Methanol Production via Indirect Gasification of Switchgrass

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1 Final Design Project CHEMICAL ENGINEERING April 2012 Methanol Production via Indirect Gasification of Switchgrass Nick Nikbakht 1, 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. 1 nikbakht@berkeley.edu 2 arjan.puniani@brkeley.edu 3 xianbo.liang@berkeley.edu

2 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). Major Assumptions Switchgrass yield (in tons per acre) 4 Land charge on a per acre basis $80 At 4 tons/acre, our production goal Reseeding rate for frost-seeded crop 25% require just under 500,000 acres of Switchgrass stand longevity (years) 11 switchgrass, which is equivalent to a 27 x 27 mi 2 Reseed longevity (years) 10 plot of land. Thus, we Operating interest rate 9% now know our production facility s lower limit of arable land. Some of the top candidates of our survey Addition of Phosphorous and Potassium Yes 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.

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 (noncarbohydrate) 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 dimethyl-terephthalate. Abstracting away the intermediate steps of synthesis gas production and separations, we assume the following reaction governs our process: [Biofuel] + x H 2 O y CH 3 OH + z CO 2 The biofuel, in our case, was computed to be [C H 5.67 O C 4.33 H 4.33 O ], 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. 3

4 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 Efficiency process efficiencies (Cf chart). Gasifier 76.1% Synthesis 60.0% Efficiencies combine multiplicatively, which means the overall efficiency of the plant is 45.7%. The equation for relating thermal efficiency (η T ) 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 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 Product Stream Implied η T (tons mn) (tons) Stoichiometry , % Thermal Efficiency , % 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 mediumcalorific 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 4

5 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 H 2 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 CO 2 through absorbers and strippers. Since we abstracted the switchgrass as a nearly 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. 5

6 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 mol% of dextrose and 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: Dextrose TMT CO CO H2O H CH 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. 6

7 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. 7

8 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 + H 2 O CO 2 + H 2 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/m 3 /pa 2 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 CO 2 produced in the reactor, we need a cascade of absorbers and strippers to remove CO 2 from the syngas (via methanol as a solvent). Methanol Production Once we achieve a 2.06:1 (H 2 : 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: 8 CO + 2H 2 CH 3 OH 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 CO 2 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 9

10 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 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, for minimum expected capacity (or for the direct ASPEN simulation), but around $168,042, 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, are required for our initial investment in direct costs. Indirect costs bring up the total 10

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18 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 18