Category Archives: Biofuels and Bioenergy

Ethanol Fermentation and Feedstock

The ethanol fuel manufacturing process is a combination of biochemical and physical processes based on traditional unit operations. Ethanol is produced by fermentation of sugars with yeast. The fermentation crude product is con­centrated to fuel-grade ethanol via distillation. The organisms of primary interest to industrial fermentation of ethanol include Saccharomyces cerevisiae,

S. uvarum, Schizosaccharomyces pombe, and Kluyveromyces spp., among which Saccharomyces cerevisiae is most commonly utilized.

Feedstock for ethanol fermentation are either sugar or starch-containing crops. These "biomass fuel crops" (tubers and grains) typically include sugarbeets, sugarcane, potatoes, corn, wheat, barley, Jerusalem artichokes, and sweet sorghum. Sugar crops such as sugarcane, sugarbeets, or sweet sor­ghum are extracted to produce a sugar-containing solution or syrup that can be directly fermented by yeast. Starch feedstock, however, must go through an additional step that involves starch-to-sugar conversion, as is the case for grain ethanol. Needless to say, sugar crops are simpler to convert to ethanol than starch crops. Therefore, the ethanol production cost, excluding the feed­stock cost, is substantially lower for sugar crops than for starch crops.

Dilute Acid Prehydrolysis

Lower temperature operation with reduced sugar degradation is achieved by adding a small amount of mineral acid to the pretreatment process. The acid increases reaction rates at a given temperature and the ratio of hydroly­sis rate to the degradation rate is also increased.

A compromise between the reaction temperature and the reaction time exists for acid-catalyzed reactions. As for autohydrolysis, however, condi­tions explored range from several hours at 100°C to 10 seconds at 200°C with a sulfuric acid concentration of 0.5 to 4.0%. Acid catalysts have also been used in steam explosion systems with similar results. Xylose yields gener­ally range from 70 to 95%. However, sulfuric acid processes produce lignin that is more condensed (52% of the lignin extractable in dilute NaOH) than that produced by an autohydrolysis system. Sulfur dioxide has also been investigated as a catalyst to improve the efficiency of the pretreatments. Use of excess water increases energy consumption and decreases the concen­tration of xylose in the hydrolyzate, thus decreasing the concentration of ethanol that can be produced in the xylose fermentation step. In a study by Ojumu and Ogunkunle [51], production of glucose was achieved in batch reactors from hydrolysis of lignocellulose under extremely low acid (ELA) concentration and high-temperature condition by pretreating the saw­dust by autohydrolysis ab initio. The maximum glucose yield obtained was reported to be 70% for the pretreated sawdust at 210°C in the eighteenth minute of the experiment. This value is about 1.4 times the maximum glu­cose level obtained from the untreated sawdust under the nominally same condition [51].

The acid hydrolysis process has a long history of over 100 years. As an alternative to dilute acid hydrolysis, concentrated acid-based hydrolysis pro­cesses are also conceivable and available. However, these types of processes are generally more expensive to operate and render handling difficulties [52]. Sulfuric acid is the most common choice of catalyst; however, other mineral acids such as hydrochloric, nitric, and trifluoroacetic acids (CF3COOH) have also been used.

Fast Pyrolysis of Biomass

Pyrolysis of biomass is an important process option either as a pretreatment for gasification (i. e., the first stage of two-stage gasification) or as an indepen­dent process such as fast pyrolysis. The former is usually aimed at producing biosyngas, whereas the latter is intended to produce a liquid fuel product. Typical biomass pyrolysis takes place actively at around 500°C and produces a liquid product via fast cooling (shorter than two seconds) of volatile pyro­lytic products. The liquid product produced is called bio-oil or pyrolysis oil. Bio-oil is considered greenhouse gas neutral, because it only puts back into the atmosphere what was initially removed by the plant during its lifetime [50]. Bio-oil is nearly sulfur-free.

As can be seen from the prevailing pyrolysis temperature of ~500°C, pyrolysis of biomass as a process treatment is quite similar to oil shale pyrolysis [51] and coal pyrolysis [14], wherein hydrocarbon species are devolatilized and thermally cracked. Bio-oil production via biomass pyrolysis is typically carried out via flash-pyrolysis or fast pyrolysis. The biomass fast pyrolysis process [23, 52, 53] is a thermochemical conversion process that converts biomass feedstock into gaseous, solid, and liquid products via heating of biomass in the absence of oxygen or air. The prin­cipal product of typical fast pyrolysis of biomass is a liquid product of bio-oil. As the name of the process implies, the process is intended to take place very fast in a matter of a couple of seconds or shorter; that is, т > 2 s. As such, heat and mass transfer conditions in the reactor become crucially important in both design and operation of fast pyrolysis of biomass. A variety of reactor designs has been proposed and tested on pilot scales, and they include traditional fluidized bed reactors, circulating fluidized bed reactors, rotating cone reactors, vacuum reactors, ablative tubes, and more. Table 5.11 shows a partial list of operational pilot-scale biomass pyrolysis units.

A typical biomass fast pyrolysis process involves several stages of opera­tion including biomass feed drying, comminuting, fast pyrolysis, separation

TABLE 5.11

Pilot-scale Fast Pyrolysis of Biomass


Type of Reactor




Dynamotive Energy

Bubbling fluidized


Dynamotive Energy Systems,

Systems (Canada)


2011; Bain. 2004 [50, 54]

Union Fenosa (Spain)

Bubbling fluidized bed


Ringer, Putsche, and Scahill, 2006 [55]

Wellman Process Eng. Ltd. (UK)

Fluidized bed


Conversion and Resource Evolution (CARE), Ltd., 1998-2002 [56]

Resource Technology (RTI)

Fluidized bed


Scott, 1999 [57]

Red Arrow /Ensyn

Circulating fluid bed


Czernik and Bridgwater, 2004 [53]

VTT /Ensyn (Finland)

Circulating fluid bed


VTT Technical Research Centre of Finland, 2002 [58]

ENEL /Ensyn (Italy)

Circulating Transported bed

625 kg/h

Gradassi, 2002 [59]

BTG/KARA (The Netherlands)

Rotating cone

200 kg/h

Biomass Technology Group (BTG), 2011 [60]


Vacuum, stirred bed

3500 kg/h

Ray, 2000 [61]

Enervision (Norway)

Ablative tube

Bridgwater, 1999 [62]

Fortum Oy (Finland)


350 kg/h

U. S. Department of Energy — Energy Efficiency and Renewable Energy-Biomass Program, 2011 [63]

of char, and liquid recovery. Fast pyrolysis of biomass has several distinct merits as an alternative fuel process technology and they are as follows:

• The principal product of biomass fast pyrolysis is bio-oil, a liquid product. As such, storage and transportation of the product are easy.

• The process takes place very quickly in a matter of 0.5-2 seconds and as such the reactor residence time is very short.

• The process technology is very widely and universally applicable to a variety of biomass feedstock.

• The process chemistry is simple and straightforward.

• The process equipment needed is relatively simple and not complex.

• Due to the high reactor throughput and simple chemistry, the capital cost is not high.

• Small-scale process feasibility has been demonstrated. However, process economics for a small-scale power generation (<5 MWe) based on fast pyrolysis of biomass is substantially less favorable than that of a larger scale (>10 MWe) system.

However, the drawbacks of fast pyrolysis of biomass include the following:

• The moisture level of the feed biomass needs to be controlled below 10% or even lower. Otherwise, the feed water and reaction-produced water will end up in the final liquid oil product.

• The biomass feedstock needs to go through size reduction and preconditioning. The feed material has to be in particulate form in order to minimize the heat and mass transfer resistance; prefer­ably ~2 mm for bubbling bed and ~6 mm for circulating fluidized bed (CFB).

• The quality of bio-oil product is generally of poor quality. Bio-oil has a high oxygen content which makes the oil more corrosive and unstable, in addition to possessing a lower heating value. Bio-oil also contains metallic compounds and nitrogen species, which can foul and deactivate most of the fuel upgrading catalysts. The upgrading process requires a large amount of hydrogen and becomes costly.

• The process requires a very fast heating rate, which is costly in both operation and capital investment.

• The overall energy efficiency of the process is not high, by itself.

• Large-scale process operation may be subjected to significant logisti­cal burdens of feedstock collection, storage, and pretreatment.

The first biocrude in the United States was produced at a 30 kg/hr scale [64] at operating conditions of approximately 500°C and a residence time of one second, that is, at a typical fast pyrolysis condition. Since then, Canadian researchers have converted woody biomass into fuel via pyrolysis in a 200 kg/hr pilot plant [65]. The fuel oil substitute was produced (on 1,000 ton/day dry basis) at approximately $3.4/GJ, based on the 1990 fixed price (1990 U. S. dollars). At the time, the cost for light fuel oil was $4.0-4.6/GJ [66], thus indi­cating that the pyrolyzed biomass fuel was a more economical alternative. However, the skepticism of high transportation costs of the biomass to the pyrolyzer outweighing potential profits limited R&D funding for the follow­ing years. This was when the petroleum-based liquid fuel price was much lower than that in the twenty-first century. To circumvent this biomass trans­portation and logistical problem, the Energy Resources Company (ERCO) in Massachusetts developed a mobile pyrolysis prototype for the U. S. EPA.

ERCO’s unit was designed to accept biomass with 10% moisture content at a rate of 100 tons/day. At this rate, the system had a minimal net energy efficiency of 70% and produced gaseous, liquid, and char end products. The process, which was initially started using an outside fuel source, became completely self-sufficient shortly after its startup. This was achieved by implementing a cogeneration system to convert the pyrolysis gas into the electricity required for operation. A small fraction of the pyrolysis gas is also

Подпись: Oil and Gas 12,370 lb/hr 1022 lb/hr Char 146 lb/hr Ash


A schematic and material balance for ERCO’s Mobile Pyrolysis Unit.

used to dry the entering feedstock to the required 10% moisture. A simpli­fied version of ERCO’s mobile unit is shown in Figure 5.3 [67].

The end-products are pyrolysis oil and pyrolytic char, both of which are more economical to transport than the original biomass feedstock. The aver­age heating values for the pyrolysis oil and char are 10,000 BTU/lb and 12,000 BTU/lb, respectively [67]. The pyrolysis gas, which has a nominal heating value of 150 BTU/scf, is not considered an end product because it is directly used in the cogeneration system. If classified based on the coal syngas cri­terion, typical pyrolysis gas of biomass would be classified as a low-BTU gas whose usual criterion for the heating value is less than 300 BTU/scf [5]. The mobility, self-sufficiency, and profitability of the system lifted some of the hesitancy of funding research on the pyrolysis of biomass. In addition, ERCO’s success led to additional investigation of "dual," or cogeneration sys­tems, which produce both useful heat and electric power, that is, combined heat and power (CHP).

In a fluidized bed fast pyrolysis reactor, fine particles (2-3 mm size) of biomass are introduced into a bed fluidized by a gas, which is usually a recir­culated product gas [68]. High heat and mass transfer rates result in rapid heating of biomass particles via convective heat. Char attrition and bio-oil contamination may take place and char carbon could appear in the bio-oil product. Heat can be supplied externally to the bed or by a hot fluidizing gas. In most designs, reactor heat is usually provided by heat exchanger tubes through which hot gas from char combustion flows. The reactor effluent

leaves from the top of the reactor as noncondensable gas, char, bio-oil vapor, and aerosol. The fluid bed may contain, in addition to biomass particles, flu­idizing media such as hot sand particles for enhanced heat transfer. Aerosol is generally defined as a suspension of solid and liquid particles in a gas. The term aerosol includes both the particles and the suspending gas and the particle size may range from 0.002 to larger than 100 pm [49]. There is some dilution of the products due to the use of fluidizing gas, which makes it more difficult to condense and then separate the bio-oil vapor from the gas exiting the condensers. The general operational principle is quite simi­lar to that of the traditional gas-solid fluidized bed reactor [69]. The fluid­ized bed fast pyrolysis process has been pilot tested by several companies including Dynamotive [54], Wellman [56], and Agri-Therm [70], as listed in Table 5.11. Special versions of fluidized bed reactors popularly used in bio­mass fast pyrolysis are bubbling fluidized bed (BFB) and circulating fluid­ized bed (CFB) reactors. A principal difference between the two types is that the former has a freeboard space above the fluid bed (as shown in Figure 5.4), whereas the latter achieves a full entrainment of the particle-fluid mixture in the reactor.

Bubbling fluidized bed reactors have long been used in chemical and petro­leum processing. An earlier version of bubbling fluidized sand bed reactor was utilized by the WFPP (Waterloo Fast Pyrolysis Process) [71, 72]. Larger units based on the BFB are a 200 kg/hr system by Union Fenosa (Spain) and a
400 kg/hr system by DynaMotive (Canada). Both systems were based on the WFPP developed at the University of Waterloo (Canada) and designed by its spin-off company Resource Transforms International (RTI) in Canada [55]. Bubbling fluidized beds (BFBs) occur when the incoming carrier gas velocity is sufficiently above the minimum fluidization velocity to cause the forma­tion of bubblelike structures within the particulate bed. In such a condition, the bed appears more or less bubbling.

A variety of different designs has also been introduced. The particulate or granular bed in the BFB may be composed of biomass particles only without any inert media and all the process heat can be supplied by hot fluidizing gas. Alternately, the granular bed can contain a hot heat-transfer solid medium such as indirectly heated sand, which enhances heat trans­fer efficiency and generally allows for a larger throughput for the process. An earlier version of the DynaMotive system used natural gas to heat their pilot-scale reactor, but most of modern designs use the exothermic heat of char combustion to supply the necessary heat to the pyrolysis reactor. The BFB is, in principle, self-cleaning as the by-product char is carried out of the reactor by the product gases and oil vapors [55]. For this process feature, the density of char will have to be less than that of fluidizing media so that the by-product char will literally "float" on top of the bed. The allowable particle size range for this type of reactor is quite narrow and should be carefully managed. Furthermore, the bio-oil produced has a carbon contamination possibility, inasmuch as the oil vapor has to pass through the char-rich layer on its way out of the bed [55]. The gas flow rate for this reactor will have to be determined based on the dimensions of fluidizing media and the desired residence time of gas and oil vapor in the freeboard section of the reactor, which is above the bed. The residence time is generally between 0.5 and 2.0 seconds. The BFB is currently most popularly used for both fast pyrolysis and gasification processes of biomass. In order to achieve a short residence time for volatiles, a shallow bed depth, a high gas flow rate, or usually both are utilized [57, 73]. A high gas-to-biomass feed ratio is adopted for necessary fluidization and a short residence time, which in turn results in product dilu­tion and lowers the thermal efficiency of the process.

Circulating fluid bed reactors and derivative types of reactor design are frequently utilized for fast pyrolysis of biomass. The basic concept of CFB reactors involves efficient and rapid heat transfer in a convective mode and short residence times for both biomass particles and product vapors. Fine biomass particles are introduced into a circulating fluidized bed of hot sand. Hot sand and biomass particles move together with the transport gas which is usually a recirculated product gas. In reactor engineering, this transport gas is usually referred to as a fluidizing gas. In practice, the residence times for solid biomass particles are not uniform and only a little longer than the volatiles. Therefore, solid recycling of partially reacted feed would become necessary or very fine particle size of biomass would have to be used. Most CFB reactors are dilute phase units and their heat transfer rates are high,

Gas, Oil Vapors, and Aerosol



Circulating fluidized bed reactor for fast pyrolysis of biomass.

but not as high as particularly desired, because the mode of heat transfer is gas-solid convective heat transfer [57, 74]. If a twin-bed reactor system is used, that is, the first for fast pyrolysis and the second as a char combustor to reheat the circulating solids, as shown in Figure 5.5, there is a strong pos­sibility for ash carry-over to the pyrolysis reactor and ash buildup in circulat­ing solids [57].

The ash attrition and char carry-over problem could also be high and if not controlled properly, some level of contamination of bio-oil products is also possible. One of the main advantages of the CFB is the possibility to achieve a short and controllable residence time for char [75]. The alkaline compounds in biomass ash are known to possess a catalytic effect for crack­ing organic molecules contained in volatile vapors, thereby potentially low­ering the volatile bio-oil yield. Red Arrow and VTT processes are based on circulating fluid bed reactors.

A rotating cone fast pyrolysis system (Figure 5.6) operates based on the idea of intensive mixing between biomass particles and hot sand parti­cles, thereby providing good heat and mass transfer. This type of reac­tor requires very fine to fine biomass particle size. This process does not

image65Aerosols and

Hot Sand Oil VaPors



A schematic of rotating cone reactor for fast pyrolysis of biomass.

require any carrier gas for its operation and therefore the reactor size can be made compact. The BTG’s fast pyrolysis process developed by Biomass Technology Group (the Netherlands) is based on a modified rotating cone reactor [60].

In this reactor, efficient heat transfer between the hot sand particles and biomass particles is accomplished, and a good portion of the process heat is retained in the hot sand particles. A wide variety of different biomass feedstock can be processed in the pyrolysis process which is operated at 450- 600°C. Before entering the reactor, the biomass feedstock must be reduced in size to a particulate form finer than 6 mm, and its moisture content to below 10 wt%. Sufficient excess heat is normally available from the pyrolysis plant to dry the feed biomass from 40-50 wt% moisture to below 10 wt%. A sche­matic of rotating cone reactor is shown in Figure 5.6.

From the BTG process, up to 75 wt% pyrolysis oil and 25 wt% char and gas are produced as primary products [60]. Because no "inert" carrier gas is used in this process, no additional gas heating is required and the pyrolysis prod­ucts are undiluted vapor. This undiluted vapor flow allows the downstream equipment to be of a minimum size. In a condenser, the oil vapor product is rapidly cooled yielding the oil product and some permanent (noncondens­able) gases. In only a few seconds of process treatment, the biomass is trans­formed into pyrolysis oil. Biomass char and hot sand used in the reactor are recycled to a combustor, where char is combusted to reheat the sand. After reheating the sand by char combustion heat, the sand is recirculated back


to the reactor. The permanent gases (or noncondensable gas) can be utilized in a gas engine to generate electricity or simply flared off. In principle, no external utilities are required for operation of the process, that is, energy — wise self-sufficient. A schematic of a BTG process for fast flash of biomass is shown in Figure 5.7.

The vacuum pyrolysis process operates under low pressure (vacuum or atmospheric) and has principal process merits in its processability of larger biomass particles as well as its short residence time for volatiles. These bio­mass particles are taken out using a vacuum pump from the reactor regard­less of the particles’ residence time.

Due to the lack of convective gas flow inside the reactor, however, the heat and mass transfer rates are slower than those for the fluidized bed reactors, hence requiring a longer residence time for biomass particles in the vacuum reactor. A longer biomass residence time in turn makes the reactor and equipment size inevitably larger. Biomass in a vacuum reactor moves down­ward by gravity and rotating scrapers through the multiple hearth pyrolyzer with the temperature increasing from about 200°C to 400°C, as shown in Figure 5.8. Pyrovac’s pyrocycling™ process is based on vacuum pyrolysis technology [61]. According to Pyrovac, their process technology has the fol­lowing features:



A schematic of vacuum pyrolysis reactor for biomass fast flash.

• There is no need to pulverize or grind the feed biomass materials. Particles of up to 20 mm can be fed without any difficulty.

• The process system can be operated under torrefaction mode (<300°C) or pyrolysis mode (>450°C). The first mode generates biochar as a principal product, whereas the latter mode produces bio-oil.

• The process can be operated under vacuum or atmospheric con­ditions to enhance the production of either bio-oil or biochar. The process operation as well as its product portfolio is more versatile compared to competing process technologies.

• The process adopts a moving and stirred bed reactor.

• Molten salt heat carrier at 575°C is in indirect contact with biomass feedstock, thus aiding in efficient heat transfer.

• There are two heating plates with internal raking systems.

• The process uses two condensing towers. The first tower mainly col­lects heavy bio-oil and contains little water and acids. The second tower mainly recovers the aqueous acidic phase.

The ablative pyrolysis process is based on the heat transfer taking place when a biomass particle slides over a hot surface, as shown in Figure 5.9. High pressure applied to biomass particles on a hot reactor wall or surface to provide good contact is achieved by centrifugal or mechanical motion [68].

image68Bio-oil Collection


A schematic of ablative pyrolysis reactor for fast pyrolysis of biomass.

This type of reactor does not require a small particle size and can handle large particles without difficulty. The reactor does not require any carrier gas or sweep gas. The reactors of cyclonic type have difficulty in achieving sufficiently long residence times for the biomass particles that are required to allow a high degree of conversion.

Therefore, it is usually necessary to recycle partially reacted solids back to the reactor, as is the case with a circulating fluidized bed reactor. A high degree of char attrition also takes place and tends to contaminate the prod­uct bio-oil with a high level of carry-over carbon [57]. Some variations of the ablative pyrolysis process include the cone type and plate type for hot sur­faces. These processes are mechanically more complex and difficult to scale up, because the moving parts are subjected to the high temperatures required for pyrolysis. Furthermore, the loss of thermal energy from the ablative pro­cess in general could be high, inasmuch as the hot surface needs to be at a substantially higher temperature than the desired pyrolysis temperature.

Fast pyrolysis of biomass can be achieved in an auger-type reactor or auger reactor, in which an auger or an advancing screw assembly drives the biomass and hot sand through the reactor barrel. The operational prin­ciple is very similar to that of a polymer twin-screw extruder, as shown in Figure 5.10. This type of reactor achieves a very good mixing of materials



An auger type reactor for biomass fast pyrolysis.

in the reactor, thereby enhancing the heat and mass transfer efficiency. In an auger-type reactor for biomass fast flash, the usual mechanism of heat transfer is via direct heat transfer between hot sand and biomass particles.

At the outlet of the reactor, solids including hot sands, char, and ash are separately recovered from the oil vapors and aerosol. Hot sand is reheated using char combustion heat and recirculated back to the reactor, and the oil vapor and aerosol are sent to the condenser for bio-oil recovery. The process does not require a carrier gas, but the biomass particle size is preferentially small for smooth operation. The design of auger reactors has received exten­sive benefits from the industrial practice and design experience of the poly­mer and twin-screw extruder industries [76]. Therefore, this type of process is relatively straightforward to design and fabricate and is deemed to be suit­able for small-scale production.

Reactor Technology

For a mixed feedstock, the success of the gasification process depends on the reactor technology that carries out the desired objective with variable feed properties. Combustion

For coal and biomass mixture combustion, in general three types of com­bustion systems can be identified: fixed bed combustion (i. e., stoker or grate combustion), fluidized bed combustion (bubbling and circulating), and pulverized fuel combustion (dust combustion). The rotary kiln outlined in Chapter 6 is mostly used for the hazardous waste.

A fixed bed combustor can handle both small — and medium-size coal and biomass particles. The grate-fired furnace and underfeed stokers can also handle shredded tires, plastic and polymer waste, and shredded MSW. In this type of reactor, drying, gasification, and charcoal combustion take place in the primary combustion chamber followed by incineration (usually in the separate combustion zone) of the gases produced after the addition of extra air [101]. Different types of grate furnace; fixed, moving, traveling, rotating, and vibrating are available with a maximum capacity of 20 MWe. Underfeed stokers are used in small — and medium-scale systems up to boiler capacity of 6 MWe [101].

The fixed bed boilers are normally used for parallel and indirect co-firing [1]. For a capacity of less than 20 MWe, they have low investment and operat­ing costs. They can be used for varying particle sizes (except fines) and for any kind of wood with moisture content up to 10-60 wt% and high ash content. Although fixed bed boilers can be used for a mixture of woods, they cannot be used for a blend of wood and straw or grass which have different combustion behaviors and ash melting point. For a nonhomogeneous mixture, an increase in temperature may cause ash melting and resulting corrosion [102].

The underfeed stoker has low investment cost for a small boiler (<6 MWe) [1] and low emissions at partial load operations due to good fuel dosing [101]. This kind of combustor is only suitable for fuels with low ash content and high ash melting point. It also has less flexibility with respect to particle size (<50 mm).

In fluidized bed combustion, the bed contains a mixture of inert mate­rial (about 90-98% silica sand and dolomite) and fuel (about 2-10% of coal and biomass) particles, and they are fluidized by flowing air that is passed through a perforated plate at the bottom of the bed. The bed can either be operated as a one pass-through bubbling fluidized bed or as a circulating fluidized bed where the unconverted solids are recycled back in the reactor. The bed material provides high thermal inertia and stability in the combus­tion process. Good mixing and heat transfer by materials give good tempera­ture control. The particle size for a bubbling fluidized bed (BFB) is generally less than 80 mm and that for circulating fluidized bed (CFB) is less than 40 mm [101, 103]. Low combustion temperature (800-900°C) prevents ash sinter­ing and resulting defluidization [101, 103]. The bed materials remain at the bottom of the reactor in BFB, whereas in CFB, the material is carried upwards with flue gas and separated in a hot cyclone or U-beam separator, and fed back into the combustion chamber [101]. In CFB, light soot (unburned car­bon) may leave cyclone along with ash. This contaminated ash may restrict its further use [104].

Fluidized bed combustors can be converted from coal into biomass/coal co-combustion with a very small investment, and they have large flexibil­ity in calorific value and moisture and ash contents of the feedstock. The combustion temperature in these reactors is low, and they emit low CO (<50 mg/Nm3) and NOx (<70 mg/MJ) which can be further reduced to 10 mg/MJ after using SCR [1]. The combustors can have efficiency up to 90% even with low-grade fuels. Sulfur can be removed by a direct injection of limestone in the bed. For a mixed feedstock, if a common feeder for the mixture is not possible, the installment of separate feeders for different feedstock may be expensive. Slagging and fouling on the combustor walls and tubes can occur when the feedstock has high alkali content. High alkaline and aluminum contents can also cause bed agglomeration. Similarly, high chlorine content can cause corrosion on heat transfer surfaces [1].

Fluidized bed combustors are only cost-effective for capacity >20 MWe for bubbling fluidized bed and >39 MWe for the circulating fluidized bed [1].

These types of combustors have a low flexibility for particle size and bed materials can be lost with ash. They also have a high dust load in flue gas. Low temperature makes fuel combustion incomplete and in CFB unburned carbon content can appear in the ash [1, 101, 103, 104].

Pulverized fuel or dust combustion systems are fed pneumatically with fuels such as sawdust and fine shavings [104]. Fuel quality needs to be rather constant with low moisture content (<20 wt%) and particle size of 10-20 mm. This type of combustor gives increased efficiency due to low oxygen excess and when appropriate burners are used, it gives high NOx reduction [1].

Introduction to Biofuels and Bioenergy

1.1 Definition

Bioenergy is energy derived or obtained from any fuel that is derived or originated from biomass which includes recently living organisms and their metabolic by-products. Similarly, biofuels are defined as fuels made from biomass resources, or their processing and conversion derivatives [1]. Biomass is defined as all plant and animal matter on the Earth’s surface. Therefore, harvesting biomass such as crops, trees, or dung and using it to generate heat, electricity, or motion, is bioenergy [2]. Biomass is a very broad term that is used to describe materials of recent biological origin that can be used as an energy source or for their chemical ingredients. According to this definition, biomass includes crops, trees, algae, and other plants as well as agricultural and forest residues. It also includes many other materials that are regarded as waste by most people, including food and beverage manufacturing efflu­ents, sludges, manures, industrial organic by-products, and organic fraction of household waste [2]. The word "recent" in the defining statements of bio­mass is of significance, because it eliminates any logical ground for fossil fuels to be considered as such.

Biomass has a number of different end uses such as heating (thermal energy), power generation (electrical energy), and transportation fuels. The term bioenergy is usually used for biomass energy systems that produce heat or electricity, whereas the term biofuels is typically used for liquid fuels for transportation [2]. For example, biofuels include corn ethanol, cellulosic ethanol, biodiesel, algae diesel, biomass-derived methanol, biomass-derived Fischer-Tropsch fuels, and more.

Historically speaking, biomass is the oldest fuel known to humans in all regions of the world. In the current world, biomass is a clean and renewable fuel source that can produce heat, power, and transportation fuels. In the future world, biomass will be a sustainable energy source whose utilization will have little or minimal impact on the environment and climate change.

Biomass may be considered as a form of stored solar energy that is cap­tured through the process of photosynthesis in growing plants. Utilizing biomass as a biofuel or bioenergy source means that carbon dioxide (which
was captured from the air by growing plants) is released back to the air when the biofuel is eventually combusted. Therefore, the system based on biomass energy is carbon neutral, or at least close to being carbon neutral. The term carbon neutral means removing as much carbon dioxide from the atmosphere as we put in, thus leaving a net zero impact on the atmospheric carbon dioxide amount.

Soxhlet Extraction

Soxhlet extraction is a distillative extraction method that uses a chemical solvent and the extraction unit is equipped with a heated solvent reflux mechanism. These days, Soxhlet extraction is very widely used in chemical laboratories for extraction of a variety of materials (chemical, biological, and polymeric), where the desired compound has a limited solubility in a chosen solvent. Interestingly, the original Soxhlet extraction process was invented in 1879 by Franz von Soxhlet for the extraction of a lipid from a solid material [37].

Oils are extracted from the algae via repeated washing, or percolation, with an organic solvent such as hexane or petroleum ether, under hot reflux in a specially designed glassware setup equipped with a condenser, a dis­tillation path, a siphon arm, a thimble, and a distillation pot [35]. Soxhlet extraction is meant to be a laboratory process for a small-scale operation, but it is very useful for initial technology development. The extraction process is usually slow, taking an hour to several days, depending upon the specifics of the extraction process conditions as well as the specific solvent chosen for the extraction. The energy efficiency per unit mass of product yield is inevitably very low.

Industrial Cleaning of Ethanol Plant

Periodic shutdowns due to maintenance and repairs as well as emergency stoppages are inevitable with all fuel and chemical production plants. In this regard, an ethanol plant is no exception. Certain pieces of machinery involv­ing ducts, pipes, headers, stacks, and valves tend to foul more quickly and frequently than other pieces, thereby plugging up the associated equipment, developing leaks, and potentially disabling the entire process. Therefore, highly efficient industrial cleaning of ethanol plants during periods of shut­down becomes an important issue. Several cleaning options are available and also conceivable, including chemical-based cleaning, hydro-cleaning, com­pressed air cleaning, and dry ice blasting. The dry ice blasting process is used to clean the ethanol plant and its vital components during the shutdown peri­ods [25]. One of its advantages over hydrocleaning is that there is no water left behind where the system has to be dry; other advantages include the non­toxicity of carbon dioxide as well as the availability of carbon dioxide on the plant site if captured during the fermentation stage. The need for ethanol plant cleaning is universally applicable to both wet milling and dry milling plants.

Ethanol Extraction during Fermentation

In spite of the considerable efforts devoted to the fermentative alcohols, indus­trial applications have been delayed because of the high cost of production, which depends primarily on the energy input to the purification of dilute end-products, the low productivities of cultures, and the high cost of enzyme production. These issues are directly linked to inhibition phenomena.

Along with the conventional unit operations, liquid-liquid extraction with biocompatible organic solvents, distillation under vacuum, and selec­tive adsorption on the solids have demonstrated the technical feasibility of the extractive fermentation concept. Lately, membrane separation pro­cesses that decrease biocompatibility constraints have been proposed. These include dialysis [85] and reverse osmosis [65]. More recently, the concept of supported liquid membranes has been reported. This method minimizes the amount of organic solvents involved and permits simultaneous realiza­tion of the extraction and recovery phases. Enhanced volumetric produc­tivity and high substrate conversion yields have been reported [86] via the use of a porous Teflon® sheet (soaked with isotridecanol) as support for the extraction of ethanol during semi-continuous fermentation of Saccharomyces bayanus. This selective process results in ethanol purification and combines three operations: fermentation, extraction, and re-extraction (stripping) as schematically represented in Figure 4.13. As shown and suggested, novel process ideas can further accomplish maximized alcohol production, energy savings, and reduced cost in production.


Pyrolysis is a major process for waste disposal in which waste is thermally degraded in an inert environment. Pyrolysis is usually carried out at a lower temperature (250-900°C) and at low pressure. As shown in Table 6.4 [27-29], there are various modes of pyrolysis depending on the method of pyrolysis and the operating conditions. The heating value of pyrolysis gas typically lies between 5 and 15 MJ/m3 based on MSW and between 15 and 30 MJ/m3 based on RDF [27].

Conventional pyrolysis reactors are: fixed bed, fluidized bed, entrained flow, moving bed, rotary kiln, ablative reactor, and so on. Generally waste pyrolysis generates a large number of compounds, and these need to be tracked for their effective uses. Generally, the pyrolysis process involves three stages: (a) A feed preparation and pretreatment step that includes grinding and drying. The grinding improves the feed quality and subse­quent heat transfer. The drying improves gas-solid contact, heat and mass transfer, and reactions in the reactor. (b) A pyrolysis reactor that generates


Waste Pyrolysis Technologies, Operating Conditions, and Major Products




Temp. (°C)

Heating Rate

Major Products




Very Low




15 min-2 hrs





Slow pyrolysis

5-30 min

About 600


Charcoal, oil, gas





Charcoal, oil, gas



5-30 min



Charcoal, gases


Fast pyrolysis

0.5-5 sec

About 650

Fairly high


Fast pyrolysis

0.1-2 sec




Flash pyrolysis

Less than 1 sec



Oil, gases

Ultra pyrolysis

Less than 0.5 sec


Very high


Vacuum pyrolysis

2-30 sec





Less than 10 sec

Less than 500





Less than 10 sec

Greater than 700

High Oil, chemicals

Source: Modified from Bridgwater and Bridge, 1991. A review of biomas pyrolysis and pyroly­sis technologies. In A. V. Bridgwater and G. Grassi, (Eds.) Biomass Pyrolysis Liquids Upgrading and Utilisation, London: Elsevier Applied Science, pp.11-92; Bridgwater, 1995 / Thermal biomass conversion technologies, Biomass and Renewable Energy Seminar, Loughborough University, IK, March; and Huber, Iborra, and Corma, 2006. Synthesis of transportation fuels from biomass: chemistry, catalysis, and engineering, Chem. Rev., ACS.

gas, solids containing mineral and metallic compounds, and liquids. (c) An upgrading of pyrolysis gas, liquids, and solids to generate more useful fuel, chemicals, and materials. The impurities in the products such as the pres­ence of arsenic and other materials can make the product very difficult to use and the process useless. These impurities are removed using appropriate purification technologies [30].

The pyrolysis process can also (a) recover organic fractions such as methanol as a material/fuel, (b) recover char for external use, (c) generate more efficient electricity using gas engines or turbines, and (d) reduce flue gas volume after combustion which may reduce flue gas treatment capital costs. The pyrolysis process is used for MSW and sewage sludge, decontamination of soil, synthetic waste, and used tires, cable tails, and metal and plastic materials for substance recovery. In general, pyrolysis is a very versatile process and has been extensively used for waste con­version. When the pyrolysis process is used to generate fuels or chemi­cals, a subsequent upgrading of pyrolysis oil is often carried out. Also, as shown in Table 6.4 [27-29], pyrolysis is often carried out in the presence of hydrogen (i. e., hydropyrolysis) to improve quality and quantity of the oil production.

A novel Chartherm process is a pyrolytic distillation process designed to maximize the useful recovery of both materials and energy from waste [7]. The process was developed by Thermya Co. in France with an industrial capacity of 1,500 kg/hr wood waste. The advantages of this process are (a) low operating temperature (300-400°C) compared to conventional pyrolysis and gasification temperature of 500-1500°C, (b) avoidance of tar and dioxin emissions and thereby reducing gas cleaning equipment, and (c) possibilities of recovering both energy and materials from the waste feed [31, 32].

The process of pyrolysis in its different formats has been extensively used to treat different types of waste. Some of the typical recently reported litera­ture studies on this subject are depicted in Table 6.5. This table by no means gives a complete account of all reported studies.

Crop Oils, Biodiesel, and Algae Fuels

First-generation biofuels refer to the fuels that have been derived from biologi­cal sources such as starch, sugar, animal fats, and vegetable oils, whereas sec­ond generation biofuels are derived from lignocellulosic crops. The oils under the category of first generation biofuels are obtained using conventional pro­duction techniques. Some of the most typical types of first generation bio­fuels include vegetable oil, conventional biodiesel, bioalcohols, biogas, and synthesis gas (biosyngas).