alexa Conversion of Biomass and Waste to Value-add Products: Challenges and Opportunities

E-ISSN: 2252-5211

International Journal of Waste Resources

Reach Us +44-1753390542

Conversion of Biomass and Waste to Value-add Products: Challenges and Opportunities

Sherif Elshokary1*, Sherif Faraga2,3, Osayed Abuelyazeed1, Bitu Hurisso4 and Mostafa Ismail1
1El-Mattaria, Helwan University, Cairo, Egypt
2Ecole Polytechnique Montreal, P.O. Box 6079, Station Centre-ville, Montreal, QC, H3C 3A7, Canada
3RMTech for Environmental Solutions Inc., Canada
4Chemistry Department, Atlantic Centre for Green Chemistry, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
*Corresponding Author: Sherif Elshokary, Faculty of Engineering at El-Mattaria, Helwan University, Cairo, Egypt, Tel: +201020002590, Email: [email protected]

Received Date: Oct 23, 2018 / Accepted Date: Nov 08, 2018 / Published Date: Nov 15, 2018

Abstract

The significant decrease of the clean electricity cost over the last few years makes new heating techniques such as microwave; plasma, induction heating, and ultrasound are the central heating mechanism of several future industrial processes. This strategy leads to achieve two primary goals, decrease the adverse environmental impacts from the traditional heating techniques such as combustion of hydrocarbons, and take advantage of such technologies on product quality and yield, energy consumption, process efficient and other aspects. This work reviews the current investigations of the conversion of non-food biomass and non-recyclable waste to commercially viable products using non-conventional heating techniques. Physicochemical, biochemical, and thermochemical conversion techniques are debated, and the last is demonstrated in further details.

Keywords: Biomass; Wastes; Greenhouse impacts; Conversion technologies; Heating techniques

Introduction

Due to the scarcity of sustainable oil resources and the massive demand for global energy, tremendous effort by scientists and engineers have been made to look for sustainable alternative resources for the production of energy and chemicals. However, there are several questions still unanswered about greenhouse emissions impacts and developing renewable alternative energy resources, among other aspects. Scientific researchers have demonstrated that burning of fossil fuels is the primary source of greenhouse emissions that trap heat and make the planet much warmer.

Over the last 150 years, human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere [1].

Figure 1 illustrates the primary sources of greenhouse gas emissions in the United States, which is from burning fossil fuels for the heat, transportation, and electricity production purposes.

international-journal-waste-resources-emissions

Figure 1: Total greenhouse gas emissions in 2015-case study US [2].

Since the mid-19th century, the global dependence on fossil fuels has released more than 1100 Gt CO2 into the atmosphere. According to the IEA published Energy perspectives (2006), approximately 70% of total emissions to the atmosphere (CO2, CH4 and NOx) are mainly as a result of fossil fuel combustion for heat supply, electricity generation and transportation sector [3,4]. Also, the combustion of fossil fuel emits a lot of the life-threatening gases including hydrocarbons, sulfur oxides, nitrogen oxides, and carbon monoxide. Cars and trucks are regarded as the primary sources of emissions of carbon monoxide even though catalytic converter equipped muffler (exhaust system) have reduced these emissions significantly. Coal releases the most significant amount of carbon dioxide compared to other fossil fuels where it contributes 83% of the greenhouse emissions from the sector of electric power stations [5,6].

The primary objective of this review article is to discuss the up-todate technologies employed to produce a value-added product from the low-value feedstock, which would lead to enhancing the economy as well as decrease the negative environmental impacts.

Greenhouse Effect

Carbon dioxide

CO2 represents 82% of the total emission of greenhouse gases and enters the atmosphere on account of burning fossil fuels, solid waste, and from some chemical reactions as well. Global atmospheric concentrations of CO2 have grown up to 44 percent mainly because of the combustion of fossil fuels [7]. Over the last decade, the earth’s surface temperature has increased by 0.75°C as a result of the heat trapped due to the greenhouse gases [8]. Since the industrial revolution, the rise of the carbon dioxide levels has led to 30% increase in the water acidity of oceans where the ocean absorbs around 25% of emitted carbon dioxide which forms carbonic acid due to reaction with seawater [9-11]. Consequently, the acidification of the oceans grows by increasing the level of carbon dioxide in the atmosphere, which impacts the marine life.

Methane

CH4 is ejected into the atmosphere by human activities such as the production and distribution of natural gas and petroleum oil, transport of coal, a by-product of coal mining and incomplete combustion of fossil fuel (IPCC 2007). Methane is also produced through anaerobic biodegradation of organic substances from agricultural practices, animal wastes, and landfills. Since the mid-18th century, the level of CH4 in the atmosphere has raised about 162 percent. Meanwhile, in the early 21st century, the increasing rate of CH4 level decreased to near zero and had recently increased again to about 5 ppb/year (IPCC 2007).

Sulfur oxides

SOx are released during the combustion process of sulfur-containing fuels. The volume of SO3 of 1 ppm of air is enough to cause coughing and choking. Sulfur trioxide dissolves in water, raindrops, and moisture from the atmosphere forming sulfuric acid. Sulfuric acid can corrode or deteriorate many types of materials. Furthermore, the respiratory system can be harmed due to the inhalation process of sulfuric acid droplets.

Nitrogen oxides

Nitrogen oxides are produced during the combustion process when nitrogen molecules in the air chemically reacted with the fuel molecules and oxidized. NOx also results of lightning, microbial activities of soil, and biomass burning. The transportation sector is responsible for more than 50 percent of nitrogen oxides emissions. Burning of solid waste, agricultural and industrial processes also expellees NOx. Since the mid-18th century, the level of NOx in the atmosphere has heightened around 21 percent and, now it is continuing to grow. NOx has more impact than CO2 at trapping heat in the atmosphere, which is estimated to be 300 times worst. Nitrous oxide has a considerable negative effect on the ozone layer and is considered the more powerful substance that causes its degradation [7,11,12]. At high concentrations of nitrogen dioxide, it is a noxious gas that can cause death. Also, nitrogen oxides can also react with water and oxygen making nitric acid, which is one of the strongest acids and makes huge corrosion issues for materials. Both Sulfur oxides and nitrogen oxides react with water vapor found in the air to form acid rain, which dramatically impacts the plants, soil, and the health of the living organism. It is, also, responsible for damage to metallic buildings [13,14].

There has been a growing interest in biomass to fulfill the massive demand for oil and environmental protection where biomass is clean, abundant and alternative available renewable energy resource as well as carbon neutral. CO2 released from biomass combustion can be captured through the photosynthesis process; therefore, the utilization of biomass can lessen global warming [15,16]. The International Energy Agency (IEA) expects that the bioenergy will provide about 10% of the world's principal energy supply by 2035, and biofuels can substitute about 27% of world transportation fuel by 2050 [17].

Types of Biomass

Biomass is made out of three structural arrangements of natural polymeric materials: cellulose (~50 %), hemicellulose (~20-40%) and lignin (~20-40%), all are on the dry basis [18,19]. The most vital strategies to convert biomass into fluid fuel are biochemical transformations, which are employed by the enzymatic process; and thermochemical processes, which utilizes heating and in some case a catalyst. The biochemical change requires a feedstock that contains sugars or starches and water content in an overabundance of 40% in the raw materials. The thermochemical techniques are most appropriate for dry biomass (dampness content <10%) which is rich in lignin, for example, wood and agricultural wastes.

In general types of biomass have been classified under the following categories [20-22]:

Agricultural residues: Remaining of cultivated nutritional and industrial crops consisting of dry lignocellulosic wastes and livestock waste such as liquid and solid manure.

Energy crops: High yield crops cultivated explicitly for energy purposes, namely dry lignocellulosic woody energy crops, oil energy crops, dry lignocellulosic energy crops, and starch energy crops.

Forestry residues: Residuals of the forest or wood processing plants, such as bark, wood blocks, wood chips and log residues.

Industrial residues: Industrial and manufacturing residues, undesired by-products or secondary products, including wood, food and petrochemical residues.

Biomass Conversion Technologies

Massiveness, low energy density, and awkward type of biomass are the key hindrances the fast switch from fossil to biomass fuels. Not at all like gas or oil, biomass cannot be managed, stored, or transported effortlessly. This comes up with a principal motivation for the conversion of biomass into oil and gaseous fuels, which are extra energy dense and can be managed and stored more efficiently. This conversion can be accomplished through one of the two main courses: “Thermochemical” and “Biochemical” conversion. The burden of bulkiness and other defects of biomass are brought under control to a certain degree using the production of more suitable cleaner fuel via torrefaction and carbonization [23]. Figure 2 illustrated the different options of conversion of biomass into fuel and chemicals.

international-journal-waste-resources-biomass

Figure 2: Different options for conversion of biomass into fuel gasses or chemicals.

Physicochemical conversion

Physicochemical process converts biomass into bio-oil as an endproduct through two main methods: mechanical extraction and transesterification. Extraction is a mechanical change process in which oil is created from the seeds of different biomass products such as groundnuts, cotton, and so forth. The products of this step are oil and lingering strong, which is reasonable for creature grain. Transesterification is a chemical transformation through which unsaturated fats and oils are reacted with short-chain alcohols to produce biofuel. This procedure lessens the thickness of the unsaturated fats and makes them burnable.

Biochemical conversion

Biochemical conversion is a process in which biomass molecules are transformed into smaller molecules by microorganisms or catalysts without requiring much extra energy. This procedure is much slower than the thermochemical change process, however, does not need much outer energy. The three principal ways for biochemical transformation are absorption (anaerobic and aerobic), fermentation and enzymatic/acidic hydrolysis. The fundamental results of anaerobic absorption/assimilation are methane-rich gas and carbon dioxide in accession to a solid residue. The byproduct from anaerobic digestion can serve as fertilizer for agriculture. The product of fermentation is liquid mainly ethanol. Lignocellulosic feedstock requires hydrolysis pretreatment (acid, enzymatic, or hydrothermal) to break down the cellulose and hemicellulose into simple sugars needed by the yeast and bacteria for the fermentation process [23-25].

Anaerobic digestion is an organic procedure, which produces biogas. It is performed on the waste (T=30-35°C or 50-55°C) by two kinds of microorganisms and it includes two stages: (i) breakdown of complex organics in the loss by corrosive framing microscopic organisms into easier mixes, including volatile acids (e.g., propionic acid); and (ii) the change of these acids by methane creating microscopic organisms into CO2 and CH4 called "biogas". Commonly the two stages are performed in a solitary tank, and biogas contains for the most part CH4 (∼60%), CO2 (∼35%) and a blend of H2, N2, NH3, CO and H2S (∼5%). The heat value of biogas is around 22 MJ/m3 for a blend of CH4, CO2, and inserts with a composition ratio of 60, 35, and 5 percent, respectively. The study of digester-based energy conversion systems containing high moisture and high ash animal biomass (gathered with soil), have mostly handled with capturing biogas from biological systems like anaerobic digesters. The level of CH4 might be sensibly anticipated utilizing atom conservation equations for the reaction between digestible solids and H2O [26,27]:

CH1.98O0.83N0.086S0.0084(s)+0.09H2O(l)→0.54CH4(g)+0.46CO2(g) +N0.086S0.0084(s)

Even though almost no water is expended (0.09 kmole of H2O for each extraordinary carbon atom in fuel, the microorganisms can survive just in weaken slurry of water and absorbable or volatile solids (VS). Component protection yields 54% CH4 and 46% CO2. One ton of fluid compost with 5% dry matter creates around 20 m3 biogas. The digester productivity is characterized as the proportion of volatile solids changed over into gas to VS nourished in. Fermentation is the process that the biomass can likewise be aged to yield fluid alcohol fuels. It is expected that half biomass is cellulose and all cellulose is changed over into fermentable glucose, at that point with reaction:

C6H12O6(s)+3H2O (l)=3C2H5OH (l)+3O2 (g) Or 1 kg cellulose + 0.3 kg of H2O=0.77 kg ethanol + 0.53 kgO2;

Ethanol yield per ton biomass is 125 gallons for each ton of biomass which is near the announced hypothetical estimation of 124 gallons/dry ton of corn grain. With 10,000 m2 and solar irradiation of 1200W/m2 and photosynthesis effectiveness of 6%, the biomass yield is 1460 tons for every year per hectare expecting a heat value of 2,800,000 kJ/kmole, and with a sub-atomic weight, M=180 kg/kmole. Comparing it with switchgrass whose production is (37 tons/hectare/year), the highest ethanol yield is (48.1 m3/hectare/year) with a heating value (HV) of 7.37 kJ/m3. With a higher heating value (HHV) of gasoline around 32.9 kJ/m3, considering that one liter of ethanol is equivalent to 0.22 liter of gasoline [28].

Thermochemical conversion

Thermochemical conversion is the process that converts different kinds of biomass into gases, which are then blended into the converted chemicals or utilized specifically for thermal decomposition process as an energy source. Production of thermal energy is the primary driver for this transformation course that has five wide pathways, ignition, carbonization/torrefaction, pyrolysis, gasification, and liquefaction. Ignition includes high-temperature exothermic oxidation to hot pipe gas. Carbonization is a process for the production of charcoal at a temperature range (500-900°C) in an oxygen-free atmosphere. Torrefaction is a related process where biomass is heated to a lower temperature range of 200-300°C without or little contact with oxygen. Gasification includes chemical reactions in an oxygen-inadequate condition delivering item gases with heating values. Pyrolysis includes fast heating in the aggregate absence of oxygen. In liquefaction, the large molecules of solid biomass are decomposed into fluids having smaller molecules [23].

Hydrothermal

Hydrothermal processing is an attractive process for high water contents biomass to get valuable and energy-dense products. It includes liquefaction and gasification in a water-rich media happening over a scope of temperature and pressure conditions. Hydrothermal liquefaction (at 280-380°C, 70-300 bar for 10-60 min) converts biomass feedstock into crude bio-oil while hydrothermal gasification (at ~350°C and 21 MPa with a metal catalyst) can also be integrated with hydrothermal liquefaction to convert residual carbon to high energy methane gas [29].

Combustion

Combustion is the burning process of solid biomass in an abundance of air to deliver heat where it is utilized to change over the synthetic energy scattered in biomass into heat energy. The heat discharged is directly the most significant wellspring of human vitality utilization, representing over 90% of the energy from biomass. Heat and electricity are two principal forms of energy derived from biomass. Heat and power are two central types of energy obtained from biomass through various processes and equipment, for example, heaters, stoves, steam turbines, boilers, and so forth [23,24,30].

A biomass molecule that enters a hot combustion chamber is quickly heated from the outer layer to its inside center. Heat energy is transferred from the heating source to the external molecule layer through radiation convection from flue gases and conduction from the hot biomass bed while conductive heat exchange brings heat inside the molecule. The temperature increments unexpectedly occur in the external layer yet gradually distribute towards the center of the molecule, and consequently, moistness dissipation starts in the outer layer and continue towards within with a vanishing front which is considered to happen expectedly when the layer achieves 105°C. Water vanishes and its development breaks the particles creating miniaturized scale and mesopores through which steam is launched out.

The dried layers increment facilitate their temperature , yet can't burn because oxygen does not achieve the inward layers, inevitably hemicellulose first and cellulose-lignin after begin to disintegrate thermally. Long polymeric chains are split into smaller ones which vaporize or wind up lasting gases leaving the particles through similar ways took after by steam. This mixture of syngas and tars constitute the volatile substance of the biomass and when launched out into the combustion chamber reacts with oxygen delivering flaring combustion. Volatile extraction and combustion proceeds while the pyrolysis front moves towards the inward center of the particles, the same as the vanishing front did beforehand, leaving a charred layer outwardly which consumes when in contact with oxygen. Char combustion does not deliver a fire, and it is especially moderate since oxidation happens just in the solid-gas limit layer leaving a layer of protecting fiery debris which will, in the end, be expelled by mechanical activities of the flue gases joined with gravity to enable oxygen to assault another fresh layer of char. This arrangement of events which happen consistently amid the combustion of solid fuel, for example, biomass, relies upon the temperature came to by a specific region of the molecule and on the presentation to oxygen, and are outlined in Figure 3 [27].

international-journal-waste-resources-combustion

Figure 3: Schematization of the combustion of solid biomass particle: (a) heating and drying; (b) devolatization; (c) combustion [27].

Gasification

Gasification is a thermo-chemical process conducted under high temperature (590-800°C), pressure (in several cases) and gaseous mediums include air, oxygen, subcritical steam, or a mixture of thereof. Biomass feedstock is firstly volatized with restricted oxygen required for complete combustion to deliver gases and oils. The volatile materials are subsequently converted to versatile and energy efficient gases rich in hydrogen, carbon monoxide, methane, and carbon dioxide as far as union gas. These gasses would then nourish under a joined cycle gas turbine energy plant to produce power. Gasification displays higher efficiencies that regulate combustion, typically extending starting with 38 to 45 percent electric effectiveness and 84 to 92% for a cogeneration framework. Contaminated compounds with high mineral and hard matter are accumulated in the bottom of the gasifier vessel and then sent to disposal units [31]. Recently, gasification from fossil fuel is producing a more significant amount of gases over that of non-fossil fuels like biomass. The manufactured gasses will increase the heating value of the fuel by dismissing noncombustible parts in nitrogen [23,32].

The primary applications of the gasification products are presented in Figure 4.

international-journal-waste-resources-gasification

Figure 4: Applications of the main gasification products (reproduced from [33]).

The two principal components of synthesis gas, carbon monoxide, and hydrogen, are the building item of the hydrocarbon chemistry. The scope of items promptly possible from blend gas reaches out from bulk chemicals like ammonia (NH3), methanol (CH3OH) and Fischer- Tropsch products, through industrial gases to utilities such as clean fuel gas and electricity, as demonstrated in Figure 4. A considerable lot of these primary substances are just intermediates towards different products nearer to the purchaser showcase such as acetic acid derivations and polyurethanes. Synthesis gas is a moderate that can be created by gasification from an extensive variety of feedstock and can be transformed into a similarly extensive variety of products [33].

Pyrolysis

Pyrolysis takes place in the total absence of oxygen and thermally decomposes biomass into three main products; solid (biochar), liquid (bio-oil) and non-condensable gases by rapidly heating biomass above 300-400°C. Pyrolysis can be categorized into fast, intermediate, and slow processes based on the heating rate of the feedstock. Fast pyrolysis produces mainly bio-oil, whereas slow pyrolysis produces some gas and solid. Figure 5 stated the general products of pyrolysis.

international-journal-waste-resources-pyrolysis

Figure 5: General pyrolysis products.

Fast pyrolysis is the process in which the feedstock is instantaneously heated to the pyrolysis temperature. The greatest yield of fluid can be accomplished under particular conditions, including the high heating rate, short residence time and high temperature [32,33]. Higher temperatures and extended residence time can cause thermal breaking of hydrocarbon mixes and diminishing bio-oil yield. Pyrolysis at low temperatures (ordinarily below 300°C), is utilized to overhaul its fuel quality [34]. The higher heating rate will build the yield of the fluid oil which is more with enhanced H/C proportion and diminished O/C proportion. This relation between the conversion of liquid oil and the heating rate is not linear, so the very high heating rate may decrease the yield of liquid oil [34]. The main applications of the fast pyrolysis products are presented in Figure 6.

international-journal-waste-resources-applications

Figure 6: Common applications of the fast pyrolysis products.

Slow pyrolysis is a traditional process at a slow heating rate about 0.1-1°C s-1 that leads to a higher yield of solid material (biochar) and low yield of both oil and non-condensable gases under a long residence time [35]. Slow pyrolysis with relatively lower reaction temperatures as well as longer residence times would produce fairly equal amounts of liquid, solid char, and gas products. Therefore, to increase the charcoal yield, low temperature and low heating rates are necessary, whereas if the liquid is the desired product, a combination of moderate temperature, short residence time, in addition to a high heating rate are required [36].

Flash pyrolysis is the fast pyrolysis with a faster heating rate about >1000°C s-1 under a very little reaction time. The flash pyrolysis provides a high yield of oil better than other processes, but it needs a very small particle size biomass compared with other processes [37]. Table 1 illustrates a comparison between the pyrolysis conditions and the product yields of different pyrolysis modes. Fast pyrolysis produces the maximum oil yield and minimum solid residue compared to the other techniques at almost the same average temperature. In fact, this aspect shows that the process meets the economics, product yields and quality requirements for the bio-oil production and, therefore, receives noticeable interest from industries [38,39].

Process Process parameters Liquid Solid Gas
Temperature °C Residence time Heating rate
Torrefaction 200-300 10-66 min Slow 0 - 5% ~ 80% ~ 20%
Slow Pyrolysis 400-500 hrs-days Slow 25- 30% 25- 35% 30- 40 %
Intermediate pyrolysis 450-500 5-30 s Medium 40- 50% 25-30 % ~ 25%
Fast pyrolysis 400-650 1-2 s High 60-75% 12- 20% 13-20 %
Flash pyrolysis 350°C-850°C ~1 s Very high ~77% ~15% ~12%

Table 1: Applied conditions and product distribution of different pyrolysis modes (wt. %) [28,37,40-43].

Impacts of the Operating Parameters on the Pyrolysis Products

The operating parameters of a pyrolysis process influencing composition and yield of volatiles are classified as:

Extremely influencing parameters (temperature and biomass types)

Temperature is considered the most significant influencing parameter in pyrolysis of biomass. Low and very higher temperatures lead to the creation of char and gases respectively, but the intermediate pyrolysis temperatures (500-550°C) commonly maximize the oil yield. Final pyrolysis temperature can cause 10-20% variants in yield of oils. For loosely structured biomass this range tends to vary from (470-550°C) not (500-550°C). Lignin model compounds need higher temperature (550-650°C). Even at a very high temperature (800-900°C), the yield of lignin pyrolysis does not exceed 60% biomass conversion. Low temperature does not allow the secondary pyrolysis reactions to occur while at relatively high temperature the biomass after the primary pyrolysis reactions experiences the secondary pyrolysis reactions. In the secondary pyrolysis reactions, the biochar has been formed during primary reactions gets further decomposed into oil and non-condensable gases which lesser the char yield [44].

Impacts of the Operating Parameters on the Pyrolysis Products

The operating parameters of a pyrolysis process influencing composition and yield of volatiles are classified as:

Extremely influencing parameters (temperature and biomass types)

Temperature is considered the most significant influencing parameter in pyrolysis of biomass. Low and very higher temperatures lead to the creation of char and gases respectively, but the intermediate pyrolysis temperatures (500-550°C) commonly maximize the oil yield. Final pyrolysis temperature can cause 10-20% variants in yield of oils. For loosely structured biomass this range tends to vary from (470-550°C) not (500-550°C). Lignin model compounds need higher temperature (550-650°C). Even at a very high temperature (800-900°C), the yield of lignin pyrolysis does not exceed 60% biomass conversion. Low temperature does not allow the secondary pyrolysis reactions to occur while at relatively high temperature the biomass after the primary pyrolysis reactions experiences the secondary pyrolysis reactions. In the secondary pyrolysis reactions, the biochar has been formed during primary reactions gets further decomposed into oil and non-condensable gases which lesser the char yield [44].

Biomass types are considered a significant parameter impact on yield of pyrolysis oil. Generally, cellulose and hemicelluloses incline to emit more volatiles. Lignin is hard to fragment even at a higher temperature and is found in the most composition of char residue. Compositional variants in biomass are modifying composition and yield of pyrolysis oil.

Another principle for the selection of the biomass is the lignin and cellulose content existing in biomass. Thermal degradation of cellulose starts at low temperature; gradual decomposition and charring occurs while at high temperature quick volatilization takes place [45].

It proposes that at high temperatures cellulose leads to the formation of volatiles while low temperature it produces char. Lignin starts to decompose at higher temperatures as compared to cellulose. The rate of decomposition of lignin reduces at higher temperatures [46].

All this discussion recommends that for a developed char yield from the pyrolysis, the low moisture biomass with high lignin content should be selected. Among the various process parameters, Cellulose content existing in the biomass supports the formation of tar while high lignin content is favorable for the char production.

Moderately to low influencing parameters

Heating rate: Heat fundamentally delivers the required temperature to the biomass for the fragmentation of the several bonds in the biomass. Higher heating rate causes a fast heat transfer to flow through the biomass and results in a quick fragmentation of biomass into gaseous products. At the high heating rate, secondary reactions become dominant which causes high volatile yield while at low heating rate leads to the production of char [47].

Particle size: Different biomass particle size produces different product yield because the particle size controls the heating rate and temperature distribution inside the particle. Large particle size, the heat supplied to the outer surface of particle does not have the ability to pass through rapidly and then more time will be needed for the biomass to reach the pyrolysis temperature which means it has decelerated the heating rate while in small particles the heat transfer is much faster allowing a high heating rate. Large biomass particles lead to low biomass conversion and volatiles yield whereas low heat transfer rate in the larger particles provides, the more biochar production as compared to the small particle size biomass. Small biomass particles are preferable for uniform heat distribution [48,49].

Residence time: Residence time has an impact on both primary and secondary pyrolysis reaction during the pyrolysis process. Longer residence time concerning high temperature raises the affinity of the formed char towards the vapors which increases the char yield. Furthermore, it allows the recombination and polymerization resulting in higher char yield. The short residence times gives maximum oil while longer residence times lead to low liquid yields due to secondary reactions. On the other hand, liquid yield displays only a little drop for longer residence times [50,51].

Carrier gas: Carrier gas provides an inert environment for the pyrolysis process, and its flow rate impacts the pyrolysis product distribution. The carrier gas sweeps the vapors produced by the thermal cracking of the biomass. High carrier gas flow rate successfully lessens the vapor residence time that leads to more reduction in char yield helps to minimize secondary cracking and repolymerization of vapors. The very high flow rate is not required for high biochar yield because once an optimum value is achieved further flow rate increase does not affect the residence time significantly. Low or moderate flow rates of carrier gas are a suitable condition to obtain high liquids [52].

Moisture content: Initial moisture contents and mineral matter have negative effect parameters for energy inputs and volatiles yield, respectively. At high heating rates and moisture content biomass, the quality of pyrolysis oil is lesser compared to that achieved from low heating rates and dry High moisture content in biomass leads to the formation of tar which consequently reduces the char formation but low moisture content is appropriate for biochar production.

Furthermore, high moisture content in biomass increases the energy consumption during the pyrolysis process whereas most of the energy supplied to the biomass is consumed in taking out the moisture rather than in raising the temperature. Biomass has more than 30% moisture content is not suitable for pyrolysis [53].

A combination of rapid biomass heating rates, moderate pyrolysis temperature and short residence times maximizes the yield of liquid. Size of feed particles, vapor residence times, the rate of biomass heating, carrier gas flow rates, mineral matter contents, and initial moisture impact on yields and composition of volatiles.

Due to the above discussion and the major effect of pyrolysis temperature on the pyrolysis final product, the impact of heating mechanism used in pyrolysis process must be taken into account.

Impacts of the Heating Mechanism on the Pyrolysis Products

Over the last few decades, different technologies for providing the heat energy required to perform the thermal decomposition process has been developed, to fulfill the demands of the product quality and yields as well as to meet society’s needs to protect the environment and reduce energy consumption. Microwave-assisted thermochemical degradation of biomass is the most important technology that has been established in pyrolysis applications. Heating by exposure to electromagnetic irradiation can avoid most conventional heating (CH) issues and limitations [54,56], most importantly the temperature gradients inside and outside the heated material and the char layer formation that begins in conventional pyrolysis (CP), which is considered one of the significant problems in the field.

Also, the cost of renewable energy is now falling so fast that it should be a consistently cheaper source of electricity generation than traditional fossil fuels within just a few years. According to a new report published by the International Renewable Energy Agency (IRENA), the fossil fuel-fired electricity cost range in 2017 was estimated to from a low of USD 0.05 per kilowatt-hour (kWh) to a high USD 0.17/kWh, depending on the fuel type and the country. The global weighted average LCOE (Levelized Cost of Electricity) of new hydropower plants commissioned in 2017 was around USD 0.05 per kilowatt-hour (kWh), while for onshore wind plants it was around USD 0.06/kWh. For new bioenergy and geothermal projects, the global weighted average LCOE was around USD 0.07/kWh. Consequently, the utilization of clean electricity in the field of heating techniques is more efficient and less costly than that of fossil fuels.

Microwave heating

Microwaves are introduced as electromagnetic waves, situated amongst infrared and radio waves, of wavelengths from 1 mm to 1 m, comparing to frequencies between 300 MHz and 300 GHz. The industrial and residential microwave stoves operate under conditions range between 900 MHz and 2.45 GHz to avoid interference with telecommunications and radar transmissions. Microwave heating is one of the most necessary techniques for heating materials.

Microwave heating gives a number of advantages over conventional heating, including higher heating efficiency and power density, energy efficiency, efficient heat transfer, high speed startup, less process time and process control, more uniform heat distribution, and faster internal heating, making no pollution, which have made them a high demand technology in industrial and household applications. Microwave heating can reach high temperatures in a small amount of the time required for conventional heating, and the unique features of microwave heating can be utilized to enhance yields, to alter selectivity, and to perform reactions that don't happen by conventional heating. Maxwell's equations govern the distribution of electromagnetic (EM) energy in radio frequency (RF) and microwave (MW) heating systems with appropriate boundary conditions defined by the configuration of the systems and the interfaces between the treated materials and remaining space.

The dielectric properties of the materials such as dielectric constant (κ ‘ dielectric loss, κ) and Dielectric Permittivity as well as other properties of material like density (ρ, kg/m3), conductivity (k, W/m- °C), specific heat (Cp, J/kg-°C), heat transfer coefficient (h, W/m2-°C), initial temperature (T0, °C), surrounding temperature (Ta, °C), and radius of cylinder (R, m) are the leading property parameters of the Maxwell equations.

Therefore, these parameters fundamentally impact the proficiency of EM energy coupled into the materials, EM field distribution, and change of EM energy into thermal energy inside those materials. Microwave power consumption and temperature spreading inside any material are subjected to microwave beam that relies upon the electromagnetic field because of microwave power absorption. The following equations are Maxwell's equations that govern the distribution of electromagnetic (EM) energy microwave (MW) heating system.

image(1)

image (2)

image(3)

image (4)

image(5)

image(6)

Where E refers to the electric field, B referrers to the magnetic flux density, H is the magnetic field, J is the current density, D is the flux density and q the electric charge density, σ is the electrical conductivity, μ is the magnetic permeability and the electric permittivity [57-59].

Microwave heating is considered as a significant method in most of the biomass pyrolysis processes reported by various studies. Microwave pyrolysis of palm oil fiber for both biochar and hydrogen production has been found that the yield of H2 is more sensitive with the change of N2 flow rate, but the yield of biochar is more sensitive with the evolution of reaction temperature. The maximum H2 yield can be achieved at 700°C, the power of 508 W and N2 flow rate of 1200 cm3/ min. Biochar yield is most extreme at the temperature of 450°C power of 400 W and N2 flow rate of 200 cm3/min. However, both H2 and biochar yields together are optimized at 450°C, 400 W power and N2 flow rate of 955.25 cm3/min [51]. A particulate activated carbon, and mesoporous aluminosilicate bed of microwave heating offers a promising approach for the conversion of used frying oil (UFO) (left from frying chickens at temperature range 170 to 200°C for five days) into biofuel products with enhanced properties. The utilization of particulate and activated carbon as the reaction bed gave a high return of a biofuel (~73 wt. %), low nitrogen and sulfur content and free of carboxylic acids. It gives high calorific value (46 MJ/kg), low or zero outflows of NOx and SOx during the utilization of the fuel in ignition process in the combustion process.

Furthermore, the utilization of enacted carbon bed brings about the most remarkably energy recovery (88-90%) from the utilized broiling oil. Microwave pyrolysis process temperatures (350-550°C) assumed an essential part in the determination of the kinds of pyrolysis products through at 350°C the solid mass remained in the reactor demonstrating the most noteworthy yield (77-96 wt. %).

Moreover, at 400°C or more, the pyrolysis products were overwhelmed by biofuel (~73 wt. %) and smaller portions of pyrolysis gases (~35 wt. %). At 500°C or more, the yield of pyrolysis gases was found to be enhanced by the increase temperature. The microwaveheated reactor bed of activated charcoal demonstrates great guarantee as an enhanced pyrolysis way to deal with the conversion of UFO to deliver a biofuel product with attractive properties [52].

A consecutive two-advance quick microwave-assisted pyrolysis (fMAP) conditions (catalyst bed temperature and catalyst loading) were looking into utilizing a packed bed catalyst reactor with HZSM-5 zeolite as the catalyst. Pyrolysis at a temperature of 550°C produces the maximum bio-oil and aromatic hydrocarbons with the increment in the catalyst loading. These outcomes demonstrate the direct linear relationship between the bio-oil and aromatic hydrocarbon yield. The aromatic hydrocarbons proportion of the bio-oil was found to grow with increasing catalyst bed temperature and achieved its greatest value of 26 wt. % at 425°C. The yield of coke has grown up with an increment of catalyst to biomass ratio and lowered catalyst bed temperature. By raising pyrolysis temperature, the bio-oil yield enhanced up to 33 wt. % until 550°C and then drop with increasing pyrolysis temperature to 23 wt. % at 650°C while the yield of residual solid falls from 30 wt. % to 20 wt. % at the same temperature. Consequently, due to the increment of pyrolysis temperature, the yield of aromatic hydrocarbons has increased to 26 wt. % at 550°C [53].

Plasma

Plasma is a phase where a critical number of atoms and/or molecules are electrically, thermally or magnetically ionized. In general, plasma refers to the energized gaseous state comprising of particles, atoms, ions, metastable, and energized condition of these and also electrons, such that the concentration of positively charged particles is nearly the same as negatively charged species. Plasma is considered the fourth state of matter where it results in negatively charged electrons and positively charged ions that occurs when the gaseous atoms have separated into positive and negative charges by adding energy to gas, with the goal that some of its electrons leave its atoms. Physically, plasma can be partitioned into hot plasma and lowtemperature plasma. In low-temperature plasma, the electron temperature is 10 to 100 times greater than the gas temperature. On the other hand, due to the very low density and heat capacity of the electrons, the very high temperature of electrons does not signify that the plasma is hot. This means that although the electron temperature raises over several ten thousand K, the gas temperature remains at 100 K. Depending on the gas pressure, two unique types of electrical discharge in gases are known which are regularly alluded to as plasma treatment. Corona discharge is produced at gas pressures equivalent to or close to the atmospheric pressure with an electromagnetic field at high voltage ( >15 kV) and frequency (20-40 kHz) that extended for most useful applications. Moreover, glow discharge is created at gas pressures of 0.1-10 MPa, electromagnetic field in a lower voltage of 0.4-8.0 kV, and broad frequency range (0-2.45 GHz). Nature of gas used, flow rate, system pressure, discharge power, duration of treatment, aging of the plasma-treated surface, temperature change during plasma treatment and sample distance are the main factors affecting on plasma types and characteristics [54].

The plasma charged particles have a substantial capacity to react to electromagnetic fields. The conductivity of Lorentzian plasma, which is the basis for most industrial plasmas of interest, is described by equation (7) and given a unit (S/m).

image(7)

Where ‘e’ is the charge of an electron in coulombs, ‘ne’ is the electron number density per cubic meter, ‘m’ is the mass of the electron, and ν is the electron collision frequency per second [60]. Plasma is considered as an essential process of particular biomass pyrolysis process and investigated by many researchers. Heat-insulated gliding arc plasma reactor is utilized for onboard hydrogen generation with high energy proficiency with low cost through oxidative pyrolysis (OPR) conversion of methanol-utilizing air as an oxidant. Methanol transformation of 88% is accomplished in the warm plasma reactor under states of O2/C=0.3, S/C=0.5, specific energy input (SEI)=24 kJ/ mol, the energy efficiency of 74% and energy cost of 0.4 kWh/Nm3 where the O2/C ratio is the main factor affecting on this process. The type of plasma also has a significant impact on the percentage of alcohol conversion where microwave plasma gives 99% for ethanol and 97% for methanol; gliding arc plasma gives 65% for ethanol and 92% for methanol [61].

Thermal plasma pyrolysis deals with various kinds of polymer wastes (PVC, PE, PP, ABS) for producing hydrogen-rich gas which increases its concentration (75% H2 concentration in gaseous products) due to increasing the arc current and subsequently the arc temperature. Moreover, lack of CO in gaseous products and high purity carbon black nano-spheres in solid products are the main advantage of using plasma pyrolysis process. The oxygen-free thermal plasma pyrolysis has an excellent potential for development of some industrial waste disposal installation, not only to produce advanced materials, but also to remove the polymer wastes in an environmentally friendly way [62]. Three models combines xylose, hemicellulose, 4-O-MeGlcA (4-O-methyl glucuronoxylan) and Oacetyl xylose were investigated in thermal hydrogen plasma (a temperature of 2000-3500°C). The primary products of hemicellulose pyrolysis in hydrogen plasma are CO, H2, CO2 and C2H2 where hemicellulose and lignin produce CO and H2, but CO2 mainly comes from hemicelluloses, while higher lignin content gives higher C2H2 yield. Finally, the reaction mechanism of biomass pyrolysis in thermal hydrogen plasma was interpreted which showed that active hydrogen (H) in plasma assumed a crucial part in dehydrogenation rea ctions [58].

Thermal plasma pyrolysis converts asphaltene to expensive gas products (including acetylene 45 wt. %, hydrogen, methane, etc.) and carbon black. The process investigates the impact of reaction temperature (1300 K to 2000 K), plasma composition (H2, He, N2 and Ar) and particle size (25 μm to 200 μm) on heating rate as well as on devolatilization time of asphaltene particle. For 50 μm asphaltene particles and H2 thermal plasma, the results showed that the heating rate would surpass 107 K/s at 2000 K, and the time needed to finish the devolatilization process is 0.4 ms. The increase of H2 in Ar/H2 thermal plasma would strikingly enhance the heating rate as well as reduce the time of devolatilization. From the viewpoint of properties of heating exchange between the plasma phase and particles, H2 thermal plasma is better than He, followed by N2 and Ar. The devolatilization time increases sharply with increasing particle size. Consequently, the devolatilization time and the heating rate have a linear relationship at a certain final temperature [59].

Induction heating

Induction heating is a complicated technique of heat transfer, electromagnetic and metallurgical phenomena. Heat transfer and electromagnetics are firmly interlinked where the physical properties of heat-treated materials strongly depend upon magnetic field intensity as well as temperature. The metallurgical phenomenon is, in addition, a nonlinear function of temperature, heating intensity, chemical composition, and other factors. Both the electromagnetic and heat transfer phenomena are tightly coupled to the interconnected nature of the material properties such as specific heat, thermal conductivity, and electric resistivity that are functions of temperature and magnetic permeability which is, in turn, a function of magnetic field intensity, temperature, and frequency [63]. The dynamics of induction heating is described by Maxwell’s equations described in the equations above. An alternating current in a coil produces a time-varying magnetic field in its surroundings with equal frequency to the coil current. This magnetic field causes eddy currents on the surface of the work piece, located inside the coil. The resultant eddy currents are opposite in direction to the coil current, as due to the Lenz’s law. The eddy currents heat the conductor according to the Joule effect [64]. The inductionheating reactor is employed to assess the water content, elemental composition, chemical composition and energy content of the pyrolysis products of pine sawdust, cellulose, and lignin at different temperatures (500-700°C). The maximum oil yield for pine sawdust (55%), cellulose (44%) and lignin (22%) were achieved at 600°C, 500°C and 500°C respectively. From the pyrolysis of biomass, the water content of oil obtained from lignin (76%) was altogether higher than that of the pine sawdust and cellulose (69%). The reduction in char yield for cellulose, pinewood, and lignin were 4%, 15%, and 9% respectively because of lowering pyrolysis temperature from 500°C to 700°C.

Regarding gas yield, cellulose, pinewood, and lignin displayed an increase in the gas yields due to the pyrolysis temperature raised from 500°C to 700°C [65]. Induction heated fixed-bed reactor was used for producing valuable products from coconut shell, sugarcane bagasse, and rice straw through fast pyrolysis process. The yields of pyrolysis products and their chemical compositions were tested under particular conditions (pyrolysis temperature (400-800°C), heating rate (100-500°C/min) and holding time (1-8 min). The total yield of oil products sharply quantified as the pyrolysis temperature was gone up from 400 to 500°C (i.e. 15% versus 38% in rice straw; 23% versus 47% in sugarcane bagasse; 12% versus 36% in coconut shell) containing large amounts of water (73-92 wt. %). Generally, the larger yield of solid product was achieved at the higher heating rate, but on the other side, at the longer holding time, the lower yield of pyrolysis char product was observed. It was discovered that the yield of oil production from sugarcane bagasse was higher than those of coconut shell and rice straw. Employing the higher pyrolysis temperature of >500°C, the faster heating rate of >200°C/min, and longer holding time of >2 min., has resulted in the oil yield of about 50% [63]. Three sewage sludge from the fructose-manufacturing, milk-derivative and lager preparing manufacturing plants were reused as the supply materials in an induction heating pyrolysis system. The bio-oil products were obtained at a heating rate of 300°C/min and a pyrolysis temperature of 25-500°C. The obtained pyrolysis bio-oils had exceptionally complex blends of natural mixtures and contained a considerable amount of nitrogenated as well as oxygenated compounds, for example, aliphatic hydrocarbons, phenols, pyridines, pyrroles, amines, and ketones. These natural hydrocarbons containing nitrogen as well as oxygen ought to begin from the protein, and nucleic acid textures of the microbial living beings display in the sewage sludge. The non-condensable devolatilization parts were likewise made out of nitrogenated and oxygenated mixtures, however, contained little divisions of phenols, H-indoles, and fatty carboxylic acids. Then again, the products in the non-condensable gas items were mainly carbon dioxide, carbon monoxide and methane [66].

Solar heating

Solar energy is the energy source of solar heating and cooling systems. The solar heat gained could be transferred to solar heating or cooling applications which is so-called a solar thermal system [67]. Solar radiation is changed over into thermal energy in solar thermal power plants through a concentrator, then into electricity, as in fossil fuel-based plants. These sorts of plants are typically called Concentrated Solar Power plants (CSP). Thermo-chemical processes can be achieved with the support of solar energy either as direct thermal energy or using electricity generated from solar energy. Production of biofuels with the support of solar energy enhances the processes. Hydrogen production from water and other sources has been demonstrated as an alternative clean energy source. The thermal energy is utilized to accelerate the biological processes and thus reduce the waiting time or increase the production cycle. The net heat absorbed by the collector (QABS) can be expressed by the following equation:

image(8)

where α is the hemispherical absorptivity of the absorber, C is the geometrical concentration ratio of the collector, G is the direct normal irradiance [W/m2], ε is the hemispherical emissivity of the absorber, σ is the Stefan-Boltzmann constant [5.67e–08 W/m2K4], Tabs is the average temperature of the absorber and Tamb is the sky temperature or the temperature viewed by the absorber [68].

Under specific conditions (pyrolysis temperature (600- 2000°C), heating rate (10-50°C/s) and radiatively-induced fast pyrolysis, a 2D single-particle model is used not only to allow monitoring the evolution of gas and tar compositions but also to deal with extensive variety of feedstock’s and operating conditions using CFD tool ANSYSFLUENT and validated against experimental data obtained in a solar facility. Both the experimental and simulation results illustrated that an increase in the HR decreases tar yield and producing more gas. Additionally, the influence of HR on H2 distribution is significant as temperature increases, unlike CO where the heating rate influence is negligible for the temperature range analyzed. Although the kinetics adopted in this work is appropriate for both low and moderate HR as well as temperature levels, they can be connected for the case of fast pyrolysis of thermally thick particles with reasonable accuracy [69].

The pyrolysis products yield of beech wood was investigated at temperatures ranging from 600 to 2000°C and heating rates from 5 to 450°C/s through a lab-scale solar reactor. Firstly, the surface area and pore volume increased with temperature to the highest at 1200°C whereas decreased permanently at 2000°C. Char properties were impacted by solar pyrolysis temperature and heating rate where the degree of char carbonization has been enhanced with temperature increment and heating rate higher than 50°C/s [70].

The solar pyrolysis parameters such as temperature (600-2000°C), heating rate (5-50°C/s), argon flow rate (6-2 NL/min) and pressure (0.44-0.14 bar) of beech wood were investigated for raising the lower heating values (LHVs) of the gas products. The temperature dramatically affects the last product distribution and gas composition in solar pyrolysis while the pressure has the least possible impact on the product distribution. Higher CO and H2 yields occurred at the plateau temperature of 1200°C, a heating rate of 50°C/s and at atmospheric pressure which consequently leads to increase the total gas LHV dramatically. A great LHV value (10,376 ± 218 kJ/kg) of gas produced from solar pyrolysis of wood was found at 1200°C, 50°C/s, 12 NL/min and 0.85 bar operating conditions. This result attests that the calorific value of feedstock is upgraded during solar pyrolysis process when taking into consideration the energy contents of the other pyrolysis products (28% bio-oil and 10% biochar). Consequently, it can be stated that solar energy is stored in the pyrolysis products [69].

Upgrading the Pyrolysis Products

Due to the drawbacks of pyrolysis bio-oil, such as low heating value, high corrosiveness, high viscosity, and poor stability, upgrading of biooil before the practical application is necessary to acquire high-grade fuel [71]. Researches demonstrated that pyrolysis bio-oil could be upgraded using various techniques, such as hydrodeoxygenation, hydrogenation, catalytic cracking, catalytic pyrolysis, steam reforming, supercritical fluids, molecular distillation, esterification, and emulsification, etc. Conducting such research is very important to give information for the application of bio-oil from fast biomass pyrolysis in the world.

Moreover, the commercialization of bio-oil upgrading technology requires to be enhanced further.

The ways to upgrade the pyrolysis products are:

• The one-step hydrogenation–esterification (OHE) technique is much better than the traditional method due to the use of bifunctional catalysts. It is demonstrated the effectiveness of the bifunctional catalyst system for united hydrogenation/ esterification and a synergistic influence between metal sites and acid sites over respective catalysts. Furthermore, some procedures were occupied to mend the catalytic performance of the bifunctional catalyst. Noticeably, the new hydrogenation technique is much better than the traditional method due to the use of bifunctional catalysts. It is showed that the solid acid catalyst had a high catalytic activity to convert organic acids into esters effectively [71-76].

• Hydrodeoxygenation of bio-oil with high oxygen content have to be further promoted using amorphous catalysts, more novel, and economical catalysts. The catalyst activity could be further enhanced at appropriate conditions. Consequently, this new kind of amorphous catalyst will be a potential candidate for the HDO process due to its several advantages, such as sample preparation, high thermal stability and high HDO activity in addition to low cost [77,78].

• Catalytic pyrolysis can improve the production and quality of biooils by using the suitable catalysts while it faces some critical problems, such as reactor clogging, catalyst deactivation, coke production and high water content in bio-oils, etc. [79-82]. The integrated upgrading process of catalytic pyrolysis and catalytic cracking has the superiority of increasing the liquid yield and improving the fuel quality over the separate processes [83,84].

• Molecular distillation method is suitable for the separation of biooil and is not limited by its poor properties, but is energyconsuming generally. Researches demonstrated that the combined process had the superiority of improving the liquid yield and enhancing the fuel quality over the separate processes [85-87].

• Supercritical fluids (SCFs) are not an economically feasible technique to upgrade bio-oil on a large scale due to the high cost of organic solvents such as ethanol, methanol, water, and CO2. This technique takes complete advantage of the unique and superior properties of supercritical reaction media, such as liquid-like density, heat transfer and faster rates of mass, gas-like diffusivity, dissolving power and viscosity. SCFs can be not only used as a reaction condition to produce bio-oils, but also can be used as a superior medium to upgrade bio-oils, and have shown great potential for producing bio-oils with much higher caloric values and lower viscosity [88,89].

• Acidic catalysts such as NaCl, LiCl, ZnCl2, ZSM-5, ZnO, KCl and FeCl3.6H2O not only enhance the biochar formation but improve the characteristics of the pyrolysis product [90].

Conclusion

The key conclusions of this review paper include processing lowvalue renewable materials to high-value intermediate/end-products is a new approach to address the issues of both the energy and environment crisis. Thermochemical conversion, including pyrolysis, gasification, and combustion are promising techniques for converting biomass feedstock to energy and chemicals. Pyrolysis of biomass is the most promising technique for biomass processing for the production of fuels as it can deal with a considerable quantity of feedstock and other reasons. The high heating rate of feedstock along with short residence time of produced vapor result in fast pyrolysis process. To ensure the success of a pyrolysis plant, technical and economic aspects should first be evaluated and the effect of the critical factors on profitability is highly recommended to be well understood. Starting with a feedstock that has a low-value will dramatically decrease the minimum selling price of the end-product. This aspect, in addition to others, will lead to the pyrolysis product being in a competitive position among the traditional products.

Challenges and Recommendations

The techniques of biomass pyrolysis have been intensively explored, and there are still many challenges. To achieve breakthroughs and unravel the basic complexity of the biomass pyrolysis network, sustained efforts are recommended to an emphasis on the following issues.

• Enhancement of catalytic pyrolysis.

• To develop the catalytic performance with a greater selectivity of desirable products and lower coke generation, the improvement of an advanced catalyst system is still a significant issue in future research on biomass catalytic pyrolysis. Catalyst acidity/basicity, hydrothermal stability, porosity, and resistance to deactivation are a basic property that must be taken into thought. Meanwhile, any analysis is required to explore the elemental reaction mechanisms of reactants on active sites, and also the accumulation of inorganic minerals discharged from biomass on catalysts. Additional acceptable catalysts and dependable, steady and developed reactor systems ought to be developed within the future. Throughout the upgrading of bio-oil using varied catalysts, the mechanism on catalyst deactivation required to be any explained, and also the catalysts with high sturdiness, strong renewable ability, and high efficiency ought to be developed desperately.

• Extensive work is required to reveal the correlation between the biomass structure and also the pyrolysis reaction. The introduction of multiscale biomass fragments with some typical functional groups as model compounds are going to be of great help to comprehensively decouple the quality of the pyrolysis reaction network. The strategies for the directional synthesis of oligomers with specific linkages and functional groups are necessary to rise understand the evolution of the standard structures of biomass. to accumulate a lot of complete pyrolysis data, extraction strategies with minimal damage to the biomass elements got to be developed, and systematic management experiments supported the extracted elements are counseled.

• The researches on the way to mix pyrolysis reactors with reaction conditions organically and the way to efficiently use upgrading technologies of different oils for reference have to be compelled to be done in the long run so new ideas would be found.

• Fast and reliable qualitative and quantitative techniques to categorize pyrolysis-derived compounds. The present technologies (TG-FTIR and TG-MS) are supported TG analyzer and can't offer sufficient heating rates to simulate the important quick pyrolysis process. Restricted by the equipped analytical equipment, they can only characterize the evolution of a little fraction of pyrolysis products, and therefore the quantification is limited. SVUV-PIMS that was improved in recent years shows great promise to realize this goal. They are often accustomed to observe the vital intermediate products, such as free radicals, that this data is proscribed. However, large molecular organics debris from biomass pyrolysis continues to be a problem, because it is extremely simple to block the gas line of PIMS. Therefore in-situ measure instrument for large molecular intermediates and products is critical for biomass pyrolysis mechanism exploration.

• Determination of reliable and consistent macroscopic kinetic parameters and also the development of a comprehensive kinetic mechanism model for biomass pyrolysis. The computational simulation of biomass pyrolysis about heat and mass transfer needs lot of uniforms and a reliable solid kinetic model. However, because of the poor understanding of their chemical structures and reaction pathways, computational simulations of the pyrolysis of hemicellulose and lignin are still missing and need to be selfaddressed within the future. It is iimportant to create a comprehensive kinetic mechanism model by bridging large kinetics and microscopic kinetics, within which the part interactions and also the catalytic impacts of inorganic minerals also are regarded.

• Molecular simulation of the pyrolysis reaction to replicate the important biomass conversion method. There’s no satisfactory methodology to handle the pyrolysis of structural fragment compounds with over 1000 atoms or the catalytic reaction of medium-sized compounds on the catalyst surface. The molecular dynamics methodology has shown sensible performance once simulating the pyrolysis of cellulose that contains a large-scale periodic structure. The cluster model methodology was developed to simulate catalytic pyrolysis on the zeolite. Within the future, the advanced Quantum Mechanics/ Molecular Mechanics methodology is also potential for simulating the key reaction pathways throughout the pyrolysis of large-molecule compounds.

• Synchronization of biomass pyrolysis with the pretreatment of biomass feedstock and the post-processing of pyrolysis products. The optimization of several pretreatment techniques is recommended to be considered to improve biomass pyrolysis. To enhance the upgrading of pyrolysis products, pyrolysis parameters, and catalysis system is suggested being optimized synergistically for pyrolysis products application, containing the production of advanced liquid fuels or value-added chemicals.

• Throughout the researches on emulsification, looking for more economical and abundant surfactants as substitutes for the high priced surfactants remains an interesting topic to investigate

References

Citation: Elshokary S, Faraga S, Abuelyazeed O, Hurisso B, Ismail M (2018) Conversion of Biomass and Waste to Value-add Products: Challenges and Opportunities. Int J Waste Resour. 8: 360. DOI: 10.4172/2252-5211.1000360

Copyright: © 2018 Elshokary S, et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.

Select your language of interest to view the total content in your interested language

Post Your Comment Citation
Share This Article
Article Usage
  • Total views: 666
  • [From(publication date): 0-0 - Jan 17, 2019]
  • Breakdown by view type
  • HTML page views: 644
  • PDF downloads: 22

Post your comment

captcha   Reload  Can't read the image? click here to refresh