Cost of Educts and Side Products

Biogas and oxygen are the educts, while lights, ethane, and ethylene are the products. The considered range of costs for these streams has been previously reported in Chapter 2.4.2, except for the light gases, which is further discussed in the following sub-chapter. The ow rates of all streams entering and exiting BG-OCM process together with their respective price ranges and cash ows are reported in Table 4.14. The values labeled as "Best" represent the best case scenario, i.e., cost of educts at their lower bounds and cost of products at their upper bounds. The values labeled as "Worst" represent the opposite, while the values labeled as "Average" use the average between the best and worst values for each educt and product.

Table4.14:Flowrates,costranges,andcashowsforalleductsandsideproductsoftheBG-OCMprocessEductorFlowFlowRateCashowsinUSDyear −1ProductRateUnitsPriceRangePriceUnitWorstAverageBestBiogas131,400,000Nm 3year −10.0220.0525USDNm −3-6,898,500-4,894,650-2,890,800Oxygen22,781kgyear −10.0400.100USDkg −1-2,278-1,595-911Ethane3,342,063kgyear −10.04670.0686USDkg −1156,201192,752229,303Lights48,782,447kgyear −12.05.0 1USDGJ −1HHV4,817,8098,430,29112,043,273TOTAL-1,927,2683,726,7989,380,8641HHVgas=49.4MJkg −1=35.17MJm −3@288K&101.325kPa

4.6 Economic Evaluation Biogas represent a much larger share of the total educt cost rates than oxygen.

This is mainly because, in adiabatic regime, a very high methane to oxygen ratio is applied in the reaction so oxygen consumption is relatively low. Any reduction cost in biogas production and treatment would, thus yield more signicant impacts to the nal cash ow.

The contribution of ethane as a side product has been found to be rather small. It is possible to recycle ethane to the OCM reactor and increase ethy-lene output, but this also reduces the overall selectivity because more C2 hy-drocarbons will be fully oxidized. Ethane is actually an intermediate product of the oxidative coupling reaction, which is further dehydrogenated into ethylene.

Given that ethylene is more reactive and more easily oxidized than ethane, the best approach is likely to target ethane production in the OCM reactor and add a catalytic or thermal Ethane Dehydrogenation (EDH) reactor downstream.

The highest contribution to the overall educt and side products cash ow is found to be from the light or o-gases. Low methane conversions on the OCM reactor means that most of the methane entering the process exists in this stream. Therefore, the cost of this stream, which is closely linked to the overall energy cost, is of paramount importance to determine the feasibility of BG-OCM in combination with a CHP unit as considered herein. For the worst case scenario, the net cash ow is negative, which would require a higher ethylene price to cover it. On the other hand, on the average and best case scenarios, sales of light gases contribute greatly to the net cash ow of the plant, which is already positive even without revenues from ethylene sales.

Light Gas Specications and Cost

As discussed in Chapter 2.4.2, the sales price of the lights stream is highly dependent on its specications and heating value. In this work, its cost is assumed to be a fraction of the natural gas price in Brazil (see Chapter 2.4.2).

Brazilian pipeline natural gas specications are dictated by resolution ANP No. 16-2008 by Agência Nacional do Petróleo, Gás Natural e Biocombustíveis (ANP), which is the federal agency responsible for regulating petroleum, natural gas, and biofuels industries. The resolution denes the value Wobbe index (Eq.

4.5) with the Higher Heating Value (HHV) dened as per ASTM D3588 (60°F or288 Kand 101,325 Pa). The motor octane number is dened by Eq. 4.8 and used to calculate the number of methane with the linear correlation in Eq. 4.7.

W obbe_index= HHV

√︁Ggas

(4.5) G_gas = ρ^{ST P}_gas

ρ^{ST P}_air = M W_gas

M Wair (4.6)

NCH4 = 1.445·M ON−103.42 (4.7)

M ON = 137.78·x_CH4+ 29.948·x_C2H6 −18.193·x_C3H8

−167.062·x_C4H10+ 181.233·x_CO2+ 26.994·x_N2 (4.8) In Equations 4.5 to 4.8:

W obbe_index is Wobbe index in kJ m⁻³

HHV is the gas higher heating value at288.15 Kand101,325 PainkJ m⁻³ G_gas is relative density of the gas at standard temperature and pressure

conditions

ρ^{ST P}_i is the gas/air density at standard conditions in kg m⁻³ M Wi is the gas/air molecular weight inkg kmol⁻¹

M ON is the motor octane number

xi is the molar fraction of componentiin the gas NCH4 is the methane number

Table 4.15 shows the simulation stream results for the lights stream. Table 4.16 compares the main characteristics of the lights stream to the natural gas specications by ANP. They have very similar higher heating value, Wobbe in-dex, methane number, methane content, inerts content, as well as hydrocarbon dew point. The gas' heating value and Wobbe index lie close to the speci-cation's lower bound, because natural gas also has higher hydrocarbons that contribute to its heating value. The lights stream contains CO, which is not typically present in natural gas and, therefore not mentioned in ANP 16-2008.

Also, the gas pressure is lower than that of pipeline natural gas, since the gas has been expanded and used for refrigeration. Therefore, the lights stream could not be directly exported/transported via pipeline and must be used in an adjacent CHP unit. Its sales price is assumed to be range from 2.0 USD GJ⁻¹ to 5.0 USD GJ⁻¹.

4.6 Economic Evaluation

Table 4.15: Stream results for the lights by-product stream

Variable Value Units

Vapor Fraction 1

-Temperature 30.00 °C

Pressure 2.00 bar

Average MW 16.81 kg kmol⁻¹ Mole Flows 331.02 kmol h⁻¹ Mole Fractions

H2 0.015 mol mol⁻¹

N₂ 0.013 mol mol⁻¹

O₂ 0 mol mol⁻¹

CH4 0.904 mol mol⁻¹ C₂H₄ 5.80E-04 mol mol⁻¹ C₂H₆ 8.69E-08 mol mol⁻¹ C3H6 0 mol mol⁻¹

CO 0.068 mol mol⁻¹

CO₂ 0 mol mol⁻¹

H₂O 0 mol mol⁻¹

Table 4.16: Comparison between natural gas characteristics specied by ANP 16-2008 and those obtained for the light gases stream

Characteristic ANP 16-2008 Lights Stream Units Higher heating value¹ 35.0 - 43.0 35.17 MJ m⁻³

Wobbe index¹ 46.5 - 53.5 46.13 MJ m⁻³

Methane number ≥65 591.16

-Methane content ≥85.0 90.4 mol%

Inert concent (N₂+CO₂) ≤6.0 1.3 mol%

Hydrocarbon dew point² ≤0 -89 °C

1At288 K and101.325 kPa

2At4.5 MPa