Effects of Dry Reforming and Biogas Composition - American

Article pubs.acs.org/IECR

Optimal Process Operation for Biogas Reforming to Methanol: Effects of Dry Reforming and Biogas Composition Borja Hernández and Mariano Martín* Department of Chemical Engineering, Universidad de Salamanca, Plz Caídos 1-5, 37008, Salamanca, Spain ABSTRACT: We optimized the operation of the process that reforms biogas with CO2 and/or steam for the production of methanol using a mathematical optimization approach. The raw biogas is cleaned up before reforming. Part of the biogas is used to provide energy for the process. Next, the unreacted hydrocarbons and CO2 are removed. Subsequently, syngas composition may be adjusted, using either water gas shift reaction or membrane-pressure swift adsorption. Finally, methanol is synthesized. The process is modeled using mass and energy balances, chemical and phase equilibria, and rules of thumb. The problem is formulated as an NLP problem with simultaneous heat integration for the optimal biogas composition and methanol production. Two objective functions are considered: a simplified production cost and an environmental one developed based on carbon footprint. Biogas is expected to have around 50−52% of CH4 and 45−47% of CO2, depending on the objective function. The production cost of methanol is $1.75/gal, for a plant size that uses 10% of the potential biogas to be produced in Madrid, Spain, with an investment of $46 MM. CH4 + CO2 ↔ 2CO + 2H 2

1. INTRODUCTION Biogas is an interesting fuel that is produced from anaerobic digestion (AD) of organic wastes. Its composition is mainly methane, with a certain amount of carbon dioxide and other species in smaller quantities like H2S or NH3.1 Therefore, it can be considered as a source of methane. Methane and natural gas have been used not only as an energy source or as a fuel, but also as a raw material for the production of syngas. Syngas is a mixture of CO and hydrogen that can be used as a building block for a large number of chemicals from methanol or ethanol, to synthetic fuels via Fischer−Trospsh technology. Therefore, biogas can also be used as such to produce syngas. There are a number of alternatives to process methane. We can use steam to reform methane. It is an endothermic reaction that produces a large H2 to CO ratio:

We see that the hydrogen to CO ratio varies widely depending on the method used. This ratio is key for the proper production of different species. For instance, the production of methanol requires a H2/CO ratio of 2, the production of FT-liquids suggests values from 1 to 2 depending on the length of the hydrocarbon chain and the operating conditions at the FT reactor. The production of ethanol and DME, if direct synthesis is used, needs a H2/CO ratio of 1.2,3 The commercial application of dry reforming has been hindered due to the need for a source of CO2.4 However, biogas already has CO2 within its composition. Because of the low ratio H2/CO obtained, there are a number of alternatives to modify the composition. The first one is related to the biogas composition itself. Furthermore, dry reforming of methane has also been combined with other reforming modes such as the use of steam reforming,5 that also mitigates the deposition of carbon due to dry reforming alone.6 Since dry reforming is an endothermic process, its combination with partial oxidation has been suggested to avoid energy consumption.7,8 Finally, a combination of steam, partial oxidation, and dry reforming has also been suggested by Song et al.9 The use of biogas as raw material for the production of methanol via methane dry or hybrid dry and steam reforming is an interesting alternative. By combining both reforming modes, the H2 to CO ratio can be modulated to achieve any value

CH4 + H 2O ↔ CO + 3H 2

Alternatively, oxygen can be used. The process, known as partial oxidation, is exothermic but with a lower yield to hydrogen: CH4 +

1 O2 ↔ CO + 2H 2 2

Autoreforming combines both steam reforming and partial oxidation, so that the process is adiabatic. The reaction goes as follows: CH4 + aH 2O + bO2 ↔ xCO + y H 2 + zCO2 + ....

Received: March 15, 2016 Revised: May 16, 2016 Accepted: May 18, 2016

Lately, another technology has been added to the portfolio, dry reforming using carbon dioxide. It is also an endothermic process with a low yield to hydrogen: © XXXX American Chemical Society

A

DOI: 10.1021/acs.iecr.6b01044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Superstructure for the production of methanol from biogas reforming.

purified using a gas−liquid separation system consisting of a flash and a distillation column

required in synthesis. Partial oxidation is not considered for methanol purposes since only by using all the methane through this path is it possible to obtain high H2 to CO ratios. In this way the CO2 within the biogas would not be captured either and pure oxygen is needed for the operation. Furthermore, renewable methanol is produced from waste or byproducts, such as previous attempts when glycerol was reformed used to produce methanol.10 Moreover, the composition of the biogas is the result of the composition of the biomass digested, the residence time, and the temperature of operation. Typically, thermophilic digestion is selected, around 55 °C for 15−20 days, because of the higher yield and conversion to biogas.11 Thus, in this work we optimize the production of methanol from biogas determining the optimal composition of the biogas and whether dry reforming alone or hybrid reforming is suggested. A mathematical optimization framework is proposed to model the process from the biogas reforming into syngas and its use for methanol production, for the optimal operation of such plants. The paper is organized as follows. Section 2 describes the process and the flowsheet. In section 3, the modeling features of the different units are depicted. Next, in section 4 the results of the operation and the economic evaluation are presented and discussed. Finally, we draw some conclusions.

3. PROCESS DESIGN METHOD: MODELING ASSUMPTIONS The optimization of the production of liquid fuels and hydrogen is formulated as a nonlinear programming (NLP) problem. The units are modeled using mass and energy balances, chemical equilibrium, rules of thumb, experimental data, and design equations with special attention to the reactors (biogas reforming, methanol synthesis). The components in the system belong to the set J = { Wa, Met, CO2, CO, O2, N2, H2, H2S, CH4, MetOH}. In this section we present the main assumptions for modeling the different units based on extensive literature research so that the models are simple but reasonably accurate. The solution to the problem yields the steady state operation of the production facility described in section 2. The objective is to maximize the production of methanol depending on either the main operating costs such as raw materials usage and energy consumption or the CO2 emissions. 3.1. Biogas Dry Reforming. The anaerobic fermentation of biomass, wastes, and other organic residues generates methane and carbon dioxide, biogas, whose composition depends on the biomass used and the operating conditions. Apart from methane and CO2, nitrogen, H2S, and NH3 are produced.1,11,12 On the basis of these facts, for the sake of simplification, we assume that biogas is mainly methane, CO2, H2S, NH3, and N2. The first stage consists of removing H2S. A bed of Fe2O3 operating at 298−373 K is used. The H2S is captured as presented in the following reaction:

2. OVERALL PROCESS DESCRIPTION The biogas is processed to remove the hydrogen sulfide that poisons the catalysts in the synthesis stage, see Figure 1. Next, it is reformed to produce raw syngas. Part of the gas is needed to heat up the furnace since dry and steam reforming are endothermic. The traces of hydrocarbons are removed in a PSA system with a bed of Silica gel. Next, the composition may need to be adjusted to a molar ratio of H2/CO around 2. Three alternatives are considered: water gas shift reactor, bypass and hybrid membrane/PSA for H2 (with a bed of oxides). The split fraction depends on the performance of the reformer which may make this composition adjustment redundant. The third section corresponds to the removal of sour gas, CO2. A PSA system with a bed of Zeolite 5A is selected for its removal. However, to allow for certain concentration of CO2 to remain for synthesis purposes, a bypass is also allowed. Once the syngas is purified, consisting mainly of H2 and CO, it is used in the synthesis reactor to produce methanol. The operating conditions are to be optimized (ratio of H2 and CO and operating temperature at the reactor). Finally, methanol is

Fe2O3 + 3H 2S → Fe2S3 + 3H 2O

The bed can be regenerated using oxygen. Next, the biogas consisting mainly of methane and CO2 is fed to the reformer. Actually, a fraction of the methane is burned so as to provide the energy for the endothermic reactions, see Figure 2. The model of the methane reformer is based on the chemical equilibrium given by eqs 1 and 3 CH4 + CO2 ↔ 2CO + 2H 2

(1)

CH4 + H 2O ↔ CO + 3H 2

(2)

CO(g) + H 2O(g) ↔ CO2(g) + H 2(g)

(3)

where the equilibrium constants are given by eqs 4−6 from the literature:13,14 B


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Industrial & Engineering Chemistry Research fc(CH4 , Spl3, reformer) = (Q (methane reformer) + Q (he1))/LHV

(11)

The gas coming out of the reformer is sent to the purification stage for the removal of hydrocarbon traces. In order to avoid the deposition of C on the catalyst surface, causing catalyst deactivation, we must ensure:15 n H2 − nCO2

Figure 2. Detail of the reforming stage. [31.447 − 29580/ T ]

kp = e

=

PCO·PH2 PCH4·PCO2

kp = 10[−11650/ T + 13,076] =

kp = 10[1910/ T − 1,784] =

nCO + nCO2

PCO·PH32 (5)

PCO2·PH2 PCO·PH2O

(6)

Atomic mass balances are as follows: mol CH4|in = mol CH4 + mol CO + mol CO2|out 4·mol CH4 + 2·mol H2O|in = 4·mol CH4 + 2·mol H2 + 2·mol H2O|out mol H2O|in = mol H2O + mol CO + 2·mol CO2|out (7)

Since the first equilibrium depends on the pressure, we use compressor 1, assumed to be a polytropic compressor, to adjust the pressure for the reaction in case it is needed. The amount of natural gas burned is computed assuming that the energy required, Q(methane reformer), is provided as lower heating value (LHV) of methane. Q prod =

∑ fc(i , reformer, mix2)·(ΔHf + ∫ i

Tout

Tref

Cp dT ) (8)

Q reac =

∑ fc(i , reformer, mix2)*(ΔHf + ∫ i = in

Tin

Tref

Cp dT ) (9)

Q (methane reformer) = Q prod − Q reac

(12)

3.2. Cleanup. First, water is removed in a two-step process using a flash before compression and a dephlegmator right after (he 7 and Flash2 in Figure 3). Next, a PSA system is used to remove the last traces of hydrocarbons and nitrogen. It operates at 298 K and 4.5 bar using silica gel as adsorbent.16 For this unit we compress the gas, assuming polytropic behavior, and cool it down. Because of the low temperature, water condenses before the PSA adsorption beds and it is discharged. The PSA system is modeled as two beds, one operating and the second one in regeneration, to allow for continuous operation of the plant, see Figure 3. Next, the syngas composition is adjusted for synthesis. We allow a water gas shift reaction, a bypass, and a hybrid membrane−PSA system to tune the H2 to CO ratio. The removal of CO2 is the last cleaning stage for the preparation of the syngas. However, CO2 must only be partially removed from the gas so as to achieve a composition from 2% to 8% in volume.17 To achieve this concentration, we consider a bypass so that only part of the gas stream is treated in a PSA system to absorb the excess of CO2 using Zeolite 5A or 13X. The operating conditions are 298 K and 5 bar. Thus, the stream is compressed and cooled down. In this process water condenses too. The amount of water condensed is given by the saturation conditions of the exiting gas, computed using Antoine correlation, while the water vapor accompanying the CO2 is adsorbed too. The cycle is short and the adsorption capacity is around 0.1 kg of CO2 per kg of zeolite which allows the removal of 95% of the CO2, see Figure 4. 3.3. Synthesis. Figure 5 presents the detail of the reaction and recycle section of the flowsheet. Methanol is produced from synthesis gas (a mixture of CO2, CO, and H2) and the reaction is catalyzed by a catalyst composed of CuO−ZnO− AlO. The three main reactions that take place are the following:

(4)

PCH4·PH2O

≥3

(10)

Figure 3. Gas cleanup section. C


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H: 2n H2 + 2n H2O|in − (2n H2 + 2n H2O + 4nCH3OH)|out = 0 (17)

C: nCO + nCO2|in − (nCO + nCO2 + nCH3OH)|out = 0

(18)

O: nCO + 2nCO2 + n H 2O|in − (nCO + 2nCO2 + n H 2O + nCH3OH)|out

There are two main operating variables that the feed to the reactor must meet for the optimal production of methanol, eqs 20 and 21: (A) The ratio hydrogen to CO20

CO + 2H 2 ↔ CH3OH CO2 + H 2 ↔ CO + H 2O CO2 + 3H 2 ↔ CH3OH + H 2O

(13)

1.75 ≤

However, since only two of the reactions are linearly independent, we consider the reactions in eq 14: CO2 + H 2 ↔ CO + H 2O

(14)

The equilibrium constants are given by the experimental results of Cherednichenko18 and Bisset,19 respectively, with P in bar and T in K as presented by eqs 15−16. [PCH3OH]

1.5 ≤

[PCO][PH2]2 −3

= 10[3971/ T − 7.492log T + 1.77 × 10

[PCO2][PH2]

T − 3.11 × 10−8T 2 + 9.218]

49170 ⎤ ⎥ T2 ⎦

(20)

H 2 − CO2 ≤ 2.5 CO + CO2

(21)

The conversion is usually low and methanol and water must be separated from the gases, hydrogen, CO, and CO2 in a flash separation. We assume that almost all H2 and CO are recycled and that water and methanol are completely recovered. However, the role of the CO2 is more complex. Thus, we develop a surrogate model for the performance of the flash based on offline simulations using CHEMCAD in order to account not only for the gas−liquid separation but also for the absorption of CO2 in the liquid phase. The flash is expected to operate a moderate pressure of around 50−75 bar and temperatures from 298 to 332 K for feed compositions 0.75− 1 mol of H2, 0.3−0.6 mol of CO2, 0.3−0.6 mol of CO, 0.8−1.2 mol of methanol, and 0.8−1.2 mol of water. The fitting equation for the CO2 in the liquid phase turns out to be

(15)

⎡ 5639.5 = exp⎢13.148 − − 1.077 ln T ⎣ T

− 5.44 × 10−4T + 1.125 × 10−7T 2 +

H2 ≤3 CO

(B) The role of CO2 in the reaction mechanism has been and still is a subject of discussion in the literature. Its contribution in reaction models is not well understood. However, it is considered that the concentration of CO2 should be 2% to 8%17,21 and the ratio of the syngas components involving CO220,22 should be

CO + 2H 2 ↔ CH3OH

[PCO][PH2O]

(19)

=0

Figure 4. CO2 removal stage.

(16)

Since these kind of reactors are designed targeting the equilibrium, we assume that the approximation is fair although optimistic. The reaction is favored by low temperatures and high pressures. Today’s synthesis processes take place at low pressure (50−100 bar), since these processes use far less energy than the ones with high pressure as the synthesis gas compression is a costly operation. Furthermore, although the equilibrium conditions favor low temperatures, methanol converters must be operated at temperatures in the range 473−573 K to ensure the catalysts are active and to use the heat of reaction effectively. Furthermore, the atomic mass balance must hold:

%CO2liquid = (1.2606 × 10−5)(Tflash 2) − (9.3191 × 10−3) Tflash + 1.7685

(22)

The gases are recycled to the reactor. Since only conceptual design is performed, no impurities are assumed across the

Figure 5. Methanol synthesis section. D


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Industrial & Engineering Chemistry Research flowsheet. Thus, the purge will not have to operate. However, it is included for consistency. 3.4. Solution Procedure. The aim of this work is to evaluate the use of biogas as a source for chemicals. To optimize the composition of the biogas composition and the operating conditions across the flowsheet, two objective functions are considered. Apart from a classic economic one, we propose an environmental one. Economic Objective Function. Methanol is the main source of income. Furthermore, the recovery of methane in the cleaning stage is considered as a valuable fuel that is possible to be sold or used internally. Among the cost we account for the cost of electricity to power the compressors, the fraction of methane used to provide the energy at the reformer, since this fraction is not used to produce methanol. Furthermore, we consider the consumption of steam either as a raw material or to provide energy, the consumption of cooling water, and the cost of H2S removal. Equation 23 presents the objective function. The values for the prices of the various species are $3.71/1000 ft3 for methane,23 $0.9/gal for methanol,24 cooling water and steam are assumed to be $0.000057/kg and $0.019/ kg, while the electricity is assumed to be $0.06/kWh.25 Finally the membrane cost to remove H2S is $90/kgH2S26

∑

profit = CCH3OH· FCH3OH − Celec·

Table 1. Energy Consumption in Various Water Processing Stepsa range of energy intensity (kWh/Mgal)

∑

FCH4 −

i ∈ CH4inlet

− C H2O· MWH2O(

∑

FCH4)

i ∈ CH4recovery

Fsteam)

i ∈ steam,inlet

− Cq · QS_max + Ccw ·

∑

Q − C H2S·FH2S

j ∈ Q refrigeration

(23)

Environmental Objective Function. The environmental objective function is inspired by the metric developed by Martı ́n.27 This metric evaluates the sustainability of a process considering environmental, social and economic aspects. Among economic aspects, the production and annualized investment costs are analyzed. Among environmental items, it considers the CO2 emissions that are mitigated due to the production of renewable power and fuels. The carbon tax is used to be able to have an economic basis. Finally, the social aspects account for the generation of jobs as a result of the investment and the effect of the use of ground to fuels/power versus other alternatives. Thus, as a basis to evaluate the environmental impact, we consider the generation of CO2 due to the consumption of electricity, that due to the need for thermal energy and the production of steam, the CO2 generated due to the consumption of water, either as raw material or to produce steam, and the CO2 that ends in the atmosphere deducting that actually consumed in the production of methanol. CO2 Associated with Power Consumption. To estimate this contribution we correlate the CO2 produced per electric kWhe presented in IDAE.28 Note that the data are region and atmospheric dependent. kgCO2 /kWhe = − 0.0131(year) + 26.719

high

0 100 250 700 0 1050

14 000 16 000 1 200 4 600 400 36 200

the water processing stages. We use an average value for water supply and distribution in the case of refrigeration, using a value of 7.775 kWhe/Mgal. We use the value for CO2 produced per electric kWh to obtain the carbon footprint. Carbon Footprint Associated with the Water Used As Raw Material. In this case, we not only transport water but also treat it. Thus, the energy involved in water conditioning becomes 15825 kWhe/Mgal, for which the final carbon footprint is defined using also (CO2 kWhe) 0.3225 kgCO2/kWhe.28 Carbon footprint Associated with the Use of Steam. We need to produce stream by evaporation. A closed cycle for steam is considered so that after use, the steam condenses providing the energy, and it is evaporated again. We account for the CO2 produced due to the energy consumed to produce the steam.

W

∑

low

a Table reproduced with permission from ref 29. Copyright 2009 River Network.

i ∈ compressors

− CCH4· MWCH4(

water use cycle segments water supply and conveyance water treatment water distribution wastewater collection and treatment wastewater discharge total

Fsteam = Q/λ

(25)

To compute the CO2 produced in the steam generation, a relation of CO2 per thermal kWh (kWht) is required. In the generation of steam from natural gas combustion, a value of (CO2 kWht) 0.204 kgCO2/kWht is used.28 CO2 Emitted from Biogas Combustion. A fraction of the biogas is used to provide the energy for the reforming stage. Therefore, CO2 is produced by burning the methane while the CO2 that initially accompanies the methane is also released to the atmosphere. CH4 + 2O2 → CO2 + 2H 2O

(26)

CO2 Released Across the Process. We take into account the CO2 either removed at gas processing stages or absorbed in the liquid methanol. CO2 Used along the Process. Actually this is the only term that is negative, meaning that it will not be released to atmosphere. The environmental objective function computing the carbon footprint becomes

(24)

For 2015 we use the value of 0.3225 kgCO2/kWhe CO2 Associated with the Consumption of Cooling Water. We have to account for different issues related to the processing stages of water.29 In Table 1 we present the energy intensity of E


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∑

ENV(kgCO2 ) = (CO2kWhe /(3.6 × 106)) ·

Table 4. Main Operating Variables

W

i ∈ compressors −8

∑

+ CO2kWhe · (4.914882 × 10 ) ·

variable

Q

Treforming reactor (K) Preforming reactor (bar) XCH4 αWGS PWGS (bar) TWGS (K) steam added (per 10 mol/s feed) (mol/s) CO2 removed/CO2 in PSA per 10 mol/s Feed (mol/s) TSynthesis (K) PSynthesis (bar) H2/CO in synthesis reactor CO2 ratio in synthesis reactor recycled fraction from flash 4 Tflash (K) methanol produced per 10 mol of feed (mol (%)) biogas burned per 10 mol/s feed (mol/s) cooling water (kg/10 mol feed)

j ∈ Q refrigeration

+ CO2kWhe × (4.1865 × 10−3) ·(MWH2O) ·

∑

(

Fsteam) + Fsteam·((3470650/(3.6 × 106)) ·

i ∈ steam,inlet

CO2kWht ) + MWCO2(

∑

FCO2 −

i ∈ CO2 inlet

∑

FCO2)

i ∈ CO2 out

(27)

We define the efficiency in CO2 on the basis of the actual flows of CO2 involved across the flowsheet as follows: ηCO2 = (nCO2in − nCO2out)/nCO2in

(28)

This equation is independent of the use of different energy sources to produce the electricity and the thermal energy required. The selection of these sources also determines the CO2 foot print. Heat integration has been included in the formulation using Duran and Grossmann’s30 model. Thus, the process is formulated as an NLP problem with over 1000 eqs and 1200 variables modeled in GAMS and solved using a multistart approach with CONOPT 3.0. It will be optimized using both objective functions. Next, a heat exchanger network31 is developed to determine that the heat required by the process is completely fed by the fired heater and no further steam is required. Finally, a detailed economic evaluation is carried out to estimate the production cost of methanol and the investment required by such a facility.

Table 2. Objective Function Results

economic ($/10 mol feed) environmental function (kgCO2/10 mol feed)

carbon footprint optimization

0.052 −0.017

0.051 −0.018

objective function, Table 3 the biogas composition suggested by the optimization for each scenario and Table 4 reports the main operating parameters of the major units within the process. If we compare the values of the objective function, the optimization of the economic function does not show the best environmental value, as expected. However, both values are close based on the need to use CO2 as raw material so that it is Table 3. Biogas Composition species

economic optimization


CH4 CO2 H2O N2 SH2

0.501 0.468 0.031 1 × 10−4 2 × 10−5

0.519 0.450 0.031 1 × 10−4 2 × 10−5


1273 1 0.945 1 4.5 623 1.686

1273 1 0.935 1 4.5 623 1.608

2.974 mol/ 3.131 mol 473 50 2.188 1.5 0.518 332 6.335 (99,6%)

2.804 mol/ 2.952 mol 473 53 2.031 1.602 0.462 298 6.240 (99.8%)

2.887

2.654

56.412

55.392

consumed during the process. As a result, not only the biogas composition suggested changes. The biogas is richer in methane and poorer in CO2, if the carbon footprint is to be reduced, see Table 3. In this way there is no such generation of CO2 in the digester and the gas has larger potential to fuel substitution. Moreover, in both optimizations, the fraction of nitrogen and H2S is the less value considered, meanwhile the steam fraction reaches the highest value. The main operating variables are shown in Table 4. In the first stage, dry reforming, the pressure required is 1 bar, close to values reported in the literature for hydrocarbon reforming.32 Therefore, there is no need to use the compressor. Furthermore, temperature reaches its highest limit, as expected based on the features of the equilibria. Two conversions have been obtained for the two objective functions. This fact is mainly due also to the different composition of the feed. The second stage, cleanup stage, removes the water condensed to avoid damage in the compressors downstream. Subsequently, the unreacted methane and nitrogen are captured by PSA systems. Next, the molar ratio between hydrogen and carbon monoxide is adjusted for synthesis. In this stage, stream is fed to the water gas shift reactor, which works at 4.5 bar and 623 K, since extra hydrogen is required. We see that when the economic optimization is perfomed a larger amount of steam is used to increase the yield to methanol. The last units in the cleanup stage are the CO2 capture PSA systems. The CO2 that reaches the PSA is less for the optimal carbon footprint optimization, while the percentage captured is similar in both. The final stage of the process is the methanol synthesis. The synthesis reactor operates at 50 bar in the economic optimization, meanwhile in the reduction of the carbon footprint reaches a level of 53 bar. However, the operating temperature is similar in both cases, 473 K. Another characteristic variable is the CO/H2 ratio. The results show that the ratio is a bit higher in the environmental optimization, see Table 4. The CO2/H2 ratio, given by eq 21, reaches the low value of 1.5 in the optimal economical conditions and a value of 1.6 in the environmental optimization. Finally, methanol

4. RESULTS 4.1. Process Operation. In this section we compare the results of the main operating parameters depending on the objective function used. Table 2 shows the values of the



F


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Industrial & Engineering Chemistry Research separation is required, where a flash works at the same pressure than the reactor and a temperature of 332 K in the economic optimization and 298 K in the environmental one. The economic optimization results in a higher yield to methanol. However, the purity is lower, 99.6% versus 99.8%, compared to the case when the environmental objective function is used. The last two operation variables studied are the utilities required. The results shown in the tables reveal that a larger share of biogas is needed as fuel and larger need of refrigeration water as required when the economic objective function is used. Finally we compute the CO2 efficiency of the process for the use of the different objective functions, Table 5. We can appreciate that with both objective functions, the values obtained are very similar and approximately 35% of the CO2 introduced to process is captured as fuel.

tank is obtained with the Matche cost estimator, considering the tank as sphere vessel and as a function of the unit metal weight.36 • Methanol storage tanks. The volume required in this case is estimated for a period of 15 days because it is a product more difficult to sell than biogas. Thus, the tank volume is 413.5 m3. Considering three tanks, the capacity per tank is 138 m3. An investment cost of $69,300 per tank is obtained with the Matche cost estimator using a cone roof tank.36 Figure 6 shows the breakdown of the units’ contribution to the investment cost.

Table 5. Carbon Dioxide Usage Efficiency ηCO2 (%)



33.7

34.6

4.2. Economic evaluation. An economic evaluation of the plant has been carried out in order to estimate the main costs of the plant and the final price that fuel can reach following the economic optimization variables achieved in the previous section, due to the proximity of the results of both optimizations and the typical error in cost estimation. The plant size has been designed to be fed with a flow of 12.08 MM m3/y of biogas, which is a tenth of the maximum potential biogas in the region of Madrid, Spain.33 This location was selected because of the high concentration of potential biogas production. Considering that the plant works 334 days per year (1 month is required to maintenance operations), the flow of biogas feed used for the final design is 0.42 m3/s. Investment Cost. We use the factorial method34 to estimate the investment cost of the facility. It is based on the estimation of the units involved in the process as follows: • SH2 iron foam cost has been estimated following the cost per kg of SH2 removed given by ref 23. A cost of $260 per year is estimated for the optimal composition of biogas obtained in the previous section. • The cost of hydrocarbon and CO2 removal beds has been estimated considering two components. First, the cost of the vessel using the Matche cost estimator36 and, second, the cost of the molecular sieves used to separate the components. The correlations can be found in the Supporting Information of Almena and Martı ́n35 • Flash separators have been estimated using the cost of the vessel given by the weight of the metal.35 • Compressors cost is estimated as a function of the power consumption required per unit and computed in the optimization.35 • Fired heater cost has been estimated as a cylindrical fired heater and with a heat production given by the optimization using Matche correlations.36 • Heat exchangers. Their price is given by the heat exchange area as presented in the Supporting Information in Almena and Martı ́n.35 • Biogas storage tank. The volume required to save is estimated considering a period of a day because the raw material is easily accessible and it is produce continuously. In addition, the gas will be stored in a pressure of 250 bar. The volume needed is 144 m3. The price of the

Figure 6. Distribution of units costs to investment.

Using the factorial method34 for plants that process gases, the capital cost of the plant adds up to $46 MM. The working capital is assumed to be 10% of this value, based on Sinnot’s procedure.34 Production Costs. We consider units amortization, but for the membranes which have to be replaced annually, maintenance, labor, administration and management, and other expenses. All these items are estimated based on Silla’s method.37 The cost for the raw materials and utilities is computed using the prices presented in the description of the objective function. As a summary, the contribution of costs per year is shown in Figure 7. These costs are distributed between the production of methanol, reaching a price for methanol production of $1.75/gal. The price is not that competitive with the current price of methanol, $0.9/gal,24 but getting closer.

5. CONCLUSIONS A new process has been developed and optimized for the production of fuels from biogas based on dry reforming using the CO2 that it is part of the mixture. Two objective functions are used, an economical and an environmental one. The economical optimization selects the use of dry reforming with no addition of steam until the water gas shift reactor, to tune the H2/CO rate. Furthermore, the process has an important environmental value because it is able to capture the CO2 and reduce its presence in the atmosphere. The environmental optimization shows that the CO2 emitted and the utilities required are minimized. In both cases, the biogas composition is tuned for the optimal operation resulting in values around 50% methane and 47% CO2. For the economic optimization, the production cost of methanol becomes, $1.75/gal, versus the current price of G


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Figure 7. Breakdown of production costs.

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REFERENCES

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methanol, $ 0.9/gal, showing potential for this use. The investment cost of such a plant results in $46 MM. Further development and economies of scale may improve the competitiveness of this particular use of biogas with interesting carbon footprint reduction possibilities.

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Qi = energy to preheat the stream entering equipment i (kJ/ s) QS_max = variable of the energy in Duran and Grossmann’s model (kW) TSynthesis = temperature at synthesis reactor (K) Treforming = temperature at the reformer (K) TWGS = temperature at the WGS reactor (bar) Tflash = temperature at the flash (K) T(unit1,unit2) = temperature of the stream from unit 1 to unit 2 (K) W = work (kW) XCH4 = methane conversion in reformer X = molar fraction in liquid phase Y = molar fraction in gas phase ηCO2 = CO2 efficiency αWGS = fraction of the flow processed from the splitter to WGSR λ = laten heat (kJ/kg) ΔHf = formation enthalpy of species i

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 923 294479. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Salamanca Research for software licenses and the TCUE Universidad-Empresa programme for financial support to Borja Hernández.

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NOMENCLATURE Cp = heat capacity (kJ/kmol K) CO2 kWh = emissions due to power used (kg CO2/kWh) fc(j,unit1, unit2) = individual mass flow rate from unit1 to unit2 (kg/s) fc(j) = individual mass flow of final product j F(unit1,unit2) = mass flow rate from unit1 to unit2 (kg/s) FCH4 = flow of CH4 (kmol/s) FCO2 = flow of CO2 (kmol/s) Fsteam = flow of steam (kg/s) K = equilibrium constant ni = moles of species i Pi = pressure of component i (bar) Preforming = pressure at the reformer (bar) PSynthesis = pressure at the reactor (bar) PWGS = pressure at the WGS reactor (bar) CCH3OH = price of methanol ($/kg) Celec = price of electricity ($/kWh) CCH4 = price of methane ($/kg) CH2O = price of steam ($/W) Cq = price of thermal energy ($/W) Ccw = price of cooling water ($/W) CH2S = price of operation for H2S removal ($/kg) steami = steam consumed at different equipment (kg/s) H


Article


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Effects of Dry Reforming and Biogas Composition - American

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