Process employing CO 2 capture with ionic liquids

In this section, a developing technology such as CO2 capture with ionic

liquids is employed instead of CO2 capture with monoethanolamine. The

aim is to assess the effect that this developing technology could have on process performance. Aiming at presenting results in a clear way, the ionic-liquid based CO2 capture plants will be compared with the MEA-base

capture plant (Base Case Model) in terms of energy requirements, solvent capacity and cost.

As discussed in section 4.1.4.1, the target application is biogas upgrading (up to 95 mol. % CH4) using, on one hand, ionic liquids which selectively

absorb CO2 physically and, on the other hand, an MEA solution (30 wt. %

MEA).

The mass and energy flows as well as the mass yields, 𝑌_𝑀 energy yields, 𝑌𝐸 and plant energy efficiencies, 𝜂𝐸 for the four process concepts

considered (three ionic liquids and MEA) are presented in Tables 5.8-5.11. In order to allow a fair comparison between the different concepts, the mass flow and energy content of the biogas are identical in all cases. Similarly, the CH4 concentration in the bio-methane product stream is set

to 95 mol. % in all cases.

The mass yield, 𝑌𝑀 which is a measure of the amount of raw biogas that

ends up in the upgraded bio-methane, is defined as:

𝑌

_𝑀

=

𝑀𝑏𝑖𝑜−𝑚𝑒𝑡ℎ𝑎𝑛𝑒

𝑀𝑏𝑖𝑜𝑔𝑎𝑠 Eq. 5.4

The energy yields and efficiencies of the evaluated processes are also presented in Tables 5.8-5.11 The energy yield, 𝑌𝐸 is a measure of the

energy content of the biogas that ends up in the upgraded bio-methane on a LHV basis and is given by,

𝑌𝐸 =𝑀𝑏𝑖𝑜−𝑚𝑒𝑡ℎ𝑎𝑛𝑒_𝑀 ∙𝐿𝐻𝑉𝑏𝑖𝑜−𝑚𝑒𝑡ℎ𝑎𝑛𝑒

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On the other hand, the energy efficiency, 𝜂𝐸 takes into account the total

energy input to the plant, i.e. biogas (LHV), electricity and heat, the later for the MEA process only. 𝜂_𝐸 also takes into account the total energy output from the plant, i.e. bio-methane and electricity, the later for the ionic liquid-based processes only. The energy efficiency is given by:

𝜂

_𝐸

=

(𝑀𝑏𝑖𝑜−𝑚𝑒𝑡ℎ𝑎𝑛𝑒∙𝐿𝐻𝑉𝑏𝑖𝑜−𝑚𝑒𝑡ℎ𝑎𝑛𝑒)+𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑

(𝑀𝑏𝑖𝑜𝑔𝑎𝑠∙𝐿𝐻𝑉𝑏𝑖𝑜𝑔𝑎𝑠)+𝐻𝑒𝑎𝑡 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑+𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑_0.39

Eq. 5.6

As in the base case models, the electricity consumed (grid electricity) is divided by the overall efficiency of the power cycle assumed to be 39% (Haro et al., 2013).

As for energy flows, Tables 5.8-5.11 include both the lower heating value of the raw biogas and upgraded bio-methane coming in and out of the processes respectively, as well as the electricity inputs and outputs. In the processes using ionic liquids, biogas and electricity are the only plant inputs since no other energy inputs, e.g. heat, are fed into these plants. On the other hand, the plant based on the MEA solution has heat inputs (expressed in natural equivalents LHV) for the solvent regeneration. It should be noted that while the in ionic liquid-based plants the solvent is regenerated by pressure-swing, in the MEA-based plant the solvent is regenerated by temperature swing.

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Table 5.8 Summary of inputs, outputs and results of the [C2MIm][Tf2N]concept

PLANT INPUTS

Solvent: [C2MIm][Tf2N] kg·h-1 56,997.63

Biogas

LHV biogas MJ·kg-1 20.20 Mass flow kg·h-1 3,774.46 LHV biogas kW 21,181.94 Electricity

Total electricity consumption kW 545.22

PLANT OUTPUTS Bio-methane

LHV bio-methane MJ·kg-1 43.70

Mass flow kg·h-1 1,522.14

LHV bio-methane kW 18,477.11

FUEL MASS YIELD % 40.3

FUEL ENERGY YIELD % 87.2

PLANT EFFICIENCY % 82.2

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Table 5.9 Summary of inputs, outputs and results of the [C6MIm][Tf2N]concept

PLANT INPUTS

Solvent: [C6MIm][Tf2N] kg·h-1 52,744.93

Biogas

LHV biogas MJ·kg-1 20.20 Mass flow kg·h-1 3,774.46 LHV biogas kW 21,181.94 Electricity

Total electricity consumption kW 575.23

PLANT OUTPUTS Bio-methane

LHV bio-methane MJ·kg-1 43.70

Mass flow kg·h-1 1,454.91

LHV bio-methane kW 17,660.95

FUEL MASS YIELD % 38.5

FUEL ENERGY YIELD % 83.4

PLANT EFFICIENCY % 81.5

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Table 5.10 Summary of inputs, outputs and results of the [P66614][Tf2N] concept

PLANT INPUTS

Solvent: [P66614][Tf2N] kg·h-1 44,182.20

Biogas

LHV biogas MJ·kg-1 20.20 Mass flow kg·h-1 3,774.46 LHV biogas kW 21,181.94 Electricity

Total electricity consumption kW 595.24

PLANT OUTPUTS Bio-methane

LHV bio-methane MJ·kg-1 43.70

Mass flow kg·h-1 1,263.60

LHV bio-methane kW 15,338.70

FUEL MASS YIELD % 33.5

FUEL ENERGY YIELD % 72.4

PLANT EFFICIENCY % 70.7

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Table 5.11 Summary of inputs, outputs and results of the MEA concept

PLANT INPUTS Solvent: MEAa kg·h-1 22,619.64 Biogas LHV biogas MJ·kg-1 20.20 Mass flow kg·h-1 3,774.46 LHV biogas kW 21,181.94 Natural gas LHV natural gas MJ·kg-1 48.85 Mass flow kg·h-1 359.64 LHV natural gas kW 4,880.12 Electricity

Total electricity consumption kW 6.00

PLANT OUTPUTS Bio-methane

LHV bio-methane MJ·kg-1 43.70

Mass flow kg·h-1 1,645.30

LHV bio-methane kW 19,959.73

FUEL MASS YIELD % 43.6%

FUEL ENERGY YIELD % 94.2%

PLANT EFFICIENCY % 76.5%

MEA capacityb kg MEA/kg BM 13.76

a,b

Both the MEA requirements and absorption capacity are referred to pure MEA

The process using the MEA solution produces 1,644.3 kg·h-1 of bio- methane at 95 mol. %, followed by the process using [C2MIm][Tf2N] ionic

liquid, which produces 1522.14 kg·h-1, 1454.91 kg·h-1 produced by the second concept (based on [C6MIm][Tf2N] ionic liquid) and 1263.60 kg·h-1

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difference in the bio-methane output produced by the MEA-based concept and the ionic liquid-based concepts relies on the CO2 absorption

selectivity. The MEA has a very low affinity for CH4 compared to the

affinity that it has for CO2, while in the case of the ionic liquids the CH4

affinity is higher. This causes a significant proportion of the CH4 in the

biogas to end up in the CO2-rich ionic liquid solution. The difference in the

bio-methane production rate from the ionic-liquids concepts can be explained by the fact that, although the [C6MIm][Tf2N] and [P66614][Tf2N]

ionic liquids have a higher CO2 absorption capacity, as shown in Tables

5.8-5.10, they also absorb more CH4 than [C2MIm][Tf2N]. To produce

these amounts of bio-methane, 56,997.63 kg·h-1 of the [C2MIm][Tf2N] ionic

liquid, 52,744.93 kg·h-1 of [C6MIm][Tf2N] and 44,182.20 kg·h-1 of

[P66614][Tf2N] are needed, respectively. These results demonstrate that the

[P66614][Tf2N] has the highest CO2 absorption capacity, followed by

[C6MIm][Tf2N] and [C2MIm][Tf2N], which is in agreement with experimental

data (Carvalho et al., 2010; Zubeir et al., 2015; Carvalho et al., 2009). As for MEA, 22,619.64 kg·h-1 of pure MEA are needed to produce the reported bio-methane rate. These results are expectable since MEA reacts with the CO2 while the ionic liquids only absorb CO2 physically.

As for the electricity flows, it should be noted that the values presented in Tables 5.8-5.10 account for both the biogas compressor and the pump recirculation in the case of the ionic liquid-based plants. In all ionic liquid cases the biogas compressor consumed 500.20 kW. The pump in the first process concept needs 45.02 kW of electricity, while the pumps in the second and third concepts request up to 75.30 kW and 95.04 kW, respectively; therefore, the biogas compressor accounts for the vast majority of the plant electricity consumption. These results show that the viscosity of the ionic liquid has a predominant effect on the electricity used by the recirculation pump. As for the MEA, the recirculation pump only consumes 6 kW of electricity.

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Taking into account Eq. 5.4-5.5, increasing bio-methane production will increase both the mass yield and the energy yield and thus the highest mass yield is achieved by MEA (44%), followed by [C2MIm][Tf2N] (40%),

[C6MIm][Tf2N] (39%) and [P66614][Tf2N] (34%). In this study, the theoretical

maximum value of 𝑌𝑀 is 69%. This is the maximum achievable if the

solvent did not absorb CH4 at all for upgraded bio-methane production at

95 mol. %. As for the energy yields, the highest is achieved by MEA (94%), followed by [C2MIm][Tf2N] (87%), [C6MIm][Tf2N] (83%) and

[P66614][Tf2N] (72%).

The process using [C2MIm][Tf2N] has the highest energy efficiency,

achieving a value of 82%. The process using [C6MIm][Tf2N] also achieves

82% plant energy efficiency, followed by MEA (77%) and [P66614][Tf2N]

(71%). The reason why [C2MIm][Tf2N] and [C6MIm][Tf2N] result in higher

efficiencies than MEA relies on the energy savings that the pressure-swing processes (ionic liquids) offer in contrast with the temperature-swing process (MEA). This is true despite the lower absorption capacity (kg of ionic liquid needed per kg of bio-methane produced) of the processes using [C2MIm][Tf2N] and [C6MIm][Tf2N]; therefore, it becomes clear that

every energy inputs/outputs must be considered before making conclusions based solely on bio-methane production rates or solvent absorption capacity. Fig. 5.11 summarises the mass yields, energy yields and plant energy efficiencies of all process concepts considered.

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Fig. 5.11 Mass yields, energy yields and plant energy efficiency of the evaluated biogas

upgrading processes 0 10 20 30 40 50 60 70 80 90 100

C2MImTf2N C6MImTf2N P66614Tf2N MEA

Y iel d s and eff ici encies ( % )

FUEL MASS YIELD

FUEL ENERGY YIELD

OVERALL PLANT EFFICIENCY

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6. ECONOMIC ASSESSMENT

In document Carbon Capture and Utilisation processes: a techno-economic assessment of synthetic fuel production from CO2 (Page 172-181)