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Final Design Project CHEMICAL ENGINEERING 160 11 April 2012
Methanol Production via Indirect Gasification of Switchgrass
Nick Nikbakht1, Arjan Puniani
2, and Xiangbo Liang
3
Department of Chemical Engineering, College of Chemistry University of California, Berkeley
Department of Physics, College of Letters and Science University of California, Berkeley
Abstract: In 2006, former President Bush stated, “America is addicted to oil.”
To reduce national dependence on conventional fuels, we perform a feasibility study on the production of methanol via switchgrass. Our target plant capacity aims to process 1 mn tons of switchgrass per annum, with an expected methanol production to exceed 380,000 tons/yr at 99.99% purity. The process design and economic evaluation address the conversion of switchgrass into methanol via pathways that are demonstrable by a pilot-unit level, and we attempt to match currently established and published technology, as well as biomass resource availability consistent with immediate deployment. Indirect steam gasification was selected as the technology around which the process was developed based on previous studies for the production of methanol via ligno-cellulosic biomass. The operations for methanol production can be simplified to: feed preparation, gasification, gas cleanup and conditioning, and purification. To ensure cost competitiveness, our process, market, and financial targets are all optimized to maximize the difference between cost of methanol per gallon and market price per gallon. We calculate an initial investment requirement of $187 mn, with a projected total project investment of $461 mn. If we assume a 40yr lifespan of our plant, we project a 14% rate of return, assuming a discount rate of 15%. This rate of return is consistent with projects of similar capacity and risk.
Introduction Our assignment was to model the chemical process of switchgrass conversion into methanol. The simulation program we used is ASPEN, which is a special design tool that can abstract certain chemical engineering processes using mass balances and numerical analysis. Our specific goal was to generate a final recommendation of the process design through data obtained from chemical engineering parameters and various equations of state. To enable comparison of research and development projects, we rely on several comparable economic studies to directly establish a benchmark and promote modularity depending on specific design requirements. Our methanol production goal is two-fold: primarily, we would like exceed the INEOS Bio and New Planet Energy’s Project Liberty initiative, which is projected to yield nearly 1 billion gallons of ethanol per year. Since the United States biofuels market is expected to reach $110bn, our secondary goal is to capture a market-leading position within in this lucrative space. We propose a production capacity of 533,500 tons of methanol per year (as an upper limit to accommodate certain reserve quantities), which requires 1.87 mn tons of switchgrass a year. In a later section, we will show the production costs associated with switchgrass in order to surmise cash flows, but ultimately, this project can generate $229,405,000 in revenues a year.
Feed Stock The economics and size of our plan will depend on the availability of switchgrass. Our goal of producing over 500,000 tons of methanol will require 1.87 mn tons of switchgrass for this particular process. Based on a biofuels study conducted by Duffy et al, we can expect an approximate dry switchgrass yield of 4 tons per acre (see table for assumptions). At 4 tons/acre, our production goal require just under 500,000 acres of switchgrass, which is equivalent to a 27 x 27 mi2 plot of land. Thus, we now know our production facility’s lower limit of arable land. Some of the top candidates of our survey include Kansas, Kentucky, and Oklahoma. We are bullish on Oklahoma for a few reasons. Primarily, the US government has nearly completed its campaign of converting around half of the state’s 35 mn acres of food crop into high-yield switchgrass plots. Our facility will require less than 0.01% of all available land devoted exclusively to switchgrass, suggesting an incredible economic opportunity.
Major Assumptions
Switchgrass yield (in tons per acre) 4
Land charge on a per acre basis $80
Reseeding rate for frost-seeded crop 25%
Switchgrass stand longevity (years) 11
Reseed longevity (years) 10
Operating interest rate 9%
Addition of Phosphorous and Potassium Yes
3
The composition of herbaceous feedstocks (or lignocellulose) can be roughly broken down into:
Cellulose (30-50%)—a glucose polymer linked by β-1,4 glycosidic bonds
Hemicellelulose (20-40%)—shorter, and more highly-branched polymer of
C5 and C6 sugars
Lignin (15-20%)—a polyphenolic structural constituent of plants (non-
carbohydrate)
Ash, and other extractives—residues following ignition (dry oxidation at
575±25°C)
Discussion of Stoichiometry The molecular composition of switchgrass is quite similar to wood, so we
assume our feed will consist of 56.7 mol% of dextrose and 43.3 mol% of di-
methyl-terephthalate. Abstracting away the intermediate steps of synthesis gas
production and separations, we assume the following reaction governs our
process:
[Biofuel] + x H2O y CH3OH + z CO2
The biofuel, in our case, was computed to be [C3.402H5.67O3.402 + C4.33H4.33O1.732],
and using it as our basis, we found the following coefficients:
x = 5.22
y = 5.11
z = 2.62
In the case of 100% conversion, we can expect 1 mole of switchgrass to yield
around 5 moles of methanol. Recall our goal production of 5bn gallons a year.
With this stoichiometry, our process will require 2.93bn kg of switchgrass. With
the published yields per acre, we can expect to require 1.46mn metric tons of
grass.
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As a check for our work, we looked towards published thermal efficiencies to
tease out an implied product rate. In a benchmarking survey prepared by the
DOE i , a comparable biofuels venture whose objective was transforming
feedstock into ethanol published a few
process efficiencies (Cf chart).
Efficiencies combine multiplicatively, which
means the overall efficiency of the plant is
45.7%. The equation for relating thermal
efficiency (ηT) to product stream is:
Where LVH refers to Lower Heating Value (an energy density less any moisture
content’s vaporization), and P and F referring to product and feed stream,
respectively. From the literature, we know that the LVH of methanol is
approximately 19.9 kJ/mol and wood’s is 18.5. The idea behind this calculation
is to temper our theoretical yields by using an efficiency proxy as our discount
factor. Thus, the unknown is P, and we substitute in the remaining parameters.
Earlier, we saw how a desired product stream of 500,000 tons of methanol
required 1.87 mn tons of switchgrass. When we take into account thermal
efficiency, this same quantity of switchgrass only yielded 248,000 tons of
methanol. This value is more realistic in that it accounts for some of the
thermal limitations of the processes we will be dealing with.
In sum we see that:
Method Feed Stream (tons mn)
Product Stream (tons)
Implied ηT
Stoichiometry 1.87 533,500 84.1%
Thermal Efficiency 1.87 248,000 45.7%
Process Overview
The major component of our process is the indirect steam gasification. We chose this
as the technology around which this process was developed based on other economic
studies in the Aden DOE report. Gasification involves the devolatilization and
conversion of biomass in an atmosphere of steam and/or oxygen to produce a medium-
calorific value gas. The biomass heating and gasification is accomplished through heat
transfer from a hot solid or through a heat transfer surface. The byproduct char is
combusted with air (external to the gasifier itself) to provide the energy for the
Efficiency
Gasifier 76.1%
Synthesis 60.0%
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gasification. Steam gasifiers need not an oxygen input; but since they operate at
relatively low pressures, they require product gas compression for downstream
purification and synthesis unit operations.
Of course, in industry, the gasifier is an inclusive unit. But through ASPEN, we actually
modeled it via two adiabatic reactors (GASIF, CMBSTR) and various separating
processes (SEP1, NEWSEP). Since multiple reactions occur in the gasification process,
this process decomposition into separate units is analytically useful. Of course, we
know from theory and previous application that the heat fro the endothermic
gasification reactions is supplied via a hot, synthetic circulating sand between the char
(Stream: COMBUST) and the char combustion chamber (CMBSTR).
Following gasification, the raw syngas is separated via cyclones and electrostatic
precipitators to the water gas shift reactor. Here we convert carbon monoxide and
water into carbon dioxide and hydrogen, and prepare for compression to the methanol
synthesizer. We require the WGS reactor to obtain a 2.1:1 H2 to CO ratio, and
parameters were adjusted accordingly (dimensionality and heat exchange
considerations) to approximate the ratio as close as possible. With the stoichiometry of
the indirect gasification and methanol synthesis known, our ASPEN model predicts a
methanol output of 121,716 lbs/hr for a 193,388 lbs/hr switchgrass feed.
One of the largest cost drivers is the gas cleanup and conditioning phase of the
process. For example, the syngas comes out extremely hot, and requires heat
exchange with our overall steam cycle. We vent out CO2 through absorbers and
strippers.
Since we abstracted the switchgrass as a nearly 50-50 mixture of complex
carbohydrates that compose wood, we were able to add the raw biomass feedstock as
conventional components. Some blocks (such as the distillation column) will receive
more rigor and detail than others (conversion extent in methanol synthesis). Due to
the presence of different stages of matter in our process (solids, liquids, and gases), a
single thermodynamics packages was wholly insufficient. However, we did rely on the
“RKS-BM” option for our high-temperature, high-pressure phase behavior, with the
1987 Steam Table properties used for cycle calculations.
From the heat and power perspective, we rely on a conventional steam cycle to
produce heat for the gasifier and electricity for internal power requirements. The
steam cycle is integrated with the biomass conversion process, and pre-heaters and
steam generators are integrated within the process to create the steam. We plan on
utilizing the steam to drive compressors, generate electricity, or withdrawn from
processes for injection purposes. Any condensates can be sent back to the steam
cycle, de-gassed, and combined with the make-up water.
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Indirect Gasification & Char Combustion Based on instructions, we model the indirect gasifier (GASIF) as a stoichiometric
reactor operating at 850°C. Steam is injected into the gasifier at stoichiometric
conditions, and is fed at 195°C, 25 psia, and 73,120 lbs/hr. One of our major
assumptions was modeling the switchgrass as a mixture of complex carbohydrates;
specifically: we chose our biomass feed to consist of 0.567 mol% of dextrose and 0.433
mol%. Since preprocessing was expected to raise temperatures, we modeled the
stream with 104°C inlet temperature, at 25 psia. Ideally, we would have liked to show
how conveyers and hoppers feed the biomass into low-pressure, indirectly-heated,
entrained flow gasifiers. These streams met at GASIF and the resulting hot syngas was
based on the following stoichiometry:
0.3047 Dextrose + 0.2327 TMT 1.2494 CO + 0.4186 CO2 + 0.6727 H2O +
0.8565 H2 + 0.702 CH4 + 0.196 C(char)
As we can see, the biomass chemically converts to a mixture of syngas components and
a solid char, which is mainly the fixed carbon residual from the biomass (plus any
carbon deposits on sand).
This syngas plus char slurry is fed into a double cyclone, electrostatic precipitator
complex as a means to separate the gas from the solid char. Although not shown, we
plan on solids having the solids flow by gravity into the combustion chamber from
these cyclones.
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Both the gasifier and char combustor temperatures are allowed to float, and are
governed by the energy balances maintained around the gasifier and combustor. We
noticed there existed a type of equilibrium as we fine-tuned the parameters: the more
char created, the higher the char combustor chamber; however, if our char combustor
reactor increases in temperature, our gasifier temperature increases as well, resulting
in less char. Because of this dynamic, char combustor profiles tend to find equilibrium.
This works out well since our gasifier equilibrium temperature was approximately
890°C, and the char combustor equilibrium was essentially 995°C.
From a practical perspective, we would have preferred to show that the air is injected
into
the
bottom of the reactor and serves as a sort of carrier gas for the fluidized bed (in
addition to the oxidant for our char combustion chamber). The heat of combustion will
heat the sands up to 1800°F.
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Water Gas Shift Reactor Our next process deals with process clean up and conditioning of the syngas in order to
synthesize methanol. Our alcohol synthesis requires 2 moles of hydrogen per mole of
carbon monoxide; however, our gasifier reaction produced a ratio closer to 1:1. At 1
atm, the water gas shift reaction is:
CO + H2O CO2 + H2
In order to meet this ratio, we actually considered splitting our syngas feed 50-50,
with half going to the water gas shift reactor (about 5.5e4 lbs/hr) and reacting with
3.5e4 lbs/hr of water. The water gas shift reactor was initially selected as an adiabatic
reactor, but improvements suggest a more accurate energy budget is attained with
modeling the WGS as a PFR with co-current cooling water. The presence of a catalyst
is reduced nearly ten-fold to avoid complications with thermal runaways.
Here, we will overview some of the primary kinetics behind the reaction. Of course,
our kinetic factor will be:
[
] (
)(
)
Where k = 9.6e-9 kmol/sec/m3/pa2 and E = 47,400 kJ/kmol
Our calculations indicate that the syngas experiences an average residence time of 1.9
seconds in the reactor, and the coolant water must maintain a temperature below
280°C, let which we wish for the catalyst to denature. Once we recombine the shifted
gas with the unreacted clean syngas, we found our molar ratio approach 2.06:1.
Of course, given the massive amount of CO2 produced in the reactor, we need a
cascade of absorbers and strippers to remove CO2 from the syngas (via methanol as a
solvent).
Methanol Production Once we achieve a 2.06:1 (H2 : CO) ratio in our syngas, it can be fed into our alcohol
synthesizer for methanol production. Based on the literature, we selected 4 MPa and
850°C for our reaction conditions, which was optimized for the following reaction:
CO + 2H2 CH3OH
Note that this reaction may only occur with a special catalytic blende of copper, zinc
oxide, and alumina, which catalyzes the production of methanol from carbon
monoxide and hydrogen.
9
These high pressures demanded use of an isentropic compressor to ramp up the 15 psia
post-WGS stream to 580 psia. We around 7,943,000 lbs of dirty methanol are produced
per hour from the MeOH reaction vessel, which is consistent with the reaction kinetics
for methanol synthesis. The exit stream is fed into a 5°C, 10 psia flash drum, which is
subsequently fed into a “stabilizer” distillation column to purify the methanol.
Some of our earlier models explored the effects of recycling and purge streams. We
triangulated an optimal recycle stream of ~10% of the unreacted materials in the
reactor (water and carbon dioxide) to avoid buildup.
Carbon dioxide is an acid gas that needs to be removed in the syngas conditioning
process. The acceptable level of CO2 is actually going to depend on the specific
catalyst we use for the alcohol. Of course, from theory, we are also dealing with
specifics from the synthesis reactor. We modeled our MEOHSYN reactor as isothermal,
though we concede that maintaining a constant temperature in a fixed bed reactor is
non-trivial (especially since reactions are exothermic). Temperature is the primary
driver for alcohol selectivity and product distribution; however, significant pressure
increases will shift the chemical pathways from hydrocarbon production towards
alcohol production.
We did explore certain kinetic models was used to guide conversion assumptions for
predicting methanol recycle. Some, such as Guntru[2], explored how maintaining high
partial pressures of methanol reduces production of alcohols higher than ethanol.
Safety and Other Considerations
Based on the parameters in our flow sheet design, methanol production is taking
place at a pressure of 4 MPa. The material used in regards to piping must
withstand leakage and its ability of handling fluctuation in pressure. It is important
to constantly monitor the piping around this high-pressure area. Some of the
gases produced in the plant are dangerous and must be monitored. In order to
safely handle these issues certain equipment must be installed through the plant
that detects for these gases. Also having a staff that actively monitors safety and
provides necessary information regarding safety to other personnel is
recommended.
Utility Requirements
Our plant design will have an estimated running cost of $XX per annum. In our
total running product cost, we included labor cost, operating cost, water, steam,
and electricity. We also performed estimates on the labor requirements
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associated with each process and operational costs. Of our 9 major components,
we estimate a total 200 of employees are required, and because our projections
are dependent on continuous operation, multiple shifts may be required. The
amount of required switchgrass to achieve our annual production goal is 11,000
tons, which is what needs to be kept in mind for employment.
The amount of cooling water is contingent on our steam network.
Purchased Cost of Major Equipment
This next section identifies the equipment costs that went into the process.
The major cost will from the indirect gasifier. We know the equation assumes the
form:
From ASPEN, we know that our heat duty for the gasifier is 3.45e8 BTU/hr, our flow
rate is 193,388 lb/hr, and the LHV of wood/switchgrass is 19.5. Our heat of reaction is
approximately 390,022,219 kJ/hr. After making the appropriate unit adjustments, our
indirect gasifier will cost us $49,424,138.00 for minimum expected capacity (or for the
direct ASPEN simulation), but around $168,042,072.36 for our maximum expected
capacity. Clearly, the gasification unit dominates our total equipment cost. The
remainder of the costs is associated with the sheer scale and installation costs
associated with our major processing units.
Our earlier simulations dictated that associated compressors and separators were
selected as cast iron in our ASPEN mapping function, whereas our heat exchanger and
WGS was composed of carbon-steel graded for very high pressure capacities. At the
last minute, we decided to remove our steam network given the flow sheet’s lack of
clarity, which explains why we no longer have heat exchangers.
Economic Analysis Please see below. We project about $232,669.18 are required for our initial investment
in direct costs. Indirect costs bring up the total
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Conclusion This opportunity is a very profitable one, and the plant represents a $1.37 bn enterprise. With a strong rate of return and lucrative biofuels market coverage, there is tremendous commercial viability in this indirect gasification project.
i http://seca.doe.gov/technologies/coalpower/gasification/pubs/pdf/BMassGasFinal.pdf
