What is the typical specific energy for CO₂ compression in CCUS?

DOE NETL Cost & Performance Baseline reports 80–130 kWh/tCO₂ for compression from low-pressure capture (~1.5 bara) to dense-phase pipeline conditions (~150 bara). The lower end (~80) corresponds to advanced multi-stage centrifugal trains with optimized intercooling; the upper end (~130) reflects older designs and high-CO₂-content flue gas where compression starts at near-atmospheric pressure.

How many compression stages are typical for CCUS service?

4–6 stages are typical for compression from atmospheric capture to 150–200 bara dense-phase pipeline conditions. Pressure ratio per stage is usually limited to 2.5–3.0 for centrifugal machines (aerodynamic limit) and 3.0 for reciprocating machines (mechanical limit). The final stage often crosses the critical point — at ~75 bara and 30 °C the fluid becomes a liquid and the last stage is properly a centrifugal pump rather than compressor.

Why is intercooling critical for CO₂ compression?

Without intercooling, multi-stage CO₂ compression would produce discharge temperatures > 200 °C at typical pressure ratios — beyond carbon-steel piping limits and well above efficient compressor operation. Intercooling between stages back to 35–40 °C reduces the polytropic head per subsequent stage (head ∝ T_in), reduces total compression energy by 20–30%, and keeps materials within ASME B31.3 process-piping temperature ratings.

What is the difference between CO₂ compression and pumping?

Below the critical pressure (~74 bara), CO₂ is a compressible gas and is handled with centrifugal or reciprocating compressors using polytropic head (H_p = Z·R·T·n/(n-1)·[PR^((n-1)/n) − 1]). Above ~75 bara at temperatures below 30 °C, CO₂ is essentially incompressible liquid and is handled with centrifugal pumps using simple ΔP head (H = ΔP/ρ·g). The transition typically occurs at the discharge of stage 4 of 5 in a CCUS train.

What polytropic efficiency is typical for CO₂ compressors?

Modern multi-stage centrifugal CO₂ compressors achieve polytropic efficiency η_p of 75–82% at design point (DOE NETL, MAN Energy Solutions, Siemens Energy datasheets). Reciprocating compressors used for low-flow CCUS service (e.g., enhanced oil recovery sites) achieve 75–80%. Mechanical efficiency (gearbox losses) is typically 97–99%. Older or off-design service can drop η_p to 65–70%.

CO₂ Compression for CCUS | Decarbonization

1. Overview

CCUS systems must compress captured CO₂ from low-pressure capture conditions (typically 1.2–2.0 bara at 30–60 °C) to pipeline transport conditions (100–200 bara, 30–50 °C dense phase). The compression train is one of the largest parasitic loads on the capture plant — typically 80–130 kWh per tonne of CO₂ captured, equivalent to 4–7% of net plant output for a coal-fired power station with 90% capture.

Three engineering challenges dominate CO₂ compressor design:

Pressure ratio: 75–150× compression from capture to pipeline — requires 4–6 stages with aerodynamic and mechanical limits per stage.
Critical-point transition: at ~75 bara the fluid becomes liquid-like; the last stage is properly a centrifugal pump, not a compressor.
Heat management: compression generates substantial heat that must be rejected via intercoolers to keep materials in range and reduce subsequent-stage power.

Standard / Reference	Scope
GPSA Engineering Data Book §13	Polytropic head, Schultz method, multi-stage design
ASME PTC-10 (2007)	Compressor performance test code — polytropic basis
API STD 617 (2014)	Axial and Centrifugal Compressors and Expander-Compressors
API STD 618 (2007)	Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services
DOE NETL Cost & Performance Baseline (2022)	Reference compression energy for CCUS power-plant integration
ISO 27913:2016	CO₂ pipeline transport — compression to dense phase

2. Polytropic Thermodynamics

The Schultz polytropic head equation (GPSA §13, ASME PTC-10) gives the work input per unit mass for a single compression stage:

H_p = Z_avg · (R/MW) · T_in · n/(n−1) · [PR^(n−1)/n − 1] H_p = polytropic head (J/kg) Z_avg = average compressibility factor across the stage R = 8.314 J/(mol·K) MW = 0.0440 kg/mol for CO₂ T_in = stage suction temperature (K) PR = stage pressure ratio (P_out/P_in) n = polytropic exponent

Polytropic exponent

The polytropic exponent relates to the isentropic exponent (k = Cp/Cv) through the polytropic efficiency η_p:

n = 1 / [1 − (k − 1)/(k · η_p)] k = 1.30 for CO₂ at moderate pressure (more variable near critical) η_p = polytropic efficiency (0.75–0.82 typical)

For CO₂ with k = 1.30 and η_p = 0.75: n = 1 / [1 − 0.30/(1.30 × 0.75)] = 1 / [1 − 0.308] = 1.444.

Discharge temperature

The polytropic discharge temperature is:

T_out = T_in · PR^{(n−1)/(n·η_p)}

Shaft power

Total shaft power accounts for polytropic efficiency (gas-side losses) and mechanical efficiency (gearbox/coupling losses):

P_shaft = m_dot · H_p / η_p / η_mech m_dot = mass flow (kg/s) η_mech = 0.97–0.99 typical for direct or geared drive

Why polytropic, not isentropic: The Schultz polytropic head method properly accounts for the path-dependent thermodynamic state through the stage. Isentropic head would underpredict the actual work input by ~5–10% for typical η_p = 0.75. Polytropic is the industry standard for centrifugal/axial compressors per ASME PTC-10.

3. Multi-Stage Design

The total pressure ratio from capture to pipeline is typically 75–150 (e.g., 1.5 → 150 bara = 100×). This must be split into multiple stages, each within mechanical/aerodynamic limits.

Stage count

For a target maximum pressure ratio per stage (PR_max):

N_stages = ceil(ln(PR_total) / ln(PR_max)) Then PR_{per stage} = (PR_total)^1/N

Compressor type	PR_max/stage	Stages for 100× total	Notes
Centrifugal (CCUS standard)	3.0	5	Aerodynamic limit at high tip Mach number
Centrifugal (conservative)	2.5	6	Margin for off-design and inlet variations
Axial	1.4	14	Multi-stage in single casing; rare for CO₂
Reciprocating	3.0–4.0	4	Used for low flow / EOR applications

Compressibility factor variation

CO₂ Z-factor varies substantially across the train — from ~0.95 at low P to 0.6 near the critical point and back to ~0.85 at supercritical. Industry practice is to use stage-specific Z-averages:

Stage suction P (bara)	Typical Z_avg
1–10	0.97–0.99
10–30	0.92–0.96
30–50	0.85–0.92
50–75 (near critical)	0.60–0.85 (steep)
> 75 (supercritical)	0.85–1.00

For final design, Z is computed from PR-EOS or Span-Wagner at each stage's actual conditions.

4. Intercooling

Inter-stage cooling brings the gas back to a low temperature (typically 35–45 °C, set by cooling water or ambient air) before the next compression stage. The benefits are substantial:

Polytropic head reduction: H_p ∝ T_in — cooling from 175 °C to 40 °C reduces next-stage head by ~30%.
Material limits: uncooled compression to 150 bara would produce discharge T > 300 °C — beyond ASME B31.3 carbon-steel piping limits (typically 425 °C max for short term, but cyclic limits much lower).
Water and acid removal: intercoolers also condense water vapor and trace contaminants, simplifying downstream dehydration.

Intercooler duty

Heat removed per stage is roughly:

Q_cool = m_dot · C_p · (T_discharge − T_cooled) C_p = 0.85 kJ/(kg·K) for CO₂ at moderate conditions T_cooled = 35–45 °C typical (set by cooling medium)

Total intercooler duty is summed across all stages except the final (which discharges directly to the pipeline). For the worked example below (4 intercoolers between 5 stages), the total heat removed is ~1.3× the shaft power — significant cooling water demand.

Aftercooler

An aftercooler downstream of the final stage cools the high-pressure dense-phase CO₂ to pipeline operating temperature (usually ambient + 5–10 °C). This is critical because:

Pipeline operating T is typically 35–40 °C (above CO₂ critical point) — feed must match.
Hot pipeline operation reduces CO₂ density and increases ΔP per unit mass.
Pipeline expansion joints and bends are designed for the operating T range.

Heat integration opportunity: CCUS amine-capture plants need ~3–4 GJ/tCO₂ of low-grade heat for solvent regeneration. The compressor intercooler heat is at 70–150 °C — too low for solvent reboiler use directly, but can preheat boiler feedwater or building HVAC. Integration typically captures 5–10% of compression energy as recoverable heat.

5. Pump Transition Near Critical

The CO₂ critical point (304.13 K = 30.98 °C, 73.77 bara) sits in the middle of typical CCUS compression trains. Below the critical pressure, CO₂ is a compressible gas; above it, dense liquid (or supercritical fluid). At the typical intercooler temperature of 35–40 °C, conditions immediately below T_c with enough cooling can produce a phase transition.

Industry practice exploits this by switching from compression to liquid pumping for the final 1–2 stages:

Train segment	P range (bara)	Equipment	Energy (kWh/tCO₂)
Stages 1–4 (gas compression)	1.5 → 75	Centrifugal compressor	~80–100
Knockout + cooler @ 75 bara, 30 °C	~75	Surface condenser	~ −20 (heat removal)
Final stage (liquid pumping)	75 → 150	Centrifugal pump	~5
Total compression + pumping	1.5 → 150	Hybrid train	~85–105

The liquid pumping segment is dramatically more efficient than gas compression at the same pressure ratio — ~5 kWh/t to lift from 75 to 150 bara via pump, vs ~25 kWh/t to compress the same step as gas. This is the basis for the lower end of the DOE NETL specific-energy range.

Cooling-pump pinch: The CO₂-to-liquid transition requires cooling to ~30 °C at 75 bara — but ambient/cooling-water temperatures in hot climates may be too warm to achieve the condensation. In such cases, refrigerated cooling or vapor-recompression is needed, partially offsetting the pumping efficiency gain. Site-specific cooling capability often determines train architecture.

6. Worked Example

Problem: Size a CO₂ compression train for 100,000 kg/h captured CO₂. Suction 1.5 bara at 35 °C. Discharge 150 bara dense-phase. Centrifugal, intercooler outlet 40 °C, η_p 75%, η_mech 98%, max PR/stage 3.0.

Step 1: Total ratio and stage count.

PR_total = 150 / 1.5 = 100 N = ceil(ln(100) / ln(3.0)) = ceil(4.19) = 5 stages PR_{per stage} = 100^1/5 = 2.512

Step 2: Polytropic exponent.

k = 1.30, η_p = 0.75 n = 1 / [1 − 0.30/(1.30 · 0.75)] = 1 / [1 − 0.308] n = 1.444

Step 3: Stage 1 head and power.

T_in = 308 K, Z_avg = 0.95 H_p,1 = 0.95 · (8.314 / 0.0440) · 308 · 1.444/0.444 · [2.512^0.444/1.444 − 1] = 0.95 · 188.95 · 308 · 3.252 · 0.318 = 57,200 J/kg m_dot = 100,000 / 3600 = 27.78 kg/s P_gas,1 = 27.78 · 57,200 / 0.75 = 2,118,000 W = 2118 kW P_shaft,1 = 2118 / 0.98 = 2161 kW T_out,1 = 308 · 2.512^{0.444/(1.444·0.75)} = 308 · 2.512^0.410 = 308 · 1.451 = 446.9 K = 173.8 °C

Step 4: Repeat for stages 2–5 with intercooled T_in = 313 K (40 °C). Z drops as P rises, so head per stage actually drops slightly:

Stage	P_in (bara)	T_in (°C)	T_out (°C)	Z_avg	H_p (kJ/kg)	Shaft kW	Intercool kW
1	1.5	35	174	0.95	57.2	2161	3170
2	3.77	40	184	0.95	58.1	2196	3404
3	9.46	40	184	0.92	56.3	2127	3404
4	23.78	40	184	0.75	45.9	1735	3404
5	59.78	40	184	0.60	36.7	1387	0
Total						9606 kW	13,382 kW

Step 5: Specific energy and cooling load.

Specific = 9606 / 100 = 96 kWh/tCO₂ ✓ (within DOE NETL 80–130 range) Total cooling = 13.4 MW (~ 1.4× shaft power) Cooling water at 8 °C ΔT: ~ 1430 m³/h

Result: 5-stage centrifugal train, 9.6 MW shaft power, 13.4 MW cooling. The decreasing Z-factor through the train means later stages are slightly lower-power than uniform-Z analysis would suggest. Integrating the final 75 → 150 bara segment as a liquid pump (instead of gas stage) would reduce specific energy to ~85 kWh/t and add a refrigeration load instead of a compression stage.

7. Standards & References

GPSA Engineering Data Book, 14th Ed. (2017), §13 Compressors
ASME PTC-10 (2007), Performance Test Code on Compressors and Exhausters
API Standard 617, 8th Ed. (2014), Axial and Centrifugal Compressors and Expander-Compressors
API Standard 618, 5th Ed. (2007), Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services
DOE NETL Cost and Performance Baseline for Fossil Energy (2022) — Volume 1: Bituminous Coal & Natural Gas to Electricity
ISO 27913:2016, CO₂ capture, transportation, and geological storage — Pipeline transportation systems
Schultz, J.M. (1962). "The Polytropic Analysis of Centrifugal Compressors," J. Eng. Power 84(1), 69–82.
IEAGHG Technical Report 2014/04, "CO₂ Pipeline Infrastructure"
NIST REFPROP, NIST Standard Reference Database 23, Version 10.0

CO₂ Compression for CCUS

80–130 kWh/t

2.5–3.0

75–82%

Contents

Compressor train sizing