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WORKING PAPER 2019-13
Advanced alternative fuel pathways:
Technology overview and status
Authors: Chelsea Baldino, Rosalie Berg, Nikita Pavlenko, Stephanie Searle
Date: July 19, 2019
Keywords: Thermochemical conversion; biochemical conversion; non-food feedstock; lignocellulosic biomass;
biofuels; renewable fuel; electrofuels
Introduction and summarized in this paper. For Fischer-Tropsch (FT) synthesis and
this figure, as well for all the follow- methanation, before it can be used as
Policymakers recognize the signifi-
ing figures in this paper that illustrate a transportation fuel. Hydrothermal
cant greenhouse gas (GHG) savings
the individual conversion technology liquefaction and fast pyrolysis are
that are possible by transitioning from
pathways in more detail, yellow boxes other primary conversion technolo-
first-generation, food-based biofuels indicate processes and green boxes gies that process biomass into crude
to alternative, non-food based fuels, represent an input to the process. oils, which are then fed into hydropro-
i.e., advanced alternative fuels. First- Blue boxes represent the desired cessing or other upgrading technolo-
generation biofuels are produced by fuel output, whereas gray boxes are gies to produce drop-in fuel.
converting readily available biomass intermediary products. Bold arrows
chemicals—sugars, starch or oils—into indicate primary steps in the conver- This paper synthesizes the best avail-
transportation fuels such as ethanol sion process; narrow arrows indicate able information on the feedstocks
and fatty acid methyl esters (FAME). less common, or less important, steps. that are or could be suitable for each
Advanced biofuels, on the other hand, The black, dashed box groups pro- pathway. Overall, these feedstocks
are those produced by converting cesses that can be applied to the include materials such as lignocel-
additional chemicals contained in oxygen and hydrogen that is produced lulosic biomass, municipal or indus-
the biomass feedstocks, specifically in the PtX pathway. PtX refers to elec- trial waste streams, and waste oils
hemicellulose and cellulose, which are trolysis followed by upgrading. and fats. Lignocellulosic biomass is
more difficult to extract and convert organic material with a high cellulose
into transport-grade liquid fuels. For This paper reviews primary conver- and lignin content. It includes agri-
this reason, more complex, advanced sion technology pathways, such as cultural residues such as wheat straw
conversion techniques are required. electrolysis and gasification, which and corn stover, forestry residues,
Advanced fuels also can include non- convert water and biomass, respec- and purpose-grown energy crops
biological pathways, such as power- tively, into gases, as well as cellulosic such as switchgrass and poplar. In
to-liquid or power-to-gas, which are ethanol conversion. In some cases, the case of PtX, the feedstocks are
generically referred to as PtX fuels or such as in the production of cellulosic electricity and carbon dioxide (CO2).
electrofuels. This paper provides an ethanol, the finished product from Also described is any pretreatment,
overview of some of the most promis- the pathways addressed in this paper chemical or physical, necessary to
ing advanced alternative fuels produc- can be used directly as transporta- prepare the feedstock for the conver-
tion pathways. tion fuel. In most cases, however, the sion processes.
liquid or gas carrier that is produced
Figure 1 presents the conversion tech- from the pathway requires further Although technical assessments for
nology pathways that are assessed processing and upgrading, such as each of these conversion technology
Acknowledgements: This work was generously supported by the David and Lucile Packard Foundation. Thanks to helpful reviews from Nic Lutsey, Jacopo
Giuntoli, Adam Christensen, Bruno Miller, Rob Conrado, Dave Newport, Arne Roth, Alex Menotti, and Steven Gust.
© INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION, 2019 WWW.THEICCT.ORGADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
CO2 or CO
Legend
Oxygen Process
Methanol
Methanol
Synthesis
Water Electrolysis Input
or
Hydrogen Intermediary
Methanation Methane
Product
or
Wax Desired
FT Product
Synthesis
Biomass Other
Gasification Syngas Hydro-carbons
or Wastes
Gas
Fermentation
Cellulosic Ethanol
Ethanol
Conversion
Hydro-
thermal Bio-Crude
Liquefaction
Hydro-processing
Fast Drop-in
Bio-Oil and Other
Pyrolysis Fuels
Upgrading
Waste Oils
& Fats
Figure 1: Simplified overview of the conversion pathways reviewed in this paper. Primary steps are highlighted with bold arrows. The
black, dashed box groups processes that can be applied to the oxygen and hydrogen that is produced in the PtX pathway.
pathways are available in the litera- Cellulosic Ethanol polymer chains that form the physical
ture, there is no study that the authors structure of plants. The process of
Conversion
know of that includes all of these converting lignocellulosic biomass
pathways together. This overarching to ethanol is called cellulosic ethanol
OVERVIEW OF CONVERSION
study summarizes how these tech- conversion. Cellulose is trapped inside
PROCESS
nologies convert biomass and wastes the lignin, making it more difficult
into fuel and assesses their com- Ethanol is conventionally made primar- to convert to ethanol compared to
mercial potential, citing examples, ily from sugars and starches obtained first-generation ethanol production
if they exist. It also addresses the from food crops, such as corn, wheat, from starches or sugars. Cellulose and
major costs associated with these sugarcane, and sugar beet. These hemi-cellulose are first broken down
pathways, when information is avail- sugars and starches can be easily into monomeric sugars by enzymes,
able. Obstacles that might inhibit converted into ethanol by microbes, and the sugars are then fermented
commercialization are reviewed for such as yeast. Lignin and cellulose, to ethanol by yeasts. Lignin is a by-
each of the technologies. in contrast, consist of long organic product of this process that is typically
2 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
combusted on-site for electricity gen- There are different feasible pretreat- route, and is described in the sections
eration, although the use of lignin to ment methods for various kinds of on Gasification and Gas Fermentation.
extract chemical products with higher biomass due to differences in physical The biochemical route uses a com-
added-value is being explored in bio- structure, lignin content, and other bination of either enzymes or acids
refinery concepts. considerations (Maurya, Singla, & Negi, to convert the pretreated cellulosic
2015). In particular, the pretreatment biomass to ethanol; this pathway
Feedstocks utilized process for woody biomass differs includes three steps: hydrolysis, fer-
substantially from that for agricultural mentation, and distillation.
Within lignocellulosic biomass, the cel-
biomass (Limayem & Ricke, 2012).
lulose is bundled into structures, called Figure 2 illustrates the steps neces-
Various pretreatment techniques have
microfibrils, that are surrounded by sary to convert biomass inputs into
lignin and attached to each other by been developed, each with its own
cellulosic ethanol combustion fuel:
hemicellulose. Typical proportions are benefits and drawbacks, as addressed
pretreatment, hydrolysis, fermen-
40%–50% cellulose, 25%–30% hemi- in the Obstacles to Commercialization
tation, distillation and drying, and
cellulose, and 15%–20% lignin (Menon section below. These include the
separation of distillation. Cellulosic
& Rao, 2012). Cellulose is a polymer application of physical processes
ethanol production requires external
of glucose, which means it is made (e.g., particle size reduction through
heat and energy, but in this figure
of many glucose molecules bound grinding or steam explosion), chemi-
we show only where there is poten-
tightly into chains. Hemicellulose also cals (e.g., sulfuric acid), physicochemi-
tial for recycling or export of energy.
is a polymer of sugars, but a mixture cals (e.g., liquid hot water combined
As shown, along with the primary
of sugars, including six-carbon sugars with ammonium fiber explosion,
product of ethanol, a secondary,
(such as glucose) and five-carbon where high-pressure, liquid ammonia
desired product—biogas, which is
sugars (such as xylose). is applied and then the pressure is
shown in blue—can also be produced.
explosively released), and biological
Pretreatment agents (e.g. white-rot or brown-rot Hydrolysis breaks down cellulose
fungi and bacteria), or combinations into free sugars in order to make the
Pretreatment is the first step in the of these (Bensah & Mensah, 2013). glucose within the cellulose acces-
conversion of lignocellulosic biomass sible for fermentation (see Figure 2).
into cellulosic ethanol (see Figure 2).
Chemistry of the conversion This process is also called sacchari-
Pretreatment alters the lignin and fication or cellulolysis. Hemicellulose
hemicellulose structures, exposing Lignocellulosic biomass can be con-
has more complex sugar polymers as
the cellulose to the action of enzymes verted into ethanol through either
well, but the use of enzymes to cleave
and reducing particle size to maximize thermochemical or biochemical
these sugar polymers has been found
surface area to mass ratio; this mini- pathways. In both routes, the recal-
to be cost-prohibitive (Limayem &
mizes energy consumption and allows citrant structure of lignocellulose is
Ricke, 2012).
for maximal sugar recovery (Tong, broken down into fragments of lignin,
Pullammanappallil, & Teixeira, 2012; hemicellulose, and cellulose; the hemi- Breaking down cellulose through
Limayem & Ricke, 2012). Further, agri- cellulose and cellulose are then con- hydrolysis can be done using either
cultural and forestry residues often are verted into sugars and then ethanol. sulfuric acid or enzymes. Sulfuric
collected from the ground and there- The thermochemical route, in which acid can be used in either diluted
fore contain soil and other unwanted the biomass is gasified into syngas—a or concentrated form; concentrated
materials that must be removed, mixture of gases, primarily hydrogen acid hydrolysis is more common and
usually by washing, so that they do not and carbon monoxide, that also can considered to be more practical. One
interfere with the conversion process include carbon dioxide and methane— drawback is that undesirable degrada-
(U.S. Government Accountability and then converted into ethanol, is far tion products, such as aldehyde, form
Office, 2016). less common than the biochemical during acid hydrolysis.
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 3ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
Cellulase
Micro-organisms
Ligno-cellulosic Enzymes &
(e.g. Brewer’s Ethanol Co-products
Biomass Water, or
yeast)
Sulfuric Acid
Pretreatment: Separation of
Mechanical, Distillation & Distillate;
Hydrolysis Fermentation
Chemical Drying Recovery of
and/or Fungal Residue
Legend
Process
Input Heat & Lignin
Waste-water
Energy Residue
By-Product
Internal
Energy
Anaerobic
Desired Biogas Combustion
Digestion
Product
Figure 2: Simplified overview of cellulosic ethanol conversion. Primary steps are highlighted with bold arrows. Dashed arrows indicate
optional processes. Adapted from from Limayem & Ricke (2012), Baeyens et al. (2015), and Humbird et al. (2011).
In the enzymatic hydrolysis process, consumed as soon as it is produced, in regions where ethanol is used as
cellulases, usually a mixture of several allowing cellulolysis to proceed unin- a blend it is impractical because
enzymes belonging to three different hibited. SSF also has the advantage of the water content inhibits blending
groups (endoglucanase, exogluca- requiring a single reaction vessel for with gasoline (Volpato Filho, 2008).
nase or cellobiohydrolase, and b-glu- the two processes, instead of one for To remove the remaining water, pro-
cosidase), are used to break down each. However, saccharification and ducers use more energy intensive
the cellulose. Once the cellulose fermentation have different optimal distillation methods or adsorption
has been broken down into glucose temperatures, so for this factor a com- columns (Vane, 2008). In an adsorp-
molecules, the fermentation process promise must be made. tion column, the ethanol is passed
uses microorganisms to consume across a material called a molecu-
glucose and produce ethanol as an After fermentation, the ethanol is sep- lar sieve, which selectively attracts
end product (see Figure 2). These arated from the yeast solids and most water, leaving dehydrated ethanol.
microorganisms include brewer ’s of the water by distillation (University The molecular sieve is then regener-
yeast (Saccharomyces cerevisiae) and of Illinois Extension, 2009). To do this, ated, which is to say dried, resulting in
Escherichia coli. the broth that comes from fermen- a cyclic operating scheme.
tation is heated, and ethanol, which
The accumulation of the sugar end evaporates more readily than water, It is also worth noting that one of the
products in the enzymatic hydrolysis is collected and cooled. However, it is end products of the biochemical con-
process inhibits the activity of the impossible to achieve perfect separa- version of lignocellulosic biomass into
cellulase enzymes and slows down tion using standard distillation; there- ethanol, whether acid or enzymatic, is
the reaction. To counter this, in some fore, the product of distillation, called lignin, which can be combusted and
systems, the hydrolysis and fermen- hydrous ethanol, still contains about converted to electricity and heat. The
tation steps are done together in 5% water. economics of cellulosic ethanol pro-
simultaneous saccharification and duction could also be improved were
fermentation (SSF) processes (Sun & Hydrous ethanol is sometimes used lignin to become a higher-value end
Cheng, 2002). This way the glucose is as a fuel in Brazil (E100), although product.
4 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
COMMERCIAL POTENTIAL least one has been shut down. These (Ethanol Producer Magazine, n.d.-b).
facilities use agricultural waste, energy Woodland Biofuels has a demonstra-
Commercial cellulosic ethanol produc-
crops, woody biomass, or a combina- tion plant in Sarnia, Ontario, using
tion generally falls into two categories:
tion of all three. POET-DSM Advanced woody biomass with a nameplate
smaller, add-ons to existing, first-gen-
Biofuels operates a cellulosic ethanol capacity of 2 million liters per year
eration ethanol facilities using starch
plant in Emmetsburg, IA, with a name- (Ethanol Producer Magazine, n.d.-a).
or sugar crops, or larger, stand-alone
plate capacity of approximately 76 American Process Inc. Biorefinery has
projects. Add-on facilities are some-
million liters per year using corn cobs a facility in Thomaston, GA, with a
times called “bolt-on” facilities, pri-
and corn stover as feedstock (Ethanol nameplate capacity of approximately 1
marily using cellulosic waste from the
Producer Magazine, n.d.-b). Recently, million liters per year using sugarcane
conventional ethanol facility. Bolt-on
POET filed suit against an engineering bagasse and woody biomass (Ethanol
facilities have generally had more
company for building an unsatisfactory Producer Magazine, n.d.-b). The facility
success ramping up production than
pretreatment process; in 2010 POET also is manufacturing nanocellulose
stand-alone facilities. Quad County
contracted with Andritz, Inc. to build a for use as a material in tires and other
Corn Processors is an ethanol plant in
biomass pretreatment unit, but POET consumer products (Lane, 2017a).
Galva, Iowa, with a bolt-on cellulosic
claims that the process never worked Production of co-products such as
ethanol process that uses corn kernel at commercial scale, despite one and these may improve the economic fea-
fiber sourced from an adjacent first- a half years of redesigns and multiple sibility of cellulosic ethanol production.
generation ethanol facility (Ethanol plant shutdowns (Ellis, 2017; Sapp,
Producer Magazine, n.d.-b). The Quad 2017). DuPont operated a plant in In Europe, Beta Renewables operated
County facility’s nameplate capacity Nevada, IA, with a nameplate capacity a plant in Crescentino, Italy, until its
is approximately 8 million liters per of 114 million liters per year using corn closure in late 2017. It has a name-
year of cellulosic ethanol, which is far stover until the plant was shut down in plate capacity of 50 million gallons
less than the 130 million-liters-per-year late 2017 (Ethanol Producer Magazine, per year and came online in 2012,
capacity of the adjacent corn ethanol n.d.-b). DuPont cited its merger with but it was never able to reach this
facility (Lane, 2014b). ICM Inc. is oper- Dow as the reason for the decision capacity. Beta Renewables utilized
ating a pilot bolt-on facility at a con- and is seeking a buyer for the facility wheat straw, rice straw, and Arundo
ventional ethanol plant in St. Joseph, (Scott, 2017). POET and DuPont were donax (ETIP Bioenergy, n.d.-a; Schill,
MO, with a nameplate capacity of both operating well below capacity as 2016). Clariant has a plant in Germany
nearly 1 million liters per year (Rivers, of April 2016 (Rapier, 2016). producing ethanol from agricultural
2015). The operators claim to have wastes (Clariant, 2017). The nameplate
successfully run their unit with corn At a smaller scale, several U.S. pro- capacity of the plant is approximately
kernel fiber and switchgrass. Ethanol ducers also have developed demon- 1 million liters per year (Clariant, n.d.).
producer Raízen Energia Participacoes stration projects for cellulosic ethanol
SA, using Iogen Energy cellulosic conversion at stand-alone facilities.
biofuel technology, has a $100 million Fiberight planned to convert post- OBSTACLES TO
ethanol production plant utilizing sug- recycling municipal solid waste (MSW) COMMERCIALIZATION
arcane bagasse located adjacent to a into cellulosic ethanol, but after seven
Cost
sugarcane mill in São Paulo. In 2016, years of research and development,
the plant produced 7 million liters of the company decided to instead The conversion process for cellulosic
next-generation ethanol. The facility focus on producing biogas in lieu of ethanol remains costly. Limayem and
is slated to produce 40 million liters cellulosic ethanol. Statements from Ricke (2012) emphasizes that while
per year of cellulosic ethanol using Fiberight suggest that securing invest- technology continues to improve,
sugarcane bagasse and straw (Iogen ment for MSW-to-ethanol is very dif- advanced fuels still require “the most
Corporation, n.d.). ficult (Lane, 2015b). ZeaChem has a advanced systems analysis and eco-
demonstration plant in Boardman, OR, nomical techniques designed to cope
Several large, stand-alone cellulosic using poplar, straw, and corn stover. with feedstock versatility and com-
ethanol facilities are in development, The Boardman facility has a nameplate modity.” Some research suggests that
in early operational phases, and at capacity of 946,000 liters per year cellulosic ethanol production may
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 5ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
be cheaper than other production and does not produce inhibitory com- during SSF, and actively removing it
processes using lignocellulosic feed- pounds but requires 4 to 6 weeks and improved the economics of high-solids
stocks. For example, Peters, Alberici, thus incurs costs related to storage loading processes.
Passmore, and Malins (2015) estimated space for that time period (Sun &
the costs for cellulosic ethanol to be Cheng, 2002). Cost of enzymes and other
lower than for the gasification/Fischer- chemicals
Tropsch process and hydrotreated There is also potentially promising
research on genetically modifying Both acid and enzyme hydrolysis
pyrolysis oil process, although the
plants to reduce the difficulties of pro- require expensive materials. Acid
authors emphasize that this finding is
cessing lignin, either through modify- hydrolysis requires large quantities of
indicative and not absolute.
ing the lignin’s chemical structure or acid, which can be costly to purchase
or recycle within the process. For
Recalcitrance of lignin and reducing the lignin content. This could
these reasons, research and develop-
hemicellulose be achieved, for example, through
ment has focused mainly on enzy-
down-regulating lignin biosynthesis
Pretreatment is the most expensive matic hydrolysis for the commercial
enzymes in the plant or shifting the
step in cellulosic ethanol production production of cellulosic ethanol,
plant’s energy from lignin biosynthesis
because lignocellulosic biomass is but cellulase enzymes also can con-
to the synthesis of polysaccharides
highly recalcitrant, which is to say, hard tribute significantly to the ongoing
such as sugars and starches (Mood et
to break down. Many lignocellulosic operating cost of a plant (Tong,
al., 2013).
feedstocks also are heterogenous and Pullammanappallil, & Teixeira, 2012).
their characteristics can change over Enzyme costs fall in the range of $5
High-solids loading
time, so it is important for pretreat- to $20 per kilogram (Johnson, 2016;
ment methods to be flexible and effec- Solids loading refers to the ratio at Liu, Zhang & Bao, 2015).
tive across a range of physical and which solids (i.e., biomass) are added
to the system, relative to the water that To minimize enzyme consumption,
chemical characteristics (Limayem &
is needed for the fermentation broth. A manufacturers have options such as
Ricke, 2012). However, all of the avail-
higher ratio of solids to liquid trans- recycling enzymes for use in subse-
able pretreatment options have limita-
lates to reduced water needs, smaller quent cycles (Lu, Yang, Gregg, Saddler,
tions related to cost, effectiveness, or
tank sizes, and lower distillation costs. & Mansfield, 2002). One study reports
other problems.
However, increasing solids loading that this technique can reduce enzyme
Mechanical means of pretreatment results in decreased sugar yields in costs by 50% (Du, Su, Zhang, Qi, &
can be prohibitively energy intensive. the hydrolysis step and decreased He, 2014). Another solution is to use
Using heat or chemicals to remove ethanol yields in the fermentation step. additives, such as proteins called car-
lignin and break down biomass can Kristensen, Felby, & Jørgensen (2009) bohydrate binding modules (CBMs),
be effective and is a common method. found that there is a linear, inverse cor- that can enhance the enzyme activity
Chemical pretreatment of lignocel- relation between conversion efficiency (Chundawat et. al., 2011).
lulosic biomass forms the basis of and solids concentrations between
several proprietary cellulosic ethanol 5% and 30% initial total solids content C5 sugar content
production configurations and tech- by weight (w/w). Ongoing research is Most yeasts and microbes metabolize
nologies (Bensah & Mensah, 2013). attempting to determine the drivers of glucose and other six-carbon sugars;
However, chemical pretreatment tends this relationship with an aim to identi- however, hemicellulose is mostly made
to produce compounds such as furfural fying solutions to increase sugar and of five-carbon (or C5) sugars. C5
that are toxic and inhibit microbial fer- ethanol yields. For example, Kristensen sugars, such as xylose and arabinose,
mentation. Organic solvents such as et al. (2009) found that hydrolysis are not digested by many organisms;
ammonia can avoid this problem but products, such as glucose, inhibit the consequently, enzymes that hydro-
are relatively expensive and require adsorption of cellulase at higher loads. lyze these sugars are less common.
solvent recovery systems. Biological In addition, Jin et al. (2017) found that Low-cost enzymes that can hydrolyze
pretreatment using white rot or other the presence of ethanol was the major C5 sugars have not been identified,
fungi has low energy requirements cause of decreased sugar conversion and thus hemicellulose is currently
6 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
cost-prohibitive to process into ethanol. It also can be used to make liquid and particle size and the additional energy
Additionally, research has shown that gaseous hydrocarbon fuels, such as required to achieve that particle size.
the presence of some of the C5 sugars, diesel, methane, and ethanol. Some gasifiers require a consistent
such as xylose, can inhibit the efficacy energy density of the feedstock, while
of cellulase enzymes. These issues are Feedstocks utilized others can tolerate variation. For those
seen with all lignocellulosic feedstocks that require consistent energy density,
Gasification can use a wide variety
(Limayem & Ricke, 2012). Research in torrefaction is another pretreatment
of feedstocks, including agricultural
this area focuses mainly on developing option that processes solid biomass at
residues, organic fractions of MSW,
genetically modified microbes that can low temperatures (ca. 250°C–300°C)
forest residues, lignocellulosic energy
digest xylose and arabinose (Agbogbo in the absence of oxygen. Torrefied
crops, glycerin, tall oil pitch, and black
& Coward-Kelly, 2008). Taurus Energy biomass is dry, has higher and more
and brown liquor that are residues of
has developed a strain of yeast called consistent energy density and is easier
pulp-making. Wood and lignocellulosic
XyloFerm that is being tested at the to grind than untreated biomass,
residues from forestry and agricul-
Quad County facility in Iowa (Lane, thus behaving similarly to lignite coal
ture are the main feedstocks currently
2017c). Inbicon also developed a C5 fer- (Phanphanich & Mani, 2011).
used by 80% of the commercial and
mentation technology (Ørsted, 2013). operating biomass gasification plants MSW is inherently heterogeneous, as
(Bermudez & Fidalgo, 2016). both household and commercial waste
Gasification streams contain a variety of waste
Pretreatment products including paper products,
OVERVIEW OF CONVERSION All feedstocks must be processed into food waste, and yard trimmings. For a
PROCESS a dry, more uniform material through feedstock like MSW, feed handling at
pretreatment. The objective of pre- a conversion facility must be flexible
Gasification of coal is already widely and able to accommodate a set of het-
treatment is to produce input materials
used as a power generation technol- erogeneous feedstocks. Depending on
for the gasifier with uniform physical
ogy. Increasingly, biomass is also being the gasification process selected, the
properties, high energy density, and
gasified for power, either alone or fraction of non-organic components in
low moisture content. Biomass also
co-fed with coal. In the presence of the MSW such as metal and glass will
can be perishable, and some of these
oxygen, but less than what is needed need to be sorted out and minimized
pretreatment steps stabilize the
for combustion, gasification converts in the feed to the reactor.
biomass so it can be stored or trans-
b i o m a ss o r o rg a n i c wa ste s — fo r
ported more easily. The ideal moisture
example, the organic fraction of MSW— Chemistry of the conversion
content for feedstocks for gasification
into syngas. This syngas is a mixture of
is between 10% and 15% by weight Figure 3 illustrates the steps neces-
hydrogen (H2), carbon monoxide (CO),
(Bermudez & Fidalgo, 2016). Besides sary to convert biomass inputs into
and CO2. Simply, gasification is
drying, other preprocessing steps may syngas using gasification. Gasification
include particle size reduction and requires external heat and energy, but
Biomass+ O2 + Heat à H2 + CO + CO2 + compaction of the feedstock; these in this figure we only show where there
Hydrocarbons + H2O + CH4 + Tar + Char are dependent on the specific type is potential for recycling or export of
of gasification technology employed. energy. After the feedstock has been
Ta r i s t h e u n co nve r te d o rg a n i c Research suggests that gasification pretreated, it is gasified and then
material produced after biomass has of solids works better with smaller cleaned; the primary output of this
been devolatilized during gasification. particle sizes, so, depending on the process is syngas, some of which can
During devolatilization, tar is released specific gasification technology, the be combusted on-site for energy.
in gaseous form but is condensable biomass must be ground, milled,
at lower temperatures. Char is a high- or shredded to an appropriate size There are two basic categories of
carbon solid by-product of pyrolysis, (Gaston et al., 2011). At the same gasifiers: partial oxidation and steam
gasification, and incomplete combus- time, size reduction takes energy, and reforming. For partial oxidation gasifi-
tion of biomass. Syngas can be com- there is a trade-off between improved cation, the biomass enters the gasifier
busted for electrical power generation. gasification efficiency with smaller along with an oxidizing gas, typically
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 7ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
Legend
Water, Tar,
Biomass or Oxidizing Gas Sulfur or Process
Wastes or Steam Nitrogen-Containing
Compounds Input
By-Product
Internal
Energy
Cleaning
Pretreatment Gasification Syngas
(Hot or Cold)
Desired
Product
On-Site
Ash Energy Combustion
Recovery
Figure 3: Simplified overview of gasification.
oxygen, air, steam, or a combina- which are devices that transfer heat to heat up, carbon-containing mol-
tion thereof. The choice of oxidizing between mediums by conduction. ecules (i.e., lignin, cellulose, and
agent depends on the quality required their decomposition products)
for the syngas and on the operating There is a fundamental trade-off break down into gaseous compo-
conditions. The quantity of oxygen between using oxygen or air as the nents (i.e., CO2, oxygenated vapor
introduced is insufficient to combust gasifying agent. Purifying oxygen from species) and condensable vapors
the biomass. Complete oxidation (i.e., air requires additional expense, but air (i.e., char, primary oxygenated
combustion) occurs if there is a perfect has a very large fraction of nitrogen, liquids, and water).
match between (a) the ratio of the an inert gas. Bermudez and Fidalgo
(2016) report that for partial oxidation 3. Gasification: The remaining char
actual amount of oxygen and carbon
gasification reactions, oxygen gasifica- reacts with CO2, water, and oxygen
from the biomass in the reactor and
tion is more favorable than air gasifi- in the presence of heat to form CO,
(b) the stoichiometric ratio between
cation because it allows for smaller H2, and methane (CH4 ). The vola-
oxygen and carbon that is necessary
downstream equipment and facilitates tiles from the previous step may
for complete oxidation. This match
the removal of CO2 from the syngas be converted into fuel gases by
is expressed in an equivalence ratio, the secondary reactions of com-
product, which is required for proper
which divides the first factor by the bustion and reforming (Bermudez
synthesis in a Fischer-Tropsch reaction.
second. The equivalence ratio is there- & Fidalgo, 2016).
fore approximately 1 for complete Several distinct reactions occur in
oxidation. Gasification usually is con- the gasifier, throughout either steam The gasifier output is a mixture of
ducted with an equivalence ratio reforming or partial oxidation (see hydrogen and carbon monoxide, the
between 0.2 and 0.4 (Bermudez & two primary energy carriers, along
Figure 3):
Fidalgo, 2016). The reaction usually with some combination of water,
occurs at temperatures between 800 1. Dehydration: The high heat of the methane, carbon dioxide, ash, tar, and
and 1300°C. process evaporates any moisture sulfur- or nitrogen-containing com-
still present in the feedstock. This pounds (National Energy Technology
In steam reforming, the feedstock steam serves a purpose in subse- Laboratory, n.d.). Ash composition
enters the gasifier along with steam, quent reactions. varies depending on the feedstock, but
and the endothermic energy required it usually contains some trace elements,
for the reaction is provided directly 2. Pyrolysis (also known as devolatil- such as potassium, calcium, and phos-
or indirectly through heat exchangers, ization): As the biomass continues phorus. The ratio of CO and H2 depends
8 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
on the type of substrate and gasifica- whereas disadvantages include sen- of solids; complex operation; and a
tion conditions (Du et al., 2016). sitivity to the quality and size of the medium amount of tar yield compared
feedstock; feedstock size limits; low to other reactors. Circulating fluidized
A gasification reaction is endother- energy efficiency; inability to scale bed gasifiers are also best for medium-
mic, which is to say it requires heat up; and risks of corrosion, explosions, to large-scale gasification processes,
inputs from outside the system, but and fuel blockages. Minimizing tar are flexible across varying feedstock
the process can be designed so that production is important for minimiz- characteristics, and can be constructed
the required energy is provided com- ing impacts on gasifier machinery and compactly. Compared to bubbling
pletely by the recirculation of syn- for increasing product yield, because fluidized bed reactors, they have
thesis gas, the recirculation of tail tar is essentially unconverted carbon. several advantages, such as greater
gas from the Fischer-Tropsch process Updraft reactors’ advantages include gas-solid contact. Their disadvantages
downstream, or the partial combus- their simple design; suitability for include medium tar yield and complex
tion of the solid fuel (Bermudez & biomass with high moisture content, operation, as well as the potential for
Fidalgo, 2016). low volatility, high ash content, and damages due to corrosion and attri-
a variety of particle sizes; low char tion (Bermudez & Fidalgo, 2016).
Gasification technology formation; and high energy efficiency.
Disadvantages include risk of explo- In an entrained flow reactor, very finely
Several different reactor configurations
sions, fuel blockages, and corrosion, dispersed feed is added together with
can be used for gasification. The main the oxidizing gas. The small particle
as well as high tar yield (Bermudez &
types are fixed bed, entrained flow, size allows the reactions to occur much
Fidalgo, 2016).
and fluidized bed gasifiers (Bermudez more quickly and allows high conver-
& Fidalgo, 2016). Except for fluidized In fluidized bed reactors, the biomass sion efficiency with low tar production.
beds, which are essentially isothermal, is either directly injected into the hot An entrained flow reactor operates
there are different zones with varying fluidized bed or mixed with inert (or at very high temperatures, higher
temperatures and material composi- sometimes catalytic) bed material than those in a fluidized bed reactor,
tions where the conversion reactions such as quartz sand or dolomite, which causing ash to melt into slag that is
described above take place. The fosters heat transfer so that there is then collected from the bottom of the
reactor types differ in how and where a uniform temperature in the conver- reactor. Of the three primary gasifier
the feedstock and gasifying agents are sion zone. This gasifier configuration configurations, an entrained flow
introduced and in how ash is handled. is therefore able to process feedstocks reactor is the most expensive reactor
with varying qualities. Contrary to to operate, requiring high amounts of
Fixed bed reactors, also called moving
the different zones in the fixed bed oxidizing gas and high temperatures. It
bed reactors, follow the same basic
reactor, drying, devolatilization, oxida- also requires a biomass feedstock that
co n f i g u ra t i o n a s co m m o n b l a st
tion, and gasification occur simultane- is brittle enough to be broken down
furnaces. Feedstock is introduced at
ously and homogenously, producing to the necessary particle size, such as
the top and moves down through the
a synthesis gas with relatively high torrefied biomass. Despite these draw-
vessel through four primary zones: (1)
heating value. These kinds of reactors backs, the clean syngas it produces
drying, (2) carbonization, (3) gasifi-
generally perform better than fixed makes this technology promising
cation, and (4) combustion. Feed is
bed gasifiers. There are two main con- for use with biomass (Bermudez &
added to the top of the reactor in gen-
figurations of fluidized bed reactors: Fidalgo, 2016).
erally large pieces, i.e., coarse particle
bubbling fluidized beds and circulating
size, forming a bed inside the reactor. fluidized beds. Before the syngas can be used for
The oxidizing gas can be added from fuel production, it must be purified
below the bed (updraft reactor), from Bubbling fluidized bed gasifiers are and upgraded in order to prepare the
the side (cross draft reactor), or from among the most popular for biomass proper gaseous mixture for the down-
just below the top (downdraft reactor). gasification. Their advantages include stream synthesis. Syngas cleaning can
The oxidation reaction occurs nearest flexibility regarding feedstock charac- occur via two routes, known as hot
to where the oxidizer is added, with teristics (e.g., ash content and particle and cold (see Figure 3). The cold route
the heat from that reaction powering size) and feed rate, and they can be is more developed, but the hot route
the others. Downdraft reactors’ advan- constructed compactly. Disadvantages offers better energy efficiency if the
tages include their simple design and include potential back-mixing, which syngas will be used downstream at a
low tar and particulate formation, l i m i t s t h e co nve r s i o n e f f i c i e n cy high temperature. Current research
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 9ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
focuses on how to better implement to produce hydrogen and carbon a range of equipment costs that fall
this route. Cleaning removes tar, sulfur dioxide. Catalysts are used, typi- between $2,540 and $3,860 per kW.
(in the form of H2S), hydrochloric acid cally an iron oxide-based catalyst For circulating fluidized bed reactors,
(HCl), ammonia (NH3), and fly ashes. followed by a copper-based cata- equipment costs fall between $1,440
There are various processes that can lyst. Researchers are still working and $3,000 per kW. Because these
be applied downstream of the gasifica- to improve this process. cost figures are at least six years old,
tion process to produce usable trans- however, it is possible that in recent
portation fuels (Bermudez & Fidalgo, years gasification technology has
COMMERCIAL POTENTIAL
2016). These include: become less expensive.
Unlike gasification of coal, gasification
• Methanation: At temperatures of biomass and waste feedstocks for Fulcrum BioEnergy, which is devel-
of 700°C–1,000°C and with a transportation fuel production is still oping several commercial-scale MSW
nickel catalyst, carbon monoxide in its relatively early stages of devel- gasification facilities across the United
and hydrogen are converted to opment. However, the extensive expe- States, is using steam-reforming
methane and water. rience gathered from its application bubbling fluid bed reactors (Fulcrum
to coal could help the development BioEnergy, 2018). In Europe, most
• Methanol production: At temper-
of the biomass gasification industry. existing commercial plants that gasify
atures of 220°C–300°C, a pressure
There are major differences between biomass are dedicated to the produc-
between 50 and 100 bar, and with
coal and biomass gasification and tion of power or combined heat and
a catalyst of copper-zinc oxide
many of the coal gasifier technolo- power, although there are some pilot
supported on alumina, methanol
gies cannot be directly applied to projects using gasification for further
synthesis occurs through the reac-
biomass. For example, entrained flow processing into biofuels for transport
tion of both carbon monoxide and
technology is the most widely used (IEA Bioenergy, n.d.). For example, the
carbon dioxide with hydrogen.
gasifier type for fossil fuel gasifica- bioliq project in Karlsruhe, Germany,
• Fischer-Tropsch synthesis: In the tion, but entrained flow gasifiers are uses an entrained flow reactor to
presence of a catalyst, the hydro- not available on a commercial scale gasify the bio-crude being produced
gen and carbon monoxide from for biomass. Current research is trying from fast pyrolysis (bioliq, n.d.-b).
syngas are combined into liquid to address the challenge of how to
hydrocarbons, the composition enhance the production of syngas Almost all of the facilities that produce
of which can be modified accord- with high H2 content where impurities liquid biofuel from biomass gasifica-
ing to the desired specifications such as tar are minimized and capital tion at a commercial level are in the
for the finished fuel, such as and operating costs are also reduced United States and Canada. Enerkem, a
renewable diesel. See the Fischer- (Bermudez & Fidalgo, 2016). Montreal, Quebec-based company with
Tropsch Synthesis section for a long history of working with bubbling
more information. Gasification has higher capital costs fluidized bed biomass gasifiers, is now
than pyrolysis and biochemical pro- converting MSW containing mixed
• Gas fermentation: In the presence
cesses (Wang & Tao, 2016). Currently, textiles, plastics, fibers, wood and
of catalysts, syngas can be fed to
around 75% of commercial biomass other non-recyclable waste materials
acetogenic microorganisms, such
gasifiers are fixed bed reactors into chemical-grade syngas, which is
as Clostridium ljungdahli, which
(Bermudez & Fidalgo, 2016). A study then converted into methanol, ethanol
then produce ethanol (Limayem
by the Internation Renewable Energy and other chemicals. At its Westbury,
& Ricke, 2012). See the G as
Agency (IRENA, 2012) found that fixed Quebec, facility Enerkem processes
Fermentation section for more
bed gasifiers have equipment costs 17,500 tonnes per year of treated
information.
that fall between $1,730 and $5,074 wood (e.g., electric utility poles), wood
• Hydrogen production: In order to per kilowatt (kW). Bermudez and waste, and MSW into syngas. At its
maximize hydrogen production, Fidalgo (2016) suggest that bubbling facility in Edmonton, Alberta, Enerkem
for example for use in hydrogen fluid bed reactors are a promising processes 100,000 tonnes per year
fuel cell vehicles, the hydrogen technology for biofuels because this of MSW (IEA Bioenergy, n.d.). In the
concentration can be increased reactor type already has been demon- United Kingdom, BioSNG built a com-
with a water-gas shift reaction. In strated across a wide range of condi- mercial facility that will process 10,000
the water-gas shift process, steam tions. The 2012 IRENA study found that tonnes per year of waste wood and
(H2O) reacts with carbon monoxide bubbling fluidized bed reactors have other wastes to produce 22 gigawatt
10 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
hours (GWh) per year of grid quality increase product yield and selectivity used as a carbon and energy source
methane through thermal gasification and produce non-native products. by microorganisms suspended in a
and methanation (Gogreengas, n.d.). liquid nutrient solution. If hydrogen
LanzaTech is the best-known company is present, it also can be used as an
that has developed and commercial- energy source. The microbes secrete
OBSTACLES TO ized a gas fermentation process, con- fe r m e n t at i o n p ro d u c t s , s u c h a s
COMMERCIALIZATION verting waste gas from industrial pro- ethanol, 2,3-butanediol, and acetic
There are several technological issues cesses into fuels, such as ethanol, and acid into the broth. Modifications
related to cleaning the syngas so it chemicals, such as 2,3‑butanediol (Wu to the process can change both the
can be used for downstream con- & Tu, 2016). The waste gases used by nature of the reaction products as well
versions. For example, gasification LanzaTech include gaseous streams as the ratio between products and co-
produces tar, which can degrade con- from steel mills that would otherwise products. The selectivity and yield of
version equipment over time. Tar pro- be combusted for electricity and biofuel depend not only on the type
duction can be handled on the front process heat or be flared. This tech- of bacteria used, but also on process
nology also reportedly can convert conditions such as temperature, pH,
end by pretreating biomass via tor-
gaseous streams from other indus- and syngas composition.
refaction or conducting pyrolysis first
trial processes, such as oil refining and
(Phanphanich & Mani, 2011). Both of There are also different designs and
chemical plants, as well as syngas from
those technologies are under devel- configurations of bioreactors, and
gasification of forestry and agricul-
opment for use in this application, but each type has its advantages and dis-
tural residues, unsorted unrecyclable
neither has been proven cost-effec- advantages. The bioreactor design is
municipal waste, natural gas, and coal
tive at a commercial scale. Tar, along critical for gas fermentation because it
(Handler, Shonnard, Griffing, Lai, &
with other syngas contaminants, also influences the gas-liquid interface and
Palou-Rivera, 2015).
can be handled on the back end, via the gas-liquid mass transfer rate (Wu &
syngas cleaning processes. These are Tu, 2016). The continuous stirred-tank
Chemistry of the conversion
proven technologies, used in coal gas- reactor (CSTR) is a common design:
ification facilities, but they do add Hydrocarbons such as tar inhibit fer- Syngas is continuously injected into
additional steps, and thus costs. mentation and adversely affect cell the reactor through a diffuser and
growth; it is thus necessary to clean mechanical agitation breaks the large
the syngas first, as described in the bubbles into smaller ones. However,
Gas Fermentation previous section on Gasification. After CSTR is not economically feasible at a
the syngas is cleaned, it is compressed commercial scale because of its high
OVERVIEW OF CONVERSION and delivered to the reactor, where energy cost, so significant research
PROCESS fermentation is carried out by micro- efforts have been undertaken to
organisms. The steps necessary for improve the design of this kind of
Gas fermentation is a hybrid biochemi-
fermenting gas into ethanol combus- reactor. Bubble column reactors do
cal/thermochemical process where
tion fuel are shown in Figure 4. Gas fer- not require the same mechanical agi-
biocatalysts convert gases composed
mentation requires external heat and tation as CSTRs and they have higher
of hydrogen, carbon monoxide, and
energy, but in this figure we show only mass transfer rates, but they can have
carbon dioxide into liquid fuels (Griffin
where there is potential for recycling problems with mixing and coalescence.
& Schultz, 2012). Biocatalysts can be
or export of energy. Product recovery In a trickle-bed reactor, liquid flows
acetogenic microorganisms, such as down through a packing medium, with
comes after fermentation and includes
bacteria in the Clostridium genus that the syngas moving either downward
distillation and other techniques. As
use the Wood-Ljungdahl pathway (co-current) or upward (counter-
in the case of cellulosic ethanol con-
for gas fixation into several fuel current). These reactors also do not
version, a secondary desired product,
products, primarily ethanol, butanol require mechanical agitation and have
biogas (shown in blue), can also be
and methanol (Wu & Tu, 2016). Value- produced by anaerobically digesting been shown to have higher gas conver-
added co-products can include a waste organic solids. sion rates as well as higher produc-
range of chemical products including tivity compared to CSTR and bubble
acetone, isopropanol, 2,3‑butanediol, In the LanzaTech process, carbon mon- column reactors. Finally, there are
and isoprene (Liew et al., 2016). The oxide-containing gas is first deoxygen- hollow fiber membranes (HFMs), where
bacteria can be genetically modified to ated and then the carbon monoxide is syngas is diffused through micro-size
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 11ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
Other Product
Syngas or Micro-
Recovery Ethanol
waste gas organisms
Techniques
or
Cleaning &
Fermentation Distillation Co-products
Compression
Legend
Process
Organic
Input
solids
By-Product
Internal
Energy On-Site
Anaerobic
Biogas Combustion Energy
Digestion
Desired Recovery
Product
Figure 4: Simplified overview of gas fermentation to ethanol.
pores without forming bubbles. This • Adsorption: Product fuels from operate continuously at an indus-
system offers the highest volumetric gas fermentation, such as butanol, trial scale, does not harm the fer-
mass transfer coefficient, followed by first are adsorbed by certain mentation culture, and does not
trickle-bed reactors and CSTR (Wu & adsorbent materials, such as resin, require a membrane, making it one
Tu, 2016). and then desorbed, creating a of the most attractive techniques
concentrated product. for product recovery (Strods &
Wu and Tu (2016) describe several Mezule, 2017; Wu & Tu, 2016).
• Pervaporation: In this membrane-
techniques for recovering the desired based technique, a membrane
fuel and other co-products from the Another technique is the LanzaTech
selectively separates volatile conversion process, in which fer-
fermentation broth, which is the mix compounds such as ethanol and mentation products are continuously
of fermentation products. These tech- butanol; these compounds diffuse withdrawn from the reactor and sent
niques include: through the membrane, evaporate, through a distillation-based separation
and then are recovered through system for product and co-product
• Liquid-liquid extraction: A dis-
condensation. recovery (see Figure 4). Waste streams
solved compound is extracted
from a liquid mixture using a • Gas stripping: In this process, an are minimized and recycled internally;
oxygen-free gas, such as nitrogen for example, organic solids such as
solvent.
(N 2), circulates in the fermenta- spent microbial biomass can be filtered
• Pertraction: This is a liquid-liq- tion broth in bubbles that then out and digested anaerobically, and
uid extraction process in which break, inducing vibration of the with the resulting biogas, can be mixed
a membrane is placed between liquid and the removal of vola- with some of the vented gas from the
the extracting liquid and the tile compounds (Ezeji, Karcher, reactor for on-site energy recovery
extractant. Qureshi, & Blaschek, 2005). It can (Handler et al., 2015).
12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
COMMERCIAL POTENTIAL INEOS Bio converts several types of as distillation, or instead to geneti-
waste biomass including wood waste, cally engineer or breed strains that are
Compared to other technologies, gas
vegetative waste, and yard waste into tolerant of more concentrated product.
fermentation has several advantages:
bioethanol (Wu & Tu, 2016). In 2013, Bacterial activity is also inhibited by
high product selectivity, low reaction
INEOS Bio started its first commer- acetic acid produced as a by-product,
temperature, high tolerance to sulfur,
cial gas fermentation plant in Florida; thereby reducing ethanol yield. It is
and the biocatalyst is much cheaper
however, a few years later there were possible that genetically engineering
than the heterogenous catalyst used
reports that the facility was not pro- or breeding more selective bacteria
in the Fischer-Tropsch synthesis
ducing much fermentation-derived strains could mitigate this problem
process. Gas fermentation also can
ethanol, primarily due to the sensi- (IRENA, 2016).
handle a wider range of H2:CO ratios
tivity of the microorganisms to high
in the input syngas than FT synthesis Bacterial contamination is another
levels of hydrogen cyanide in the
(Wu & Tu, 2016). barrier that can have a substantial
syngas. Recently, INEOS Bio sold this
facility, citing changes in the market impact on final yields. To mitigate this,
To date, the developers exploring this
for ethanol as one of the reasons for operators must use resilient bacterial
conversion pathway have focused on
this decision (Voegele, 2016). strains and reactor systems that are
maintaining stable microbial popula-
less susceptible to contamination and
tions and setting up new reactor tech-
As of 2018, the Shougang-LanzaTech improve the removal of trace species
nologies. Reactor design still must be
Joint Venture is operating LanzaTech’s that can cause population loss (Daniell,
optimized to improve pass-through
gas fermentation technology at the Köpke, & Simpson, 2012).
rates, downsize equipment, and inte-
Shougang Group’s Jingtang Steel Mill
grate energy by using unreacted gases
in Caofeidian, China. This 60 million-
(e.g., unused H2 from the syngas) for
liter-per-year facility is converting steel
Fischer-Tropsch Synthesis
process energy recovery (IRENA,
mill waste gas to ethanol. Four further
2016). Further, Wu and Tu (2016) high- OVERVIEW OF CONVERSION
commercial projects are expected
light that mass transfer of gas-to-liquid PROCESS
to start up in 2019–2020 producing
in a bioreactor is the largest rate-lim-
ethanol from refinery and ferroalloy Fischer-Tropsch (FT) synthesis, illus-
iting parameter, posing a major chal-
off-gases in India and South Africa; trated in Figure 5, is used in conjunc-
lenge to gas fermentation.
biomass syngas in California; and steel tion with other conversion technol-
Researchers also are working on select- mill off-gases in Belgium. In Japan, ogies that produce syngas, such as
ing desirable traits in the microbes SEKISUI has demonstrated ethanol gasification and power-to-X, which
and blocking out particular metabolic production using LanzaTech’s gas fer- are addressed in other sections of this
pathways in order to improve primary mentation technology on syngas from paper. After the syngas is cleaned, FT
product yields. Specifically, there are gasified unsorted unrecyclable MSW synthesis catalytically converts syngas
efforts to genetically modify the fer- that would otherwise be incinerated to produce liquid hydrocarbons, which
mentation bacteria to improve yields, (Burton, 2017). include waxes, drop-in fuel, and other
reduce product inhibition, and make hydrocarbons, as well as tail gas. Tail
them more tolerant to operating con- gas usually is recycled if it is made
OBSTACLES TO
ditions that are uninhabitable to other up mainly of syngas, and it usually is
COMMERCIALIZATION
bacteria strains. Capital costs also combusted if it is made up mainly of
can be reduced by using new product Continuous input of the feedstock and other off-gases. FT synthesis requires
recovery technologies, such as mem- removal of the product is necessary energy, but in this figure we show only
branes (IRENA, 2016). to maintain the activity rate of the where there is potential for recycling
microorganisms, but this increases the or export of energy. This product may
INEOS Bio, Synata Bio, and LanzaTech energy necessary for product separa- require further refining before use in
are the three major companies that tion. It is therefore critical to identify the transport sector but in some cases
have, or have had, precommercial or methods to either reduce energy includes fuels that ready to use (see
commercial gas fermentation facilities. demand for product separation, such Figure 5).
WORKING PAPER 2019-13 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 13ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS
Legend
Syngas Catalyst Wax Process
Input
By-Product
Other
Cleaning & Hydrocarbons
Fischer-Tropsch Energy
Water-Gas
Synthesis
Shift
Desired
Product
Drop-in Fuel
On-Site
Tail Gas Combustion Energy
Recovery
Figure 5: Simplified overview of Fischer-Tropsch synthesis.
Chemistry of the conversion be applied to the syngas to adjust the each of the interfaces of solids (cat-
H2:CO ratio if required for FT synthesis. alysts), liquids (hydrocarbons), and
FT synthesis converts a mixture of
Ideally, the final concentration of inert gases (carbon monoxide, hydrogen,
hydrogen gas and carbon monoxide
gases (CO2, N2, CH4, etc.) should make steam, and hydrocarbons). Finally,
into hydrocarbons and water accord-
up less than 15% of the gas volume because this is a capital-intensive
ing to the following equation:
(Bergman et al., 2004). process, reactors must scale up effec-
tively. Three reactor types are con-
(2n+1) H2 + n CO à Cn H(2n+2) + n H2O
Fischer-Tropsch synthesis sidered viable for commercial scale
catalysts production: multitubular fixed bed
The n in this equation indicates that
reactors, fluidized bed reactors, and
the reaction can be manipulated to Several kinds of metals can be used
three-phase slurry reactors.
produce hydrocarbons with a range to catalyze Fischer-Tropsch synthe-
of carbon chain lengths. The hydro- sis: iron, cobalt, nickel, and ruthenium. Multi-tubular fixed bed reactors are
carbon distribution is determined by In practice, ruthenium is prohibitively easy to operate and scale up, but they
operating conditions, such as tempera- expensive, and nickel catalyzes some are expensive to construct and have
ture, pressure, type of catalyst, etc. other undesired reactions, so only iron high gas compression costs for the
Conditions can be chosen to maximize and cobalt are industrially relevant. recycled gas feed.1 They also require
the yield of a particular cut with a They have different prices, sensitivities long downtimes during replacement
higher market value, such as middle to contaminants, life spans, ideal gas of catalysts.
distillates (National Energy Technology compositions, and ideal operating con-
Laboratory, n.d.). ditions (National Energy Technology Fluidized bed reactors have higher
Laboratory, n.d.). efficiency in heat exchange and better
Prior to the catalytic reaction, the temperature control. They also require
gaseous inputs for the FT synthesis The FT reaction is exothermic, which smaller heat exchange area and lower
process require cleaning and upgrad- is to say it releases heat, so an impor- gas compression costs. In addition
ing. The process is very sensitive to tant consideration for the reactors is they are easier to construct than
contaminants, especially substances their capacity to quickly dispel heat fixed beds. Fluidized bed reactors
containing sulfur or metals because from the catalysts to avoid overheat- also allow for online catalyst removal,
they can poison the catalysts (Peters ing and catalyst deactivation, while at so there is no downtime for catalyst
et al., 2015). Syngas cleaning is the same time maintaining steady tem-
addressed in the Gasification section perature control. Reactors also must 1 Fixed and fluidized bed reactors are described
of this paper. The water-gas shift can facilitate effective mass transfer across in more detail in the Gasification section.
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