6.47 – techno-economic aspects of ethanol production from lignocellulosic agricultural crops and...

7/16/2019 6.47 – Techno-Economic Aspects of Ethanol Production from Lignocellulosic Agricultural Crops and Residues

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6.47 Techno-Economic Aspects of Ethanol Production from Lignocellulosic Agricultural Crops and Residues M Galbe, O Wallberg, and G Zacchi, Lund University, Lund, Sweden © 2011 Elsevier B.V. All rights reserved.

6.47.1 Introduction 615 6.47.2

Which

Process

Steps

Can

Be

Considered

Most

Important?

617

6.47.2.1 Pretreatment 618 6.47.2.2 Hydrolysis and Fermentation 621 6.47.2.3 Separate Hydrolysis and Fermentation 621 6.47.2.4 Simultaneous Saccharification and Fermentation 622 6.47.2.5 Separation of Solids and Liquids 622 6.47.3 Process Modeling 623 6.47.3.1 Effect of Various Parameters on the Energy Demand and Production Cost 624 6.47.3.2 Co-Location with Other Plants 625 6.47.3.3 Integration with Heat and Power Plant 626 6.47.3.4 Integration with 1G Ethanol 626 6.47.4 Conclusions 627 References 627

Glossary bioethanol Ethanol produced – in a sustainable way – from forest or agricultural materials. flowsheet design The use of computer programs for design and/or rating of complex production facilities, usually based on a conceptual drawing (flow sheet) of the process. lignocellulosic materials A common name for materials containing cellulose, hemicellulose, and lignin, such as hardwood, softwood, straw, and bagasse.

pretreatment The treatment of materials so that the structure is made more accessible to enzymatic attack, as lignocellulosic materials are by nature very recalcitrant to degradation. process integration The efficient use of heat and power is made possible by utilization of, for example, waste heat internally (in the plant), or externally (in a nearby plant),

which can diminish the requirement for expensive equipment, such as steam boilers.

6.47.1 Introduction Efforts to decrease the utilization of fossil fuels, such as oil or coal, have increased during the last decade. A number of alternatives have been put forward as suitable substitutes, for example, bioethanol, biogas, hydrogen, and dimethyl ether (DME). Some of these alternatives are already produced on an industrial scale; however, the production volumes are worldwide still very small compared

with the amount of oil-based fuels presently used. They are also produced using technologies that may not be sustainable. In the following, some techno-economic aspects on the production of bioethanol from various crops and residues are discussed.

It is difficult to design a process, which is the most efficient in every single aspect. There are far too many factors affecting the route to the desired product. This is not only true for industrial process in general, but also true for production of bioethanol from various raw materials. As a whole range of starting materials are available for bioethanol production, the processes may differ in complexity and efficiency. Regarding ethanol production, some of the most important parameters are the type, and also the cost, of raw material, the overall energy demand for the production process, the value of co-products, and the capital cost [1]. The design of the plant heavily relies on accurate and consistent data, which comprise both physical and chemical data, as well as cost estimation data. The latter poses a problem: it is always best to use data gathered from a similar industrial-scale plant when designing a new one. However, as there are no plants producing bioethanol from lignocellulosic materials in operation, all estimations must be based on lab- or pilot-plant data. Figure 1 shows the interaction which is required when a new process is designed. Procedures and ideas found in lab scale (1 l) need to be evaluated in a somewhat larger scale (10–100 l), a process development unit (PDU), where larger quantities of material can be produced for characterization of products and byproducts. In this scale, it is also relatively easy to modify a process concept, since the various units do not have to be physically connected to each other. Further upscaling to pilot scale is required to, for instance, estimate heating or cooling requirements, to study transport properties, and to evaluate material behavior at various positions in a plant (>1000 l). The final process design is established after iterating back and forth between lab, PDU, and pilot scale.

Bioethanol has been introduced on a large scale in Brazil, in the United States, and in some European countries and is projected to be one of the dominating renewable biofuels in the transportation sector within the next 20 years. Interest in lignocellulose-based

615



VerificationResearch Development

EvaluationBuilding knowledge Test of configurations

Technology testsTest of ideas Optimization

Final optimizationModeling Products and byproducts

Gathering scale-up data

Lab PDU Pilot Fullscale scale scale scale

SSF

EnzymaticStarch Liquefaction Fermentation Distillation Dehydration EtOHhydrolysis

Solid separation Drying DDGS(centrifugation)

Evaporation

WWT To Recipient

616 Wastes from Agriculture, Forestry and Food Processing

Figure 1 Interaction between various levels of scale.

ethanol production has brought about action on high political levels. For example, in the United States of America, the Energy Policy Act of 2005 requires blending of 7.5 billion gallons (≈28.4 million m3) of alternative fuels by 2012. The major part of this alternative fuel will probably consist of ethanol, and, to be able to meet these demands, lignocellulosic materials will most likely have to be utilized. The European Commission plans to replace 20% of conventional fossil fuels with alternative fuels in the transport sector by 2020. Already by the year 2010, 5.25% (based on energy content) is to be replaced by renewable fuels. Again, bioethanol is expected to be one of the main means to achieve this goal.

At present,

bioethanol

is

produced

almost

solely

from

either

sugar- or

starch-based

raw

materials

(e.g.,

cane

sugar,

corn,

and

wheat) often called first-generation (1G) bioethanol [2]. In a starch-based process (Figure 2), starch is liquefied at temperatures around 90 °C by adding hydrolytic enzymes (α-amylases). After the liquefaction step, further hydrolysis of the polymers is accomplished by addition of glucoamylases. The resulting sugar solution is normally readily fermented to ethanol using yeast, for example, Saccharomyces cerevisiae. In starch-based processes, a dried co-product, distiller ’s dried grains with solubles (DDGS) adds a value to the overall process. The DDGS is sold as an animal feed and contains compounds with a high nutritional value.

There is considerable experience regarding starch-based ethanol production. In addition, since the technology has existed for many years, the process has reached engineering maturity; therefore, new plants based on cost-estimation data and production data have a high relevance and accuracy. From a process point of view, the sugar-based ethanol production is simpler than the starch based one.

The liquefaction step and enzymatic hydrolysis steps are replaced by extraction of sugar from sugar-containing raw materials, for example, sugar beet and sugar cane, yielding a sucrose-rich solution that can be fermented quite readily. In the case of sugar cane, the juice can be accessed using crushing and pressing methods. There is no production of DDGS in this case.

Figure 2 Starch-based process. SSF, simultaneous saccharification and fermentation; WWT, wastewater treatment.



Techno-Economic Aspects of Ethanol Production from Lignocellulosic Agricultural Crops and Residues 617

However, future expansion has to be based on bioethanol from lignocellulosic materials, that is, second-generation (2G) bioethanol, such as agricultural residues (e.g., wheat straw, sugar cane bagasse, and corn stover) and forest residues (e.g., sawdust and thinning rests), as well as from dedicated crops (e.g., salix and switch grass). Production based on these raw materials generates

very low net greenhouse gas emissions, thus reducing environmental impacts. The raw materials are sufficiently abundant and also available worldwide. The availability of agricultural land for nonfood crops places a limit on the area of land that can be used for ethanol production from starch-based materials in a cost-efficient way. On the other hand, if ethanol can be produced from lignocellulosic materials, including agricultural residues, potential conflicts between land use for food and energy production can be reduced. These types of raw material are cheaper than conventional starch-containing feedstock used for ethanol production today. Furthermore, they can be produced with lower requirements for fertilizers and pesticides. The composition of lignocellulosic materials differs from one species to another. However, the main constituents are of the same type: about 50–60% carbohydrates in the form of cellulose (made up of glucose) and hemicellulose (various pentose and hexose sugars), which also can be fermented to ethanol, and about 20–35% lignin. Agricultural crops and hardwood contain more pentose sugars than does softwood. There are also valuable components such as extractives and fatty acids, which preferably should be separated prior to ethanol production. Lignin is a valuable co-product which can be used to generate heat or a solid fuel, which helps improve the overall process economics. In a longer perspective, lignin may be used for production of chemicals; it is a very complex molecule, which contains numerous aromatic compounds of which some have interesting properties. In contrast to the co-product from starch-based ethanol production (DDGS, which is used as animal feed), there is no real limitation for the use of the lignin-rich residue.

Cellulose-containing materials, such as forest or agricultural residues, have not yet been commercialized in the ethanol industry. There are several reasons for this. The main difficulty lies in the general structure of lignocellulosic materials, which creates physical and chemical barriers such as: •

The complex

structure

of

the

materials,

which

makes

hydrolysis

(acid

or

enzymatic)

difficult.

• The mixture of pentose and hexose sugars, which can cause fermentation problems, since pentoses are not readily fermented. • The formation of various compounds that may have an adverse effect on fermentation and to some extent on enzymatic

hydrolysis. These compounds may originate from the raw material itself, for example, extractives, or be degradation products from an earlier process step. The byproducts originate from carbohydrates or lignin, for example, aromatic compounds or aliphatic acids. Some of these compounds can severely inhibit – or even stop – fermentation, which of course will have a profound effect on capital and operating costs.

So far, no full-scale ethanol plants based on production from lignocellulosic materials using modern technology have been put in operation. One of the major issues is the big risk being the first to invest in such a plant. Experience from lignocellulosic production is limited; occasionally, full-scale plants have been in operation, mainly during times of war. In Germany, concentrated HCl was used to hydrolyze wood in the Bergius process for generation of the sugars. Another process worth mentioning is the Scholler process (dilute H2SO4), which was used in a couple of countries, such as the former Soviet Union, Japan, and also Brazil. This limited experience makes designing new plants difficult, as modern process technology requires other sets of data to be able to predict capital and operational costs with satisfactory precision. For instance, the byproducts from the old and modern processes are not necessarily the same. Modern technology using enzymes does not require the same severe conditions to produce fermentable sugars, which is positive for the subsequent process steps, such as fermentation. However, as the product pattern can be very different, it is inappropriate to use old process concepts for design of modern full-scale processes.

The different byproducts may have a considerable value and actually contribute to the overall profit from the process, whereas others can be a waste product that has to be treated in wastewater-treatment plants or in scrubbers to remove various compounds. In the following, some important issues regarding the design on future processes for biomass-to-ethanol production are discussed.

6.47.2 Which Process Steps Can Be Considered Most Important? Ethanol production from lignocellulose comprises several important steps [3]: hydrolysis of hemicellulose, hydrolysis of cellulose, fermentation of both C5- and C6-sugars, separation of lignin, and recovery and concentration of ethanol and wastewater handling (see Figure 3). A process based on enzymatic hydrolysis and fermentation is currently regarded as the most promising option for the conversion of carbohydrates in lignocellulosic materials into ethanol in an energy-efficient way, resulting in high yields and low production cost. The enzymatic hydrolysis and fermentation either can be run separately (separate hydrolysis and fermentation (SHF)) or combined into a simultaneous saccharification and fermentation (SSF). The latter has so far been shown to result in higher ethanol yields than does SHF. Some of the most important factors to reduce the production cost include efficient utilization of the raw material by high ethanol yields, high productivity, high ethanol concentration in the feed to distillation, and process integration in order to reduce capital cost and energy demand. The key steps for success are the conversion steps, that is, pretreatment, enzymatic hydrolysis, and fermentation (or SSF) of all sugars. It is also crucial to design a highly integrated process working at high substrate consistency to minimize the energy demand in the downstream processing, for example, distillation and evaporation. Another highly important aspect is the full utilization of all parts of the raw material. It is likely that some compounds will not be recovered,




SSFDehydration EtOH

Enzymatic

hydrolysisFermentation Distillation

Stillage Drying Lignin pellet

Lignocellulosic

biomass PretreatmentYeast

cultivation Solids separation(filtration,

centrifugation) Combustion

(CHP)Heat, power

Liquid AD Biogas

Sludge

WWT

T o R e c i p i e n t

Figure 3 Schematic representation of a lignocellulose to ethanol process. SSF, simultaneous saccharification and fermentation; AD, anaerobic digestion; WWT, wastewater treatment.

either as ethanol or as solid lignin. This opens up to introduce also a biogas-producing step, which besides converting substances such as organic acids and residual sugars also can be regarded as part of the wastewater-treatment system. By introducing such a step, there

is

also

an

option

to

use

C5-sugars

for

either

ethanol

or

biogas

production.

This

has

to

be

decided

already

in

the

design

stage.

6.47.2.1 Pretreatment Enzymatic hydrolysis, using cellulases, is regarded to be the most attractive way to convert cellulose to glucose. However, due to the recalcitrant nature of most biomass species, the enzymatic hydrolysis is very slow and it is difficult to reach high sugar yields if the raw material is not pretreated prior to enzymatic hydrolysis. The pretreatment is perhaps the single most crucial step as it has a large impact on all the other steps in the process, for example, enzymatic hydrolysis, fermentation, downstream processing, and

wastewater handling in terms of digestibility of the cellulose, fermentation toxicity, stirring power, energy demand in the downstream processes, and wastewater-treatment demands.

Pretreatment methods can (somewhat arbitrarily) be categorized as belonging to mechanical, biological, chemical, or physicochemical methods. The latter two, which are the most common, can be divided into classes depending on the pH of the pretreatment. It is difficult to clearly assign a pretreatment method to one group, because several mechanisms may be involved to break down the material: • Low pH methods, that is, addition of acids, for example, dilute acid hydrolysis and steam treatment with addition of acids. Most of the

hemicellulose is usually hydrolyzed to monomer sugars and to some extent oligomer sugars available in the liquid fraction after pretreatment. Depending on the severity, that is, temperature, acid concentration, and residence time, a part of the cellulose may also be hydrolyzed. In addition, a minor part of the lignin is solubilized as phenolic compounds, but the major part remains in the solid fraction although redistributed. These pretreatment methods usually also result in production of sugar degradation products, such as furfural and 5-hydroxymethylfurfural (HMF).

• High pH methods, for example, alkaline pretreatment, ammonia fiber explosion, and wet oxidation with the addition of alkali. These methods result in partial solubilization of hemicellulose and solubilization of the major fraction of the lignin. An exception to this is the ammonia fiber explosion (AFEX) method where a fractionation is obtained but both hemicellulose and lignin are still




in the solid fraction. The hemicellulose sugars that are solubilized are however obtained mainly as oligomer sugars. This then requires hemicellulases acting on both solid and dissolved hemicellulose.

• Methods working close to neutral conditions at the start of the pretreatment, for example, steam pretreatment and hydrothermolysis. Most of the hemicellulose is solubilized due to the auto-hydrolysis created by the acids released from the hemicellulose, such as acetic acid. However, the sugars are obtained as a mixture of monomer and oligomer sugars. This thus requires hemicellulases or acids acting on soluble oligomer fractions of the hemicellulose.

Therefore, depending on the conditions during pretreatment, various compounds will be present after pretreatment, which will have a major influence on the process design. In all methods discussed, the cellulose fraction mainly remains in the solid fraction (except in the wet-oxidation method where a large fraction ends up solubilized in the liquid) and is made more accessible for the cellulase enzymes used during enzymatic hydrolysis. The digestibility of the material, as well as the amount of the hemicellulose sugars that are solubilized and the extent of degradation that occurs, is dependent on the severity of the pretreatment. The severity increases with increased temperature and residence time and with increased catalyst (acid or alkaline) concentration. In summary, an efficient pretreatment should fulfill a number of requirements: It should • result in high recovery of all carbohydrates; • result in high digestibility of the cellulose in the subsequent enzymatic hydrolysis; • produce no, or very limited amounts of, sugar and lignin-degradation products – the pretreatment liquid should be possible to

ferment without detoxification; • result in high solids concentration as well as high concentration of liberated sugars in the liquid fraction; • require a low energy demand or be performed in a way so that the energy can be reused in other process steps as secondary heat; and • require low capital and operational cost.

To be successful, pretreatment has to be developed as an integrated part of the whole process, including enzymatic hydrolysis, fermentation, downstream processing, and wastewater treatment. Each pretreatment method has to be assessed based on the process configuration and process conditions suitable for this specific pretreatment method [4–6]. For instance, the use of hemicellulases in the enzymatic hydrolysis, instead of only cellulases, will be beneficial to pretreatment methods that result in a large amount of oligomer hemicellulose sugars. In the same way, fermentation of slurries from methods generating major amounts of inhibitors should be performed using adapted yeast. Figure 4 shows some options of how the pretreated material may be utilized. The assessment of the pretreated material should

thus also reflect the process option that is used and what product is produced from the various parts of the fractionated raw material. It is our conviction that there is no best pretreatment that is the most suitable for all types of raw materials or process

configuration options. The choice of pretreatment depends mainly on what co-products are produced besides ethanol, the process configuration including process integration, as well as how the ethanol production is integrated with external processes, for example, heat and power production or 1G ethanol production. Some pretreatment methods have been more commonly investigated than others, such as steam pretreatment, dilute-acid pretreatment, wet oxidation and AFEX.

Dilute-acid pretreatment is performed by soaking (or by spraying) the material using a dilute acid solution and then by heating to temperatures between 140 and 200 °C for a certain time (from several minutes up to an hour). Sulfuric acid, at concentrations usually below 4 wt.%, has been of most interest in such studies as it is inexpensive and effective. The hemicellulose is hydrolyzed and the main part is usually obtained as monomer sugars. It has been shown that materials that have been subjected to severe acid hydrolysis may be harder to ferment because of the presence of toxic substances.

Alkaline pretreatment is performed at lower temperature and pressure than acid hydrolysis. Soaking of the material in an alkaline solution, such as sodium, potassium, or ammonium hydroxide, followed by heating, leads to swelling of the pores in the material.

This results in an increase in the internal surface area, and a decrease in the degree of polymerization and crystallinity. Alkaline pretreatment breaks the bonds between lignin and carbohydrates and disrupts the lignin structure, which makes the carbohydrates more accessible to enzymatic attack. This pretreatment method is more effective on agricultural residues and herbaceous crops than on wood materials, as these materials in general contain less lignin. For woody materials, the concentration of alkali has to be increased considerably, thus the procedure is more like a Kraft pulping process.

Another approach is to use an organic solvent, such as methanol, ethanol, acetone, ethylene glycol, triethylene glycol, and phenol, with the addition of inorganic acid catalysts (H2SO4 or HCl). These so-called organosolv processes dissolve the lignin which is recovered in the organophilic phase. These methods require total recovery of the solvent for both economic and environmental reasons, besides, the solvent may be inhibitory to the enzymatic hydrolysis and fermentation steps. A special case is the use of ethanol as the solvent as it is produced in the same process, and this facilitates the recovery.

Steam pretreatment is one of the most widely used methods for pretreatment of lignocellulose materials. In reality, it is a chemical method very similar to dilute-acid hydrolysis, although usually performed at higher dry matter content in a steam environment. The raw material is usually treated with high-pressure saturated steam at typical temperatures between 160 and 240 °C for 1–20 min, after

which the pressure is released. The acid can either be present in the raw material or be added, such as H2SO4 or SO2, to enhance the hydrolysis. Most agricultural residues and some types of hardwood contain enough organic acids (mainly acetic acid) to act as catalysts for the hemicellulose hydrolysis, the so-called auto-hydrolysis. The latter usually starts at neutral pH and ends at a pH around 3.5–4 depending on how much acid is released. The addition of an acid to reduce the pH considerably, often below 2, results in an increased



Biomass

SSFEthanol

Enzymatic hydrolysis and Butanol

Reactor fermentation Lactic acidOther

Slurry Enzymatichydrolysis

Fermentation

Filter

Solids

Heat andPower

Electricity and heat

Pulping Pulp

Liquid EthanolButanol

FermentationLactic acidOther

FurufuralReactions Organic acids(levulinic, formic, etc)

PolymersSeparation

(building blocks)

Biogas


Figure 4 Some alternative routes for utilization of pretreated biomass.

recovery of hemicellulose sugars, and also improves the subsequent enzymatic hydrolysis of the solid residue. This may also cause further degradation if very severe pretreatment conditions are used. Steam pretreatment has been widely tested in pilot-scale equipment, for example, in the NREL pilot plant in Golden, Co (the United States) and in the SEKAB pilot plant in Örnsköldsvik (Sweden). It is also used in a demonstration-scale ethanol plant at Iogen in Ottawa (Canada), Inbicon in Kalundborg (Denmark), and

Abengoa in Salamanca (Spain), and is considered to be close to commercialization. Wet-oxidation pretreatment involves the treatment of the biomass with water and air, or oxygen, at temperatures between 120

and 200 °C, sometimes with the addition of an alkali catalyst. This method is suited for materials with low lignin content, because the yield has been shown to decrease with increased lignin content, and also because a large fraction of the lignin is oxidized and solubilized. As with many other delignification methods, the lignin cannot be used as a solid fuel, which considerably reduces the income from byproducts in large-scale production.

AFEX is also an alkaline method, which, similarly to the steam pretreatment process, operates at high pressures. The biomass is treated with liquid ammonia about 10–60 min at moderate temperatures (below 100 °C) and high pressure (above 3 MPa). Up to 2 kg of ammonia is used per kilogram of dry biomass. The ammonia is recycled after pretreatment by reducing the pressure, as ammonia is very volatile at atmospheric pressure. During pretreatment, only a small amount of the solid material is solubilized, that is, almost no hemicellulose or lignin is removed. The hemicellulose is degraded to oligomer sugars and deacetylated, which is a probable reason for the hemicellulose not becoming soluble. However, the structure of the material is changed resulting in an increased water-holding capacity and a higher digestibility. Like the other alkaline pretreatment methods, AFEX performs best on agricultural waste, but has not proved to be efficient on wood, due to its higher lignin content. According to Sun et al., the AFEX process does not produce inhibitors that may affect downstream biological processes.

A high severity in the pretreatment is often required to enhance the enzymatic digestibility of cellulose. The reason why cellulose becomes more accessible for enzymatic attack is still not fully understood. Many structural parameters have been studied, such as crystallinity and pore-size distribution, but the relations between digestibility and these factors are somewhat inconclusive and ambiguous. It is, however, established that the removal of hemicellulose enhances the enzymatic digestibility of the cellulose fibers. However, more severe conditions during pretreatment will cause greater degradation of hemicellulose sugars. The optimum conditions are often a compromise between very high digestibility and high yield of hemicellulose sugars, that is, low sugar degradation. The recovery of monomer sugars after pretreatment varies; typically, almost all glucose is recovered, while more than




70–80% of the hemicellulose sugars usually are recovered. While most of the hemicellulose usually is found in the liquid, cellulose is recovered in the solid phase after pretreatment.

The overall ethanol yield also depends on the concentration of inhibitors, which influence the fermentability. These compounds include both substances present in the raw material, for example, acetic acid from the hemicellulose, extractives, or compounds formed during pretreatment, such as the sugar degradation products furfural and HMF, and lignin degradation products. The concentrations of these and all other inhibitory substances in the fermentation step depend on the configuration of the preceding process steps.

For most of the raw materials, the pretreatment conditions resulting in the highest glucose yield differ from those yielding the maximum yield of xylose. This would suggest a two-stage pretreatment, such as using steam, in which the first stage is performed at low severity to hydrolyze the hemicellulose, and the second stage at a higher degree of severity, in which the solid material from the first step is pretreated again. This would result in a high yield both of hemicellulose sugars and of high digestibility of cellulose. Major drawbacks are, however, the higher capital cost and the higher energy demand. The overall ethanol production cost is very much dependent on the way the two pretreatment steps are performed. The key issue is whether the pressure is released or not between the two steps, and also on the dry matter concentration after the second step.

Also the origin of the raw material, especially for agricultural residues, may affect the pretreatment results. This is important to keep in mind when comparing results from different studies on the same type of raw material. 6.47.2.2 Hydrolysis and Fermentation

Two process concepts have been more frequently investigated regarding ethanol production from lignocellulosic materials. The main difference between the two is the way in which the cellulose chain is broken apart; either dilute-sulfuric acid or cellulolytic enzymes are used to hydrolyze the cellulose molecules. The raw material is treated with 0.1–3% (w/w) H2SO4 at temperatures normally ranging from 160 to 200 °C. It may be advantageous to perform dilute-acid hydrolysis in two steps, as the hemicellulose fraction is more easily degraded than is the cellulose fraction. A disadvantage of the dilute-acid process is the somewhat low ethanol yield and the necessity of using expensive construction materials that are resistant to corrosion by acid at high temperatures. The acid must also be neutralized, which leads to the formation of large amounts of gypsum, CaSO4, or other compounds that have to be disposed of.

Enzymatic hydrolysis [7] and fermentation can be performed either separately, the so-called SHF, or combined, the so-called SSF. Enzymatic hydrolysis is performed using cellulases, that is, a mixture of various endoglucanases and cellobiohydrolases, which attack the amorphous areas of cellulose and cleave cellobiose units from both ends of the cellulose chain, respectively. They are supplemented with β-glucosidase, which cleaves cellobiose into two glucose molecules. The enzymes are end product-inhibited, that is, most cellulases are inhibited by cellobiose and β-glucosidase is inhibited by glucose, so the buildup of any of these products affects cellulose hydrolysis negatively. Also, some of the compounds formed during pretreatment are toxic, for example, sugar- or lignin-degradation products, and can cause inhibition of the cellulolytic enzymes. It should be noted that the addition of hemicellulases as well, for example, xylanases, can have a higher effect on the improvement of cellulose hydrolysis than on the increase of hemicellulose sugars, in case low amounts of hemicellulose remain in the solid fraction after pretreatment. This means that the pretreatment severity can be decreased in case hemicellulases are added to the enzymatic hydrolysis. It must be pointed out that most assessments of pretreatment of various raw materials found in literature are based on enzymatic hydrolysis (or SSF)

without the addition of xylanases. Depending on the enzyme load, results showing a yield based on the glucose content in the raw material of more than 90% are common.

Fermentation is performed using a microorganism, usually yeast, which converts sugar to ethanol. The most commonly used yeast for ethanol fermentation today is Saccharomyces cerevisiae, also called baker ’s yeast. It has a high ethanol tolerance and has also been shown to be rather tolerant to inhibitors produced during pretreatment of biomass. However, it only ferments hexose sugars, that is, glucose, mannose, and, under certain circumstances, galactose, but it is not capable of fermenting pentose sugars, such as

xylose and arabinose, which are the main constituents of most hemicellulose variants. SSF can also be preceded by a prehydrolysis to diminish the viscosity in the SSF step, as is usual in the starch-based 1G ethanol

production. Whichever configuration is chosen, it is important to maintain a high concentration of carbohydrates in the hydrolysis step in order to reach a high concentration of ethanol in the fermentation vessel. This is important primarily to diminish the energy demand for distillation of ethanol, and for evaporation of the stillage stream, in case this is included in the process. Figure 5 shows the overall energy demand for a plant based on 200 000 tons yr –1 of spruce as function of the water-insoluble solids concentration.

The shape of the curve is the same for other configurations also, although the absolute value of the energy demand may vary. 6.47.2.3 Separate Hydrolysis and Fermentation SHF has the advantage that each of the two steps can be optimized separately concerning temperature and pH, and also regarding the design of the equipment including stirring. Cellulases usually have a maximum activity around 50 ºC or higher, whereas most fermenting microorganisms, for example, S. cerevisiae, do not tolerate temperatures above around 37 ºC. Conventional ethanol fermentation is usually performed below 35 ºC. It is thus obvious that running the enzymatic hydrolysis at 50 ºC results in a higher productivity than when running it at 35 ºC. However, at the temperature for maximum activity, the enzymes are also deactivated faster than at lower temperatures. This means that although the enzymatic hydrolysis is faster at 50 ºC, it may very

well be so that the sugar yield after a 48- or 72-h hydrolysis is higher at 40 ºC, or even lower temperatures, due to the enzyme deactivation.




10

12

14 P r o c e s s h e a t d u t y

M J l – 1 e

t h a n o l

16

18

20

22

24

26

28

80

85

90

95

100

105

110

115

120

P r o d u c t i o n c o s t ( %

o f b a s e c a s e )

5 7 9 11 13 15

WIS concentration during SSF (%)

Figure 5 Overall process heat duty (- - - -) and relative ethanol production cost (—) as a function of the WIS concentration in SSF for the proposed ethanol production process.

An advantage of SHF is that the fermentation is performed with a liquid broth, instead of a slurry containing solid material which is the case in SSF, which facilitates the mass transfer and makes it possible to recycle the yeast after fermentation by filtration or centrifugation.

The main drawback of SHF is that the cellulases are end product-inhibited, that is, the productivity decreases with increasing sugar concentration. This is especially noticeable when hydrolysis is performed at high consistency, which is a prerequisite to obtain high ethanol concentration in the subsequent fermentation step. The enzymes may also be inhibited by the inhibitors present in the pretreated biomass slurry such as sugar- and lignin-degradation products. Inhibition from these compounds can be even larger than the end-product inhibition in the hydrolysis of steam-pretreated SO2-impregnated spruce. In general, using SHF, the enzymatic hydrolysis is slower due to product inhibition, so the residence time will also be longer in comparison with SSF, thus the reactor

volume needs to be larger. This will add to the capital cost. Another drawback is the loss of sugars in the separation of solids and liquids after enzymatic hydrolysis. This may be diminished

by washing, which, on the other hand, will lead to dilution of sugars even if a countercurrent washing system is used. This is avoided when SSF is employed as the ethanol is separated from the slurry by stripping in a distillation column. 6.47.2.4 Simultaneous Saccharification and Fermentation

The main advantage of SSF is that the sugars formed by enzymatic hydrolysis are converted by the yeast as soon as they are released. This maintains a low concentration of sugars in the broth, which alleviates the end-product inhibition of the cellulases and also diminishes the risk for infections. If pentose sugars are present, such as xylose, and the fermenting organism is incapable to ferment pentose sugars, there is usually an increased risk for lactic acid formation, as lactic acid bacteria are able to utilize, for instance, xylose

when the glucose concentration is low. In this case, the yeast cells are suffering from limitation in substrate (pentose fermentation is covered in another article in this book).

Another advantage of SSF is the capability of the yeast to partly detoxify the slurry. These two effects result in an increased enzymatic hydrolysis productivity also compared to enzymatic hydrolysis performed at higher temperatures. This leads to higher overall ethanol productivity, which means a lower total reactor volume. It has also been shown in several studies that the ethanol

yield is higher after SSF than after SHF for both softwood and agricultural residues; yields between 80% and 85% based on the potentially available sugars are common [8].

The main drawback with SSF is that the yeast after SSF is difficult to recover as it is mixed with the residual solid, that is, mainly lignin. In spite of this, we consider SSF to be a better option than SHF for most raw materials. The use of SSF is also cost effective as it reduces the number of reactors. 6.47.2.5 Separation of Solids and Liquids Separation of the liquid fraction from the solids and recovery of the produced ethanol are operations that can affect the overall production cost to a high degree. Regarding the solids, several options are available depending on the dry-matter content of the slurry. In some instances, decanter centrifugation can be utilized to separate the solid residue from the liquid. It may be possible to



0

1

1

2

2

α ( k

g m – 1 ) ×

1 0 – 1 2

3

3

4

4

5

80 °C

50 °C

25 °C

0 100 200 300 400 500 600 700

ΔP (kPa)


Figure 6 Specific filtration resistances as a function of applied pressure drop over filter cake for a slurry from SSF of spruce (experiments performed at 25, 50, and 80 °C). reach dry-matter contents around 40% or higher, but this is highly depends on the material. The advantage of using centrifugation is the rather compact footprint of the equipment, and also the potential to have a high throughput. However, if the material is difficult to separate, there is a risk that the solids will end up having a high liquid content, which have a great impact on the energy balance in the plant, since the lignin-rich residue needs to be rather dry to be usable for incineration. The recovery of solids in a centrifuge is also affecting the overall production cost. If the liquid contains too much solids, there will be a loss of potential solid fuel, which again will be negative for the process.

Another option is to use filter separation, for example, a filter press. In this type of equipment, the slurry is separated using a mechanical filter cloth. Filtering requires large areas (or volumes, if the filter assembled as a stack), especially if the material to be filtered has a low filterability. The filtration rate is to a great extent affected by the slurry itself; some lignocellulosic raw materials

yield a material, which, under certain circumstances, is extremely difficult to filter. The filter-cake resistance becomes very high, causing the filtration rate to decrease. If such a material is filtered, the filtration area required will be huge, and, as a result, the capital cost for filter equipment increase. Figure 6 shows an example of filtration of a slurry from SSF of spruce. In general, the specific filtration

resistance

increases

with

the

applied

pressure

over

the

filter

cake,

but

decreases

if

the

filtration

is

performed

at

a higher

temperature, which is in agreement with most filtration procedures, since the viscosity becomes lower.

6.47.3 Process Modeling As stated previously, there are several different possible layouts of a lignocellulose-to-ethanol process. A flowsheet of the process is presented in Figure 3. The process can be modified in a number of ways depending on the choice of pretreatment method, fermentation layout (SHF or SSF), evaporation, and/or anaerobic digestion. In any case, the layout is very complex with a significant amount of dependencies between the unit operations which constitutes the process. Since no lignocellulose-to-ethanol plant has been constructed and operated in modern times, and research usually is directed toward studies of individual unit operations within the process, there is a need for a tool that can evaluate different combinations of the available unit operations. A possible tool responding to such a need is flowsheeting/process modeling combined with cost estimations.

Flowsheeting programs, for example, Aspen Plus, HYSYS, and ChemCad, may be used to perform rigorous material and energy balance calculations, with the use of detailed equipment models, to determine the flow rates, composition, and energy flow for all streams in the process. Because of their flexibility, the programs have many advantages when comparing different process configurations or scenarios in terms of overall efficiency, minimum energy demand, or lower production cost. In addition, they provide a powerful tool to perform sensitivity analyses, due to the ease of changing a certain parameter.

Process simulations cannot replace experiments, but constitute a useful tool in the planning and evaluation of experiments. Furthermore, they highlight factors that are sometimes neglected in experimental studies, for example, the amounts of chemicals needed in the process (catalyst in pretreatment, acid/base for pH adjustment, nutrients, and, not least, enzymes and yeast), which constitute a significant contribution to the production cost. The overall demand of steam, process water, and cooling water are other important factors. Optimization of ethanol production from lignocellulosic feedstock requires a model that includes all the major process steps, since changing the conditions in one process step is likely to affect other parts of




the process. Although no full-scale plant based on enzymatic hydrolysis has yet been built, most of the process steps (e.g., distillation, evaporation, drying, and incineration) are considered to be technically mature, that is, their operational performance is well known. Of course, the application of these unit operations in a lignocellulose-to-ethanol plant still requires to be verified on pilot scale before a full-scale plant can be constructed. However, the ethanol process includes other process steps, which are associated with greater uncertainties regarding design and performance on full scale. This is definitely true for the pretreatment step, irrespective of the pretreatment method chosen, or how it is configured. It also applies to enzymatic hydrolysis or SSF at high solids concentrations, as well as solid–liquid separation for recovery of lignin. In many techno-economic evaluations of the lignocellulose-to-ethanol process that have been performed during the past 10 years,

Aspen Plus from Aspen Technologies has been used [9, 10]. 6.47.3.1 Effect of Various Parameters on the Energy Demand and Production Cost Process modeling of ethanol production from spruce using a process concept based on SO2-catalyzed steam pretreatment followed by SSF, as shown in Figure 3, has been used to illustrate the effect of various process parameters on the energy demand and on the ethanol production cost.

The ethanol yield is the single most important parameter in reducing the cost of ethanol production as it affects both the raw material and capital costs. High energy efficiency, achieved by process integration, is also of great importance for the process to be economically feasible. In most techno-economic evaluations of ethanol production, live steam for the process is generated in a steam boiler by burning part of the solid residue. From the excess solids it is possible to generate heat and electricity or fuel pellet that can be marketed to improve the process economics. Thus, the energy demand of the process determines the amount of solid residue that adds to the income as a co-product and, therefore, it is very important for the process to be energy efficient.

The heat duty of the process depends to a large extent on the process configuration. For the process alternative mentioned above, the heat duty of the energy-demanding process steps is shown in Figure 7. The white bars represent the primary steam demand, while the gray bars represent the amount of secondary steam that is generated in each process step. The overall process heat duty, that is, the total energy demand in the form of boiler-generated steam, is the sum of the black bars. Distillation (including preheating of the SSF broth) and evaporation account for the major part of the process primary energy demand. The contributions from pretreatment and drying, with the latter assumed to work as a steam dryer, are comparatively small, due to the generation of secondary steam in these process steps.

The energy demand of the distillation step, in which the ethanol in the mash from fermentation is concentrated, is highly dependent on the ethanol feed concentration, as shown in Figure 8. The distillation step normally consists of a stripper column, in

which the ethanol is separated from all solid and nonvolatile compounds, and a rectification column, in which the ethanol is concentrated close to the azeotropic point. The implementation of heat integration, for instance, by using the overhead vapor from the stripper as heat source in the reboiler of the rectification column, significantly reduces the energy demand. Nevertheless, it is of great importance to obtain a high ethanol concentration in the distillation feed. In a starch-based process, the ethanol concentration in the stream entering the distillation step is normally above 8% (w/w). In a lignocellulose-based process, however, the aim has

Process heat duty (MJ l–1 ethanol)

–6 –4 –2 0 2 4 6 8 10

Primary

Secondary

Overall

Pretreatment

Preheating of mash

Distillation

Evaporation

Drying

Figure 7 Heat duties of the energy-demanding process steps in the proposed ethanol production process.




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Figure 8 Energy demand in the distillation step, where ethanol is concentrated to 94 wt.%, as a function of the ethanol feed concentration. The step was assumed to consist of two stripper columns (25 trays each) and a rectification column (35 trays) heat integrated by operating at different pressures. The inlet feed temperature was increased from 80 °C to the boiling temperature before entering each stripper column.

been to reach at least 4–5% (w/w) ethanol. In addition, a high ethanol concentration results in a high concentration of nonvolatile compounds, which also leads to a decrease in energy demand in the evaporation step.

Recirculation of process streams is one way of reducing the overall energy demand, which results in a decrease in overall production cost. Recirculation of part of the stream after distillation back to the fermentation step would result in an increased concentration of nonvolatiles and thus a reduction in the energy demand in the evaporation step. Recirculation of part of the stream before distillation would also result in an increase in the ethanol concentration and thus a reduction in the energy demand in both the distillation and evaporation steps. This is true for both the SSF and SHF configurations. However, it is even more beneficial to increase the substrate concentration in the SSF step. This results in reduced water consumption, which greatly reduces the energy demand for distillation and evaporation, provided the ethanol yield is maintained at a high level. Based on this, one of the main objectives of several experimental studies performed during recent years has been to increase the substrate concentration in SSF. Figure 5 shows the process heat duty (in MJ l–1) and the overall production cost as functions of the

Water-insoluble solids (WIS) concentration in SSF. The reduction in production cost is due to an increase in co-product credit and a reduction in the fixed capital cost.

Process simulations clearly demonstrate the potential reductions in production cost and energy demand that can be obtained by running SSF at higher substrate concentrations. However, given the large number of compounds involved, and because they may act synergistically, it is impossible to predict the impact of increased concentrations on the performance of the yeast and enzymes using process models. Effects on parameters such as productivity (yield and residence time) as well as yeast and enzyme dosages have to be determined experimentally, preferably on pilot scale.

Savings in energy demand can also be accomplished by changes in the process design. Evaporation is the traditional, but energy-demanding, way to concentrate the water-soluble, nonvolatile components in the stillage stream in both 1G and proposed 2G plant designs. To reduce the energy requirements for evaporation, multiple evaporation effects (five stages) are used. The layout of the evaporation has a significant effect on the overall process heat duty. In a traditional multiple-effect evaporator system, a large proportion of the energy supplied ends up as latent heat in the vapor phase leaving the last effect in the evaporator. This vapor is normally condensed using cooling water. Another option is to compress the vapor, thereby raising the temperature to a level at

which the latent heat can be utilized. The vapor can then be used as a heating medium to replace primary steam. Another very attractive option would be to replace evaporation by anaerobic digestion, in which a large part of the organic

material (unfermented sugars, acids, yeast, etc.) is converted to biogas mainly consisting of methane and carbon dioxide. Some more difficultly digested compounds would thereafter have to be treated in a traditional wastewater-treatment plant in order to reduce the organic loading on the recipient. The implementation of biogas production has been estimated to reduce the production cost by about 7%. The performance of such a system is dependent on a number of parameters such as the composition of the feed, residence time, temperature, etc. A crucial question is also how to handle the sludge from the anaerobic digestion. Further investigation is required since very limited data regarding the performance of this kind of system have been published. 6.47.3.2 Co-Location with Other Plants One approach to reduce the production cost is integration of ethanol production with another suitable plant, for example, a combined heat and power plant, a starch-based ethanol plant, or a pulp and paper mill. One of the benefits in combining with a heat and power plant is that the syrup or lignin residue can be used for steam production without prior drying. Another option is to integrate cellulosic ethanol production with starch-based ethanol production to utilize the whole agricultural crop. This will




increase the production capacity drastically, and it may also help to boost the ethanol concentration resulting from the lignocellulosic process, if the ethanol-containing streams can be distilled in the same distillation units. This can have a beneficial effect on the energy demands in the distillation and evaporation steps. It might be an economic disadvantage if the residue cannot be used for animal feed (DDGS). However, it will still have a fuel value, which will help improve the economics of the overall process.

The biorefinery concept is also an interesting option. Using chemical and biological transformations, the raw material is processed to produce ethanol and, for example, modified lignin, specialty chemicals, and maybe biogas, adding value to the main product. In this case, the income from other products improves the overall process economics. 6.47.3.3 Integration with Heat and Power Plant Integration of cellulose-based ethanol production with a combined heat and power plant (Figure 3) has been estimated to reduce the ethanol production cost by up to 20% for conditions prevailing in Sweden and it is the main strategy pursued in the Swedish cellulose-to-ethanol effort. Live steam required in the ethanol process can be generated by burning a part of the solid residue (together with the concentrated liquid from evaporation of the stillage and possibly some biogas generated in wastewater treatment). Depending on the various co-products, that is, pellets, electricity, and district heating the energy efficiency, defined as the energy output in the products (ethanol, pellets, excess electricity, and/or district heating) divided by the energy input, vary from 53% to 92% [11]. From an energy point of view, the district heating alternative is most attractive as it utilizes a large part of the low-temperature waste heat from the process. However, this option restricts the location to the vicinity of larger cities of the plant as there must be a demand for the surplus heat. 6.47.3.4 Integration with 1G Ethanol One promising alternative is to integrate 2G cellulosic ethanol production with 1G starch-based or sugar-based ethanol production to use the whole agricultural crop. Examples of agricultural residues are corn stover, wheat straw, sugar cane bagasse, and trash.

Taking it further, the two methods could be integrated at some suitable point in a plant to share some common process equipment. Figure 9 shows some possible integration schemes for a starch-based 1G plant. Due to the similarities in the two processes, several points for process integration exist. The easiest point would be after fermentation and solid residue separation before the distillation as the two processes would have separate and dedicated equipments for pretreatment, hydrolysis, and fermentation. However, by combining material streams further upstream, the equipment cost for adding a 2G technology into an existing 1G plant could be lower and the energy demand could be decreased.

PretreatmentLignocellulosicbiomass

2 G plant

1 G plant

SSF

Liquefaction

Enzymatic

Hydrolysis Fermentation

SSF

1 2 3

EnzymaticHydrolysis

Fermentation Distillation

Dehydration EtOH

Drying

ADLiquid

Stillage

Solids separation(filtration,

centrifugation)

Lignin pellet

Heat, power

Biogas

Sludge

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T o R e c i p i e n t

Combustion(CHP)

Starch

Figure 9 Integration of first- and second-generation bioethanol production (1, 2, and 3 denote some possible integration points).




Integration of the two concepts can be beneficial for both processes. As an example, the 2G ethanol production has an energy surplus in the form of lignin, which can be used in the whole plant. It is also usually difficult to reach high sugar and ethanol concentrations in the 2G ethanol production, while starch- or molasses-based ethanol production require dilution of the sugar. By combining the process flows at some point in the plant, the energy situation in the distillation can be improved compared to two stand-alone plants. Also, the energy demand for evaporation of the stillage stream when applicable (not shown in Figure 9), can be diminished for some of the process configurations. The demand for addition of nutrients will also be lower, since the 1G raw material contains sufficient levels of many of the required elements.

Integration may also alleviate some of the inhibitory effects occurring from the formation of toxic compounds in the pretreat

ment step. If the process streams are mixed prior to fermentation, the lignocellulosic streams will be diluted by the starch-based streams. To summarize, we believe that integration of 1G and 2G bioethanol production combined with Combined heat and powder

(CHP) and biogas production (see Figure 9) results in higher ethanol yield, lower energy demand, and lower production cost than by using a stand-alone 2G ethanol production. To define the most optimal way of integration requires detailed studies, for example, by flowsheeting calculations based on reliable experimental data.

6.47.4 Conclusions In summary, substantial progress has been achieved in the field of lignocellulosic fuel ethanol production, especially within research. However, the transition into a mature industrial technology requires further research and development efforts to cope with the major research challenges summarized below: • To produce ethanol at a high concentration. The most obvious is to perform enzymatic hydrolysis or SSF at high dry-matter

concentration. This requires improvement of enzymatic hydrolysis with efficient enzymes, reduced enzyme production cost, and novel technology for high solids handling. Another important factor is to utilize all the sugars available in the pretreated material, that is, including pentose fermentation. This will lead both to a higher ethanol concentration and to a lower production cost.

• Development of robust fermenting organisms, which are more tolerant to inhibitors and ferment all sugars in the raw material in concentrated hydrolysates at high productivity and with high ethanol concentration.

• The increase in concentration of inhibitory compounds with increased dry matter may lead to a decreased ethanol yield. To cope with this, an option is to separate the solid and liquid fractions and only use the solid fraction, that is the cellulose, for ethanol production. The liquid could then be used for other applications, for example, biogas production, where it may be diluted

without negative effects in the product recovery as the biogas is obtained in the gas phase. • Extension of process integration to reduce the number of process steps and the energy demand and to reuse process streams to

eliminate the use of freshwater and to reduce the amount of waste streams. • Process integration with other types of industrial processes, for example, a combined heat and power plant or 1G ethanol plant,

which will reduce the production cost further. This may result in higher ethanol concentrations to the distillation step compared with a stand-alone 2G plant.

• Integration of 1G and 2G plants also benefit from a decreased requirement for addition of nutrients since the meal, or molasses, already contain some compounds. Also, it will be easier to cope with inhibitors since the streams from the 1G plant dilute the 2G process streams.

Finally, one of the most important issues is to verify all process steps in an integrated way in pilot and/or demo scale. Not only critical process steps such as pretreatment and SSF have to be verified at large scale but also more technical issues such as filtration of lignin and the influence of process integration and recycling of process streams on fouling.

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pretreatment. Cellulose 16: 4649–4659.[5] Olsson L (ed.) (2007) Advances in Biochemical Engineering Biotechnology . Berlin: Springer. [6] Sousa LD, Chundawat SPS, Balan V, and Dale BE (2009) ‘Cradle-to-grave’ assessment of existing lignocellulose pretreatment technologies. Current Opinion in Biotechnology

20: 3339–3347. [7] Jorgensen H, Kristensen JB, and Felby C (2007) Enzymatic conversion of lignocellulose into fermentable sugars: Challenges and opportunities. Biofuels Bioproducts and

Biorefining 1: 2119–2134.[8] Olofsson K, Bertilsson M, and Lidén G (2008) A short review on SSF – an interesting process option for ethanol production from lignocellulosic feedstocks. Biotechnology for

Biofuels 1: 7 (doi:10.1186/1754-6834-1-7).



628 Wastes from Agriculture, Forestry and Food Processing [9] Wooley R, Ruth M, Glassner D, and Sheehan J (1999) Process design and costing of bioethanol technology: A tool for determining the status and direction of research and

development. Biotechnology Progress 15: 794–803. [10] Sassner P, Galbe M, and Zacchi G (2007) Techno-economic aspects of a wood-to-ethanol process – energy demand and possibilities for integration. Chemical Engineering

Transactions 12: 447–452. [11] Sassner P and Zacchi G (2008) Integration options for high energy efficiency and improved economics in a wood-to-ethanol process. Biotechnology and Biofuels 1: 4

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6.47 – techno-economic aspects of ethanol production from lignocellulosic agricultural crops and...

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