Category Archives: Biofuels and Bioenergy

Clean Air Act Amendments of 1990

The Clean Air Act Amendments of 1990 target automobile emissions as a major source of air pollution. The Act mandates the use of cleaner burn­ing fuels in U. S. cities with smog and air pollution problems. The oxygen requirements of CAAA spurred a market for oxygenates and created new market opportunities for ethanol. The Oxygenated Fuels Program targets 39 cities that do not meet National Ambient Air Quality Standards (NAAQS) for carbon monoxide (CO). CAAA mandates the addition of oxygen to gaso­line to reduce CO emissions. It requires an oxygen level in gasoline of 2.7% by weight. Control periods vary by cities because most CO violations occur during the winter season. The average control period is about 4 months. The most widely used oxygenate in the market had been a methanol-derived ether, MTBE, which was made mostly from natural gas feedstock and was rapidly being phased out from the U. S. market in the twenty-first century, as explained in the previous section.

Most major gasoline refiners are using ethanol more and more to meet gasoline oxygenate content requirements. In 1993, about 300-350 million gallons of ethanol were blended with gasoline and sold in markets covered by the OFP. In 2004, fuel ethanol consumption reached 3.4 billion gallons in the United States. In about 10 years, the U. S. production of grain ethanol has seen a tenfold increase. In 2010, corn ethanol production in the United States reached 13.2 billion gallons, which is nearly a fourfold increase in the six — year period. The CAAA also requires the use of oxygenated fuels as part of the Reformulated Gasoline Program for controlling ground-level ozone for­mation. This program requires an oxygen level in gasoline of 2.0% by weight. Beginning in January 1995, reformulated gasoline was required to be sold in nine ozone nonattainment areas year-round. Other provisions in the act allow as many as 90 other cities with less severe ozone pollution to "opt-in" to the RFG program. Under a total opt-in scenario, as much as 70% of the nation’s gasoline could be reformulated.

An oxygen level of 2.0% by weight in gasoline means that at least 5.75% by weight of ethanol needs to be blended in gasoline, based on the stoichio­metric calculation of 2.0 x (46/16) = 5.75. Therefore, 2.7% oxygen requirement pushes the required level of ethanol in gasoline to 7.76 wt% as a minimum. Thus, 10% ethanol blended gasoline, E10, sold in gas stations is consistent with the result of such calculation. Even though ethanol is clean burning and has a relatively low Reid vapor pressure of blending, it has a substan­tially lower heating value than conventional gasoline. The higher heating value (HHV) and lower heating value (LHV) of gasoline are 47.3 and 44.4 kJ/g, respectively, whereas those for ethanol are 29.7 and 28.9 kJ/g, respec­tively. However, at a level of 10% ethanol blending, the reduced energy out­put is much less appreciable and could be compensated for by better engine performance.

Cellulosic Ethanol Technology

In this section, various process stages of typical cellulose ethanol fermenta­tion technology, as illustrated in Figure 4.5, are explained.

Steam Gasification

Steam reacts with carbonaceous materials including hydrocarbons, carbohy­drates, oxygenates, natural gas, and even graphite at elevated temperatures and generates carbon monoxide and hydrogen. The stoichiometric chemical reactions in this class of reaction include:

C(s) + H2O(g) = CO(g) + H2 (g)

Coal + Steam = CO + H2

CH4 + H2O = CO + 3 ■ H2

image078
Подпись: H2

V

The first reaction represents steam gasification of carbon, whereas the second reaction is steam gasification of coal. The third reaction is steam ref­ormation of methane (or, methane steam reformation, MSR), whereas the fourth reaction is known as reformation of hydrocarbon fuels. The chemi­cal equilibrium favors the forward reaction of steam gasification of carbon, if the temperature of reaction exceeds 674°C, as explained in Section 5.3.1 and Table 5.9. This threshold temperature (and its vicinity) for forward reac­tion progress is nearly universally applicable to all hydrocarbon species including coal. As clearly shown, product hydrogen in these reactions at least partially originates from water (steam) molecules. Without separately going through a water-splitting reaction, this reaction efficiently extracts hydrogen out of water molecules and carbon atoms in the hydrocarbon molecules react with oxygen atoms from water molecules. As expected, the forward reactions, that is, steam gasification reactions as written, are highly endothermic at practical operating temperatures, requiring high energy input.

As mentioned earlier in the pyrolysis section, even when hydrocarbons are reformed or gasified by steam at elevated temperatures, thermochemical conversion due to pyrolysis is also taking place as a competing and parallel reaction to the steam gasification reaction. If a hydrocarbon feedstock, such as coal and biomass, is introduced into a reactor where steam gasification is desired at an elevated temperature, the resultant reaction usually proceeds as an apparent two-stage reaction, viz., appearing to be the pyrolysis reac­tion followed by steam gasification. Mathematically, this apparent two-stage reaction process can be explained by the result of superposition of two par­allel reactions between one very fast reaction (pyrolysis) and one slow reac­tion (gasification). In this explanation, easily pyrolyzable components are rapidly broken down in the early period (e. g., a matter of a few seconds), whereas much slower gasification takes place more steadily over a much lon­ger period of time (e. g., a matter of 0.5-3 hours). In coal gasification studies, some researchers interpreted this early stage pyrolysis result as an initial conversion [33].

Because biomass generally contains a high level of moisture, steam gas­ification reaction is nearly always present with or without a separate feed of steam into the reactor, except in the case of fast pyrolysis. In the fast pyrolysis of biomass, the typical temperature of operation is around 500°C, which is substantially lower than the steam gasification temperature, and therefore the biomass moisture is not involved in the steam gasification reaction.

Mixed Feedstock

7.1 Introduction

As the world supply and demand picture for fossil energy changes and the environmental regulations for greenhouse gas (GHG) emissions become more stringent, more efforts are being made to find alternate sources of energy. One source is renewable bioenergy which addresses environmen­tal regulations. Although bioenergy currently contributes to only a small percentage of worldwide energy production (it is about 5% of European Union energy supply and smaller in the United States), its worldwide usage is rapidly increasing. Furthermore, recent advances in sustainable waste management provide an additional opportunity for converting various cellulosic and polymer waste, rubber tires, MSW, and the like, to energy and products. Types of biomass feedstock used for energy purposes are described in Table 7.1 [1-3].

Although the use of biomass for power and fuel brings environmental benefits, its use involves high investment costs. Furthermore, the use of biomass raises concerns about the security of feedstock supply particularly for large power and fuel plants. The feedstock supply issue with biomass is caused by (a) the seasonal nature of biomass, (b) biomass resources are dispersed in many countries and an infrastructure for the biomass supply is not established, and (c) transportation of biomass can be very expensive because of its low mass and energy densities. Lower heating values and lower bulk densities compared to coal result in a much greater volume of biomass to be transported, handled, and stored and as a consequence, large biomass units (>300 MWe) are economically unattractive [4]. The argument of an inconsistent and unreliable feed supply to large plants also applies to the waste industry.

The biomass demand in stationary applications (heat and power) is the main driving force behind early expansion of bioenergy and this will remain the major demand source for bioenergy up to 2030. However, the need for more improved and efficient generation of stationary bioenergy, as well as the new efforts to use biomass and waste to generate biofuels and other products, will stimulate more and more use of biomass. These expansions

Types of Biomass Feedstock Used for Energy Purposes [1-3]

TABLE 7.1

Source

Types and Examples

Woody, forestry, and

1. Industrial waste wood from timber mills and sawmills (e. g.,

agricultural, and park

bark, sawdust, wood chips, slabs, off-cuts)

and garden waste

2. Waste from paper and pulp industry including black liquor

3. Forestry by products (e. g., wood blocks, logs, wood chips)

4. Dry lignocellulosic agricultural residues (e. g., straw, sugar beet leaves and residue flows from bulb sector)

5. Livestock waste (e. g., chicken, cattle, pig, and sheep manure)

6. Herbaceous grass and woody pruning

Dedicated energy crops

1. Woody energy crops (e. g., willow, poplar and eucalyptus)

2. Herbaceous energy crops (e. g., various types of reed grass, switch grass, miscanthus, Indian shrub and cynara cardunculus)

3. Oil energy crops (e. g., rapeseed, sunflower seeds, soybean, olive-kernel, calotropis procera)

4. Sugar energy crops (e. g., sugar beet, cane beet, sweet sorghum, sugar millet, Jerusalem artichoke)

5. Starch energy crops (e. g., barley, wheat, potatoes, maize corn (cob), amaranth)

6. Other energy crops (e. g., flax, hemp, tobacco stems, cotton stalks, kenaf, aquatic plants (lipids from algae))

Waste

1. Contaminated waste (e. g., biodegradable municipal waste, demolition wood, sewage sludge)

2. Landfill gas

3. Sewage gas

Miscellaneous

1. Roadside hay (e. g., grass)

2. Husks/shells (e. g., olive, walnut, almond, palm pit, cacao)

Source: Maciejewska et al. 2006. Co-Firing of Biomass with Coal: Constraints and Role of Biomass Pre-Treatment, DG JRC Institute for Energy Report, EUR 22461 EN; Loo and Koppejan (Eds.) 2004. Handbook of Biomass Combustion and Co-Firing, Prepared by Task 32 of the implementing agreement on bioenergy under an auspices of the international energy agency, Twente University Press; VIEWLS. 2005. Biofuel and Bio-Energy Implementation Scenarios, Final report of VIEWLS WP5, modeling studies.

will also stimulate the establishment of a new and improved supply infra­structure for biomass in the long term.

At the present time, a mixed feedstock of coal and biomass offers a possible solution to the above-described problem. Such a mixture can help mitigate environmental concerns of plants running on coal alone. On the other hand, coal can mitigate the effects of variations in biomass feedstock quality and buffer the system when there is a lack of sufficient required biomass quantity [3,5]. A mixed feedstock can be used in large units that have better thermal and economical efficiencies compared to small-scale systems. Furthermore, it is possible to adapt existing coal power plants or coal-based fuel refineries for mixed feedstock at a relatively lower cost compared to building new and dedicated systems [4].

The use of mixed feedstock started in co-firing because its major purpose was to generate heat and electricity and the compositions of gas and sol­ids were not important toward the end-use. However, now in addition to combustion, other thermochemical technologies such as gasification, plasma technology, pyrolysis, liquefaction, and supercritical technology have been further developed to generate heat, electricity, transportation fuel, chemi­cals, and materials. The use of mixed feedstock in these technologies is more complex because of the effects of mixed feedstock on the gas, liquid, and solid compositions and their subsequent use. As shown in Tables 7.2a to 7.2c [6, 7], the chemical compositions of various raw materials that can be used within a mixed feedstock vary substantially and these can significantly affect the gas, liquid, and solid product compositions.

Up till now at larger scales, co-utilization of waste and coal has received considerably more attention than co-utilization of coal and biomass. This is true for all thermochemical processes such as combustion, high severity pyrolysis, gasification (including supercritical), and plasma technology that are generating heat, electricity, or gaseous fuels and products. Investigated waste has been municipal solid waste (MSW) that has had minimal presort­ing or refuse-derived fuel (RDF) that has had significant pretreatment such as mechanical shredding and screening as well as shredded rubber tires, paper and pulp waste, and plastic waste.

In recent years, co-utilization of coal and biomass in combustion, gasifica­tion, pyrolysis, and plasma technology has been gaining significant accep­tance. Recent reviews of cofiring literature identify over 100 successful field demonstrations in 16 countries using many types of biomass in combination

TABLE 7.2A

Selected Typical Properties of Several Coal and Biomass Fuels

Type

Coal

Peat

Olive

Residue

Willow

Straw

Corn

Stover

Cotton

Gin

Rice

Husk

Olive

Husk

Ash (db)

9.7

5.5

4.5

2.55

5

3.25

14.5

20.61

1.6

Moisture

8.0

47.5

65

55

21

35

11.5

9.96

33

(wt%)

C (%db)

81.5

54

49

49

46

42.5

42

34.94

47.8

H (%db)

4.25

5.75

6

6.25

5.9

5.04

5.4

5.46

5.1

O(%db)

7.05

35

34

43

43

42.6

35

38.86

45.4

N(%db)

1.15

2

1

0.5

0.5

0.75

1.4

0.11

0.1

S(%db)

1.8

<.17

0.12

0.06

0.125

0.18

0.5

Source: Ratafia-Brown et al. 2007. Assessment of Technologies for Co-converting Coal and Biomass to Clean Syngas-Task 2 Report (RDS), NETL report (May 10) and Sami, Annamalai, and Wooldridge, 2001. Co-firing of coal and biomass fuel blends, Prog. Energy Combust. Sci., 27: 171-214.

TABLE 7.2B

Selected Typical Properties of Some Wood Products and Municipal Residue

Type

Sawdust

Hardwood

Softwood

Redwood

Switch

Grass

Tan

Oak

MSW

Tires

Black

Locust

Ash (%db)

2.6

0.36

4.61

1.67

15.5

6.1

0.8

Moisture

7.3

11.99

27

0.5

(wt%)

C(%db)

46.9

50.2

52.7

50.64

42.02

47.81

81.5

50.73

H(%db)

5.2

6.2

6.3

5.98

4.97

5.93

7.1

5.71

O(%db)

37.8

43.5

40.8

42.88

42.02

47.81

3.4

41.93

N(%db)

0.1

0.1

0.2

0.05

0.77

0.12

0.5

0.57

S(%db)

0.04

0.03

0.18

0.01

1.4

0.01

Source: Modified from Sami, Annamalai, and Wooldridge, 2001. Co-firing of coal and biomass fuel blends, Prog. Energy Combust. Sci., 27: 171-214.

TABLE 7.2C

Selected Typical Composition of Energy Products and Food Processing Residue

Type

Poplar

Eucalyptus Sugarcane (Grandis) Bagasse

Almond

Shells

Olive Pits

Walnut

Shells

Peach

Pits

Ash(%db)

1.33

0.52

11.27

4.81

3.16

0.56

1.03

C(%db)

48.45

48.33

44.8

44.98

48.81

49.98

53.0

H(%db)

5.85

5.89

5.35

5.97

6.23

5.71

5.9

O(%db)

43.69

45.13

39.55

42.27

43.48

43.35

39.14

N(%db)

0.47

0.15

0.38

1.16

0.36

0.21

0.32

S(%db)

0.01

0.01

0.01

0.02

0.02

0.01

0.05

Source: Modified from Sami, Annamalai, and Wooldridge, 2001. Co-firing of coal and biomass fuel blends, Prog. Energy Combust. Sci., 27: 171-214.

with various types of coals in boilers [8]. Significant new efforts at the labo­ratory level as well as at semi-commercial or commercial scales to gener­ate energy or useful products have been carried out and these are reviewed in this chapter. A recent position paper indicates that co-firing represents among the lowest risk, least expensive, most efficient, and shortest term option for renewable-based electrical power generation [8]. Although in gen­eral co-firing is more expensive than the power plants based strictly on coal, CO2 emission reduction, global climate change mitigation, and sustainable resource management for waste on a large scale are the main motivations behind the development of mixed feedstock strategy.

In this chapter, our discussions on various thermochemical technologies processing a mixed feedstock are divided in two parts: those that are largely generating gases such as combustion, gasification (including high tempera­ture supercritical gasification and reforming technology), high severity (i. e., high temperature and high residence time) pyrolysis, and plasma technology;

and those that are focused in generating liquids such as low severity (i. e., low contact time or low temperature) pyrolysis, liquefaction (either by water or an organic solvent), and supercritical fluid extraction (either by water or other chemical substances). Because biochemical technologies are not extensively used for fossil fuels such as coal, tar sand, shale oil, and the like, their appli­cations for a mixed feedstock are not addressed here. Gasification technolo­gies are used for a mixed feedstock on a larger scale, and the development of liquefaction technologies for mixed feedstock is still at the laboratory and small scale stages.

Combustion, gasification, plasma technology, and high-severity pyroly­sis are commercially proven technologies for a single feedstock, however, improvements are constantly being made to adapt these commercial opera­tions for mixed feedstock. Major challenges in mixed feedstock processing are biomass fuel preparation, storage, and handling [8-10]. Other problems are associated with poor or incompatible fuel quality and these include fuel feeding, co-milling, deposit formation, increased corrosion and ero­sion, and need for new fly ash (and in general slag) utilization schemes [8, 10, 11]. These challenges are not significant in a mixed feedstock with low concentration of biomass but become more important as the concentration of biomass increases, particularly when low-quality biomass is used. In these cases an economy of the overall plant may be significantly affected. As for example, herbaceous biomass and coal are not as good a mixed feedstock as wood chips and coal. Similarly, stringlike biomass (e. g., straw or switchgrass) is not as good as RDF in the form of well-mixed pellets. As shown later, these drawbacks can to a certain extent be avoided by application of appro­priate biomass pretreatments. The present chapter addresses these topics. Also, as shown later, several gasification technologies using mixed feedstock have been demonstrated commercially and these are briefly described. The development of supercritical gasification for a mixed feedstock is only at the laboratory and demonstration levels. Unlike gasification, direct liquefaction technologies are still at a laboratory or a small-scale developmental stage.

Algae Oil Extraction

Three major methods of extracting oil from algae are: (1) solvent extraction with hexane, (2) expeller/press, and (3) supercritical fluid extraction (SFE). The extracted oil can then either be used as SVO or processed by a transester­ification reaction to produce biodiesel. Algal oil is considered a "balanced" carbon neutral fuel, because any CO2 taken out of the atmosphere by the algae is returned when the algae biofuels are burned. It has been estimated

Подпись: Algae Water Suspension FIGURE 2.3 Flottweg two-stage enalgy process of algae harvesting.
by Weyer et al. [31] that the theoretical maximum limit of unrefined oil that could be produced from algae is 354,000 L • ha-1 • yr-1 or 38,000 gal • ac-1 • yr-1, and limits for the practical cases examined in their study range from 46,300 to 60,500 L • ha-1 • yr-1.

Potential Environmental Issues of Liquid Effluents

The production of ethanol also generates liquid effluent, which may ren­der a potential pollution problem or a concern of environmental stress on water systems. About nine liters of liquid effluent are generated for each liter of ethanol produced, which varies depending on the specific process adopted. Some of the liquid effluent may be recycled. Effluent can have a high level of biochemical oxygen demand (BOD), which is a measure of organic water pollution potential, and it is also acidic. Therefore, the liquid effluent must be treated before being discharged into the water stream. Specific treatment requirements depend on both feedstock quality (and type) as well as local pollution control regulations. Due to the acidity of the effluent, precautions and care must also be taken if the effluent is directly spread over fields [18].

Cellulose Hydrolysis

4.5.3.3.1 Cellulase Enzyme Adsorption

The enzymatic hydrolysis of cellulose proceeds by adsorption of cellulase enzyme on the lignacious residue as well as the cellulose fraction. The adsorption on the lignacious residue is also interesting from the viewpoint of enzyme recovery after the reaction and recycling it for use on the fresh substrate. Obviously, the recovery efficiency is reduced by the adsorption of enzyme on lignacious residue, because a large fraction of the total operating cost is due to the production of enzyme. The capacity of lignacious residue to adsorb the enzyme is influenced by the pretreatment conditions, therefore the pretreatment should be evaluated, in part, by how much enzyme adsorbs on the lignacious residue at the end of hydrolysis as well as its effect on the rate and extent of the hydrolysis reaction.

The adsorption of cellulase on cellulose and lignacious residue has been investigated by Ooshima, Burns, and Converse [71] using cellulase from Trichoderma reesei and hardwood pretreated by dilute sulfuric acid with explosive decomposition. The cellulase was found to adsorb on the ligna — cious residue as well as on the cellulose during hydrolysis of the pretreated wood. A decrease in enzyme recovery in the liquid phase with an increase in the substrate concentration has been reported due to the adsorption on the lignacious residue. The enzyme adsorption capacity of the lignacious residue decreases as the pretreatment temperature is increased, whereas the capacity of the cellulose increases with higher temperature. The reduction of the enzyme adsorbed on the lignacious residue as the pretreatment tem­perature increases is essential for improving the ultimate recovery of the enzyme as well as enhancing the enzyme hydrolysis rate and extent. Lu et al. (2002) conducted an experimental investigation on cellulase adsorption and evaluated the enzyme recycle during the hydrolysis of SO2-catalyzed steam — exploded Douglas fir and posttreated steam-exploded Douglas fir substrates [72]. After hot alkali peroxide posttreatment, the rates and yield of hydrolysis attained from the posttreated Douglas fir were significantly higher, even at lower enzyme loadings, than those obtained with the corresponding steam — exploded Douglas fir. This work suggests that enzyme recovery and reuse during the hydrolysis of posttreated softwood substrates could result in less need for the addition of fresh enzyme during softwood-based bioconversion processes [72].

An enzymatic hydrolysis process involving solid lignocellulosic materi­als can be designed in many ways. The common denominators are that the substrates and the enzyme are fed into the process, and the product stream (sugar solution), along with a solid residue, leaves it at various points. The residue contains adsorbed enzymes that are lost when the residue is removed from the system.

In order to ensure that the enzymatic hydrolysis process is economically efficient, a certain degree of enzyme recovery is essential. Both the soluble enzymes and the enzyme adsorbed onto the substrate residue must be reuti­lized. It is expected that the loss of enzyme is influenced by the selection of the stages at which the enzymes in solution and adsorbed enzymes are recirculated and the point where the residue is removed from the system.

Vallander and Erikkson [46] defined an enzyme loss function L, assuming that no loss occurs through filtration:

L _ amount of enzyme lost through removal of residue amount of enzyme at the start of hydrolysis

They developed a number of theoretical models to conclude that an increased enzyme adsorption leads to an increased enzyme loss. The enzyme loss decreases if the solid residue is removed late in the process. Both the adsorbed and dissolved enzymes should be reintroduced at the starting point of the process. This is particularly important for the dissolved enzymes. Washing of the entire residue is likely to result in significantly lower recovery of adsorbed enzymes than if a major part (60% or more) of the residue with adsorbed enzymes is recirculated. An uninterrupted hydro­lysis over a given time period leads to a lower degree of saccharification than when hydrolyzate is withdrawn several times. Saccharification is also favored if the residue is removed at a late stage. Experimental investigations of the theoretical hydrolysis models have recovered more than 70% of the enzymes [46].

Strategies for Waste Management

As indicated earlier, in a new approach to strategic resource management, the concepts of WtP and WtE are implemented for every different type of waste. As shown in Figure 6.2, there are numerous technologies now avail­able to convert waste to heat, electricity, transportation fuels, chemicals, or materials. These technologies are generally broken down into three catego­ries: thermochemical, physicochemical, and biochemical. In each of these categories, a process can be catalytic or noncatalytic. Thermochemical con­version of waste to energy is illustrated in more detail in Figure 6.3 [7, 8]. All advanced technologies such as pyrolysis, gasification, and plasma-based technologies have been developed since 1970 [9]. In the past, thermochemi­cal techniques were predominantly used to generate energy. In recent years, these techniques are also used to generate chemicals and materials [9] via var­ious methods of product upgrading. For example, gasification of biomass can produce syngas which can be further converted to a host of liquid products such as methanol, diesel fuel, gasoline, and jet fuel via Fischer-Tropsch and related syntheses such as methanol and iso — or oxy-synthesis. Liquefaction can produce liquids that can be upgraded by hydro-deoxygenation, hydro­genation, hydrocracking, or catalytic cracking to produce a host of chemicals and fuels. Pyrolysis can produce gases, liquids, or solids depending upon the reaction conditions. The gases can be used as fuel or raw material for polymers such as polyethylene and polypropylene, and others. Pyrolysis oil can also be upgraded to use as fuels or refined into a number of chemicals.

image80

FIGURE 6.2

Waste-to-energy conversion technologies. (Adapted from M. Kaltschmitt, and G. Reinhardt, eds. (1997), Nachwachsende Energietrager-Grundlagen, Verfahren, okologische Bilanzierung, Braunschweig/Wiesbaden, Vieweg Verlagsgesellschaft. With kind permission of Springer Science+Business Media.)

The solid residues from pyrolysis can be important raw materials for the construction and fertilizer industries [10].

Biochemical conversion techniques used to convert lignocellulosic waste can also generate chemicals and materials. The LCM can be fractionated into hemicellulose, cellulose, and lignin by selective solubilization of hemicellu — loses via hydrothermal processing with water or prehydrolysis with externally added mineral acids. The liquor produced by this process can contain oligo­saccharides, fermentable sugar, furfural, low molecular weight phenolics, and levulinic acid depending on the reaction operating conditions. Various prod­ucts can be extracted from the liquor. The sugar can be further fermented to produce a host of alcohols and acids such as ethanol, butanol, xylitol, butane — diol, and lactic acid, among others. The solids coming from the solubilization step mainly contain cellulose and lignin and can be further hydrolyzed by acids or enzymes to give a fermentable glucose solution and a solid phase that largely contains lignin. This solid material can either be used as fuel or a raw material for gasification or pyrolysis. The fermentable glucose solution can be converted to a host of products such as lactic acid, citric acid, succinic acid, itaconic acid, or bioplastics by the suitable fermentation process [10].

The strategy for each type of waste is to apply the appropriate technol­ogy for the desired end product. The choice of the best technology will

image81

FIGURE 6.3

Schematic overview WtE concept. (After Helsen and Bosmans, 2010. Waste to energy through thermochemical processes: Matching waste with process, Conference Proceedings on Enhanced Landfill Mining and Transition to Sustainable Materials Management, Molenheide, Houthalen — Heichteren, Belgium, October 4-6. With permission.)

depend on various factors such as environmental regulations, local eco­nomics, resources available to use the technology, and the market for the end-product. Some of the major technologies are further described later in this chapter.

Handling of Product Streams

The exit gases and solids coming out of any type of gasification unit (com­bustion, gasification, high severity pyrolysis, plasma technology, etc.) should not only meet EPA standards but also facilitate subsequent processing of gases (such as for Fischer-Tropsch) and solids (for the construction and fer­tilizer industries).

7.4.3.1 Syngas Treatment

In a combustion process, gases do not have a significant fuel value (they mostly contain CO2, O2, H2O, and N2) and as shown earlier, the main objec­tive of gas purification is to remove any impurities and particulate materi­als. Earlier sections outline the steps taken by commercial processes for this purpose. The extent to which impurities are present in syngas (by gasifica­tion, high severity pyrolysis, and plasma technology) will be a function of the nature of the process and the nature of the feedstock. The presence of minor impurities such as sulfur, chlorine, ammonia, and soot will, however, be inevitable. Because concentrations of these compounds normally exceed specification of a gas turbine or a catalytic synthesis reactor such as Fischer — Tropsch, methanol, and the like, which process the syngas (see Table 7.9), the gas cleaning is necessary.

The impurities in syngas can be poisonous to the catalysts used in FT and methanol syntheses. The required minimum for these impurities in syn­gas is outlined in Table 7.9. The definition of gas cleaning is therefore based on the economic consideration of investment in cleaning versus accepting a lower catalyst life. Generally, an investment in cleaning is less expensive than replacing expensive catalyst materials. Co-gasification adds another complexity because different products coming out of coal and biomass need to be handled. Additional compounds will include chlorides, sulfur compounds, and very toxic carcinogenic tars (unless tar is recycled back in the reactor such as in the slagging entrained flow reactor). For mixed feed­stock of coal and biomass, higher hydrogen chloride, and more toxic organic compounds need to be treated compared to those found in coal gasification alone. The gas cleaning associated with the mixed feedstock is not the same as that for a single feedstock. As long as there is a single syngas purification system, it needs to handle impurities coming from all components of the mixed feed stock whether the components of mixed feedstock are gasified together or separately.

TABLE 7.9

Allowable Concentrations of Contaminants in Syngas for Catalytic Systems

Syngas Contaminants Maximum Allowable Concentrations

Подпись:Total in each category <1 ppmv

Total in each category <10 ppmv

Almost completely removed Lower the better; however, "loose maximum" CH4 of total up to 15 vol % is acceptable Should be at the level that no condensation occurs when syngas is pressurized to the required pressure of approx. 25-60 atm for Fischer-Tropsch synthesis

Source: Ratafia-Brown et al. 2007. Assessment of Technologies for Co-converting Coal and Biomass to Clean Syngas-Task 2 Report (RDS), NETL report (May 10); Boerrigter, H. et al. 2005. OLGA Tar Removal Technology-Proof of Concept for Application in Integrated Biomass Gasification Combined Heat and Power (CHP) Systems, ECN-C-05-009 (January); and Boerrigter et al. 2004. Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer — Tropsch (FT) Systems, ECN, Petten, The Netherlans, ECN — report number ECN-C-04-056, 59 pp.

Sulfur compounds generally result in the production of sulfur dioxide, and this can be removed by several existing processes. More development is, however, being pursued to remove sulfur at high syngas temperature to improve the energy efficiency of an integrated FT plant. Co-gasification of coal with low sulfur substances such as biomass or waste reduces hydrogen sulfide partial pressure in an FT reactor and hence makes the gas cleaning system larger. The tar from wood gasification and hydrochloric acid from waste gasification may further complicate the issue, as will the presence of liquid products (e. g., tar) in syngas.

One problem for syngas treatment for gasification of mixed feedstock is the production of a relatively large amount of tar and the possibility of this condensing as the gas is cooled. For biomass, tar condensation occurs in the temperature range of 200-500°C and this can rapidly blind the filters. The tar generation is not a problem for an entrained bed gasifier because the tem­perature in such a gasifier is greater than about 2,300°F and tar cracks at such a high temperature. In processing a mixed feedstock, the composition and rates of feedstock should be designed such that the tar formation is not an issue [111]. For an entrained bed gasifier, the biosyngas is cleaned with standard techniques used for fossil syngas: dust filters, wet scrubbing tech­niques for the removal of NH3 and HCl, and zinc oxide (ZnO) filters for the removal of H2S. After adjusting the required H2/CO ratio and CO2 removal (which may require a steam or a tri-reforming step), the gas is compressed
to the required FT synthesis pressure (approximately 40 atm) and fed to the FT reactor.

In other types of gasifiers where generally the temperature in the gasifier is low, tar in the exit gas stream needs to be handled. For example, in a CFB gasifier, tar is removed by either installing a tar cracking unit operated at 1,300°C or by using OLGA tar removal technology. In a tar cracker all organic compounds in the product gas (i. e., tar, BTX, CH4, and C2-hydrocarbons) are destroyed to produce additional syngas. In the OLGA technique, tar and BTX are separated and recycled back into the gasifier until they are extinct. Lower hydrocarbons are not removed by OLGA technology. Hence, in this case wet cleaning and the filter process are followed by a reforming step to convert lower hydrocarbons into additional syngas. There are reports [109, 112] that the tar problem can also be reduced by an addition of dolomite in the gasification process. In both cases, the gas purification step is followed by a gas conditioning step before gas is introduced in the FT reactor.

In co-gasification, gas is used for further conversion to products via FT and methanol synthesis, and solids are directly used for industrial purposes. The exact species formed from the ash constituents when biomass is gas­ified depends on the reactor temperature and oxygen partial pressure. For example, in the IGCC system, the sulfur species will be present as sulfides rather than as sulfates (alkalis and alkaline earths as sulfides; Fe as FeSx), whereas Al— and Si—containing species probably would present as oxides. This means the gas coming from the gasifier may encounter contaminants in their reduced form. To some extent this also depends upon the interac­tion between solids and gas as they travel through the reactor. It is therefore important to monitor gas and solids at several locations within the various process streams.

Feedstock Preparation and Pretreatments

Pretreatment processing usually takes place in the prescreening of feed­stock, where unsuitable materials are screened out. Typical pretreatment operations include drying, devolatilization, size reduction and particle size classification, torrefaction, pelletizing, and mixing.

1.1.1 Chemical and Biochemical Reactions

Highly oxygenated and high molecular-weighted biomass organic materials need to be converted to lighter molecular-weighted and simpler chemicals via a variety of targeted and accompanying chemical reactions, depending upon the desired outcomes. Such reactions include complete combustion, partial oxidation, pyrolysis, gasification, reformation, hydrocracking, lique­faction, hydrolysis, enzymatic hydrolysis, fermentation, and more. The reac­tions listed here are not single chemical reactions and each is rather a class of chemical reactions. For example, gasification includes steam gasification, hydrogasification, carbon dioxide gasification, partial oxidation, and a com­bination of these. Conventional process technologies are more applicable to first-generation biofuels, whereas more advanced process technologies are being developed for second-generation biofuels.