflict of interest.