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Thermochemical processes for biofuels production from biomass
Sustainable Chemical Processes volume 1, Article number: 22 (2013)
The contribution of biomass to the world’s energy supply is presently estimated to be around 10% to 14%. The conversion of biomass to biofuels can be achieved primarily via biochemical and thermochemical processes. Recently, the use of thermochemical processes as pyrolysis and gasification has received great attention. The biomass composition and form of process conduction can affect greatly the efficiency of conversion for both gasification and pyrolysis. This review compiles recent thermochemical studies using several kinds of biomass to obtain biofuels and, additionally, it presents a brief description of main gasification and pyrolysis processes employed. Publications in Patent database also were reported and compiled.
Recently there has been a renewed interest in using biomass as an energy source due to the increasing demand in global energy coupled with environmental concerns of using fossil fuels. The contribution of biomass to the world’s energy supply is presently estimated to be around 10% to 14% . The conversion of biomass to biofuels can be achieved primarily via biochemical and thermochemical processes. The thermochemical processes can convert both food and nonfood biomass to fuel products via pyrolysis and gasification . Thermochemical gasification is a promising technology that can exploit the embedded energy in various types of biomass and convert to valuable products suitable for different industrial applications. Common feedstock for gasification includes agricultural crop residues, forest residues, energy crops, organic municipal wastes, and animal waste [2, 3].
Pyrolysis, like gasification, is an advanced thermal treatment that converts a material into a syngas but at lower temperatures and in the absence of oxygen. It is always also the first step in combustion and gasification processes where it is followed by total or partial oxidation of the primary products [4, 5]. Despite the calorific value of a gas derived from pyrolysis being higher than that of gasification, the volume of gas produced is usually much lower due to the lack of the oxygen carrier [6–8].
Both pyrolysis and gasification of biomass are complex processes and depends on several factors such as the composition of lignocellulosic material, heating rate and content of inorganic material etc. The amounts of cellulose, hemicellulose and lignin present in the biomass affect the pyrolysis and gasification, implying in great variation in the efficiency among different biomass and process employed. In this sense, the objective of this review is to compile the main biomass used in thermochemical studies. In addition, it will be presented a brief description of main gasification and pyrolysis processes employed as well as the main technologies cited in patent database.
In the gasification process, biomass is converted into a syngas by the partial oxidation of biomass at high temperatures . Gasification takes place at moderately high temperature and turns solid biomass into combustible gas mixtures (known as synthesis gas or syngas) through simultaneous occurrence of exothermic oxidation and endothermic pyrolysis under limited oxygen supply [2, 10]. The main components of this gas are CO, H2, CO2, CH4, H2O and N2. However a variety of tars are also produced during the gasification reaction [11, 12]. The resulting syngas can be burned to produce heat or synthesized to produce liquid transportation fuels [2, 13]. Figure 1 shows a schematic diagram whose illustrate the biomass gasification technology. Pyrolysis is the first step to occur the gasification.
For the synthesis of liquid fuels and other chemicals only a nitrogen-free syngas is suitable. Several studies on nitrogen conversion in different gasification processes are available in the literature. Fixed bed and fluidized bed gasification are more common. Different gasification agents can be applied, as air, oxygen or steam [14, 15].
The use of air as a gasifying agent is most common in industry but yields low heating value gas (4–7 MJ/Nm3) that is only suitable for heat and power applications. On the other hand, steam and oxygen can increase the heating value of syngas (10–18 MJ/Nm3) and the H2/CO ratio. A high H2/CO ratio is required for producing liquid fuels through Fischer–Tropsch synthesis and also benefits the production of H2 for use in fuel cells. However, high capital costs and complex system design have hindered the applications of steam and oxygen gasification at a large industrial scale [2, 16].
There are different gasification processes available in the literature to produce the syngas. The most important methods are described below and, the most significant studies referring gasification of biomass are compiled in Table 1.
In fluidized bed gasifiers, biomass particles are transformed into a fluid-like state through suspension in a gasifying agent, which offer the advantage of a uniform temperature distribution and better solid-gas contact and heat transfer rates. Compared with coal, biomass has lower particle density, which results in bubble coalescence within the bed and in turn a poor fluidization quality. Therefore, some inert particles (such as silica sand) are added as lubricants to facilitate the fluidization of biomass particles, or natural/artificial catalysts (such as dolomite, olivine, alkali-based catalyst, and metal-based catalyst). These particulates are introduced to improve fluidization quality and to reduce tars in the downstream process. Moreover, these gasifiers are usually equipped with cyclones to remove relatively fine particulates from the raw product gas. Depending on the fluidization pattern and combination character, these gasifiers can be further classified as bubbling fluidized bed, circulating fluidized bed, as showed in Figure 2, and double fluidized bed system. Fluidized bed gasifiers typically operate at temperatures of 800–1000°C to prevent ash from building up. Another advantage of this type of gasifier is that its high thermal inertia and vigorous mixing enables it to gasify different types of fuel, e.g. different types of biomass. This is therefore one of the preferred technologies for large-scale biomass gasification plants [10, 17, 22, 28, 29].
Dual fluidized bed gasification (DFB)
The DFB gasifier consists of two reactors, where gasification and combustion take place separately . Circulating bed material between these two reactors carries the heat from the combustion reactor to the gasification reactor . In the DFB gasifier, biomass is gasified with steam. Due to steam gasification, there is virtually no nitrogen in the producer gas and the hydrogen content amounts to about 40%. The average heating value is around 12–14 MJ/Nm3 (Nm3 = at 273.15 K and 101.325 Pa, referred to as dry gas). Producer gas from steam gasification is well suitable not only for heat and electricity production, but also for chemical synthesis .
Fixed bed gasification
The fluidized bed (FB) provides high mixing and reaction rates, accommodates variation in fuel quality and allows scaling-up of the process. Various concepts have been developed for gasification in FB . In fixed-bed gasifiers, the gas passes through the raw material while the gasifier zones are in “fixed” position where the reactions take place. Depending on the direction of gas flow, these gasifiers can be further classified as updraft, downdraft and cross-flow fixed beds showed in Figure 3.
Supercritical water gasification (SCWG)
This process has the potential to convert biomass with water contents up to 80% directly without the need for an energy-expensive drying step . An advantage of processing wet biomass hydrothermally, rather than drying it, is that doing so avoids the energy penalty associated with the phase change of the water. This leads to an improvement in economic performance compared with a conventional gasification process . These systems make use of the conditions of the critical point of water at 647.3 K and a pressure of 22.1 MPa as a favorable environment for wet biomass gasification reactions .
Plasma is considered to be the fourth state of matter, consisting of a mixture of electrons, ions and neutral particles, although overall it is electrically neutral. The degree of ionization of a plasma is the proportion of atoms that have lost (or gained) electrons. Plasma technology involves the creation of a sustained electrical arc by the passage of electric current through a gas in a process referred to as electrical breakdown. Because of the electrical resistivity across the system, significant heat is generated, which strips away electrons from the gas molecules resulting in an ionized gas stream, or plasma [15, 34]. Plasma gasification processes may reach temperatures from 2,000 to 30,000°C .
A general analysis of data compiled in Table 1 reveals that fluidized and fixed-bed reactors are more usual in gasification procedures. The variable more studied is temperature, where it is seen that at high temperature the energy production and the syngas quality increases, beyond that decrease the tars production. The particle size and the moisture have a significant influence in the gasification process, but if the particle size is reduced the syngas production increases and at high moisture contents the efficiency of gasification decrease.
Pyrolysis is an advanced thermal treatment that converts a material into a syngas at temperatures around 1000°C and in the absence of oxygen. Pyrolysis also can be described as the direct thermal decomposition of the organic matrix that could obtain solid, liquid and gas products [35–37]. Temperature is the most important factor for the product distribution of pyrolysis, most interesting range for the production of the pyrolysis products is between 625 and 775 K. The charcoal yield decreases as the temperature increases. Yield of products resulting from biomass pyrolysis can be maximized as follows: charcoal (a low temperature, low heating rate process), liquid products (a low temperature, high heating rate, short gas residence time process), and fuel gas (a high temperature, low heating rate, long gas residence time process) [8, 38, 39].
Lower process temperature and longer vapour residence times favour the production of charcoal. High temperature and longer residence time increase the biomass conversion to gas and moderate temperature and short vapour residence time are optimum for producing liquids [4, 40]. Short residence time pyrolysis of biomass at moderate temperatures has generally been used to obtain high yield of liquid products. For highly cellulosic biomass feedstocks, the liquid fraction usually contains acids, alcohols, aldehydes, ketones, esters, heterocyclic derivatives and phenolic compounds [35, 41, 42].
The pyrolysis process for fuels and chemicals could be divided in: catalytic, fast and flash. The difference between them are the process conditions which involves the solid residence times, heating rate, particle size and temperature. These can be used for a commercial production of a wide range of fuels and chemical from biomass feedstocks [38, 43, 44]. To produce these syngas four different methods are applied, which are discussed below. A compilation among of different types of biomass and pyrolysis processes is described in Table 2.
Biomass is pyrolysed at slow heating rates (5–7 K/min). This leads to less liquid and gaseous product and more of char production, at low temperature (675–775 K), and/or gas, at high temperature. Significant amount of work has been done on this process. The most used reactors in this process are fixed bed and tubular reactor .
Fast pyrolysis is a process in which a material, such as biomass, is rapidly heated to high temperatures in the absence of air (specifically oxygen). It involves fast heating of biomass but not as fast as flash pyrolysis. Heating rate is somewhere about 300°C/min. Generally, fast pyrolysis is used to obtain high-grade bio-oil. Fast pyrolysis is successful with different reactors configurations, some of them are fluidized-bed reactors, entrained flow reactor, wire mesh reactor, vacuum furnace reactor, vortex reactor, rotating reactor, circulating fluidized bed reactor. If the purpose was to maximize the yield of liquid products resulting from biomass pyrolysis, a low temperature, high heating rate, short gas residence time process would be required. If the purpose were to maximize the yield of fuel gas resulting from biomass pyrolysis, a high temperature, low heating rate, long gas residence time process would be preferred [9, 53–55].
Flash pyrolysis is the process in which the reaction time is of only several seconds or even less. The heating rate is very high. This requires special reactor configuration in which biomass residence times are only of few seconds. Two of appropriate designs are entrained flow reactor and the fluidized-bed reactor. Flash pyrolysis of any kind of biomass requires rapid heating and therefore the particle size should be fairly small. Major problem of the present reactors for flash pyrolysis are the quality and the stability of the produced oil, strongly affected by char/ash content of bio-oil. Besides the known problems concerning solid particles in the bio-oil, char fines will catalyze repolymerization reactions inside the oil resulting in a higher viscosity. Flash pyrolysis is of following types: flash hydro-pyrolysis is flash pyrolysis done in hydrogen atmosphere, it is carried out at a pressure up to 20Mpa; rapid thermal process is a particular heat transfer process with very short heat residence times (between 30 ms and 1.5 s). It is done at temperatures between 400 and 950 1C; rapid de-polymerization and cracking of feed stocks takes place; rapid heating eliminates the side reactions whereby giving products with comparable viscosity to diesel oil; solar flash pyrolysis concentrated solar radiation can be used to perform flash pyrolysis; vacuum flash pyrolysis the process is done under vacuum, the vacuum facilitates the removal of the condensable products from the hot reaction zone [56–58].
Catalytic biomass pyrolysis
From literature it was seen that liquids obtained from biomass by slow, flash or fast pyrolysis process, could not be directly used as transportation fuel. This oil needs to be upgraded as they have high oxygen and water content. These oils are also found to be less stable and less miscible in conventional fuels. Catalytic biomass pyrolysis is introduced to improve the quality of the oil produced [38, 59, 60].
Table 3 shows the latest technologies that involves the production of syngas and biofuels by gasification and pyrolysis processes from biomass. The gasification process by fluidized-bed reactor is the most common among registered technologies, as showed in Table 3. The registered patents, US 2012/0266531 A1  e US 2012/176611 A1 , use this process to make a pretreatment of biomass and to produce the syngas. Referring the technologies for pyrolysis, it is found registers for catalitic pyrolysis, fast pyrolysis e hidropyrolysis. Catalitic pyrolysis is the most used technology, where the registered patents WO 2011/064172 A1  used catalysts to make an oil, whereas WO 2012/012191 A1  used the effective pyrolysis of biomass to produce biofuels through fast thermal exchange.
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The authors thank CAPES for scholarships.
The authors declare that they have no competing interests.
All authors contributed equally in this work. All authors read and approved the final manuscript.