Carbon Capture and Storage  Low Emissions Combustor

Carbon Capture and Storage


Specifying & Selecting a CO2 Compressor

Until recently, CO2 compressors have been thought of as an existing technology that one simply buys as part of the balance of plant (BOP). Unfortunately and to the surprise of many, this is only true if you don’t care about capital or operating cost impacts.

The reality is that the CCS system and the CO2 compressor are intimately coupled and need to be thought of as a combined compression and capture system to minimize the cost impact that these systems can have on the CCS economics.

To properly size and select a compressor the following information needs to be understood and addressed.

Compressor Power & Things That Affect It

  • CCS Application Specific Issues
     -Capture system flash levels & control   requirments
       • Pressure
       • Mass flow additions
     -Water knockout
       • Process location (i.e., pressure)
       • Method – Glycol/Molecular sieve/PSA
     -CO2 compressor inlet pressure
     -Heat integration
     -Materials of construction
       • Heat exchangers
       • Piping
     -Discharge pressure
  • The basic inputs
     -Gas composition, including moisture content
     -Mass flow
     -Inlet pressure
     -Inlet temperature
     -Discharge pressure
  • Often forgotten
     -Cooling media & temperature
       • Air
       • Water-cooled
       • Process cooled
     -Interstage assumptions
       • Pressure drop
       • Design practice
       • Fluor estimate DP = P2^0.7/10; not to      exceed 5 psi
       • Intercooler/heat exchanger approach      temperature or Cold Temperature      Difference– CTD
       • 15°F CTD normal approach temperature
     -Mechanical losses
       • Compressor
       • Gearbox
     -Sparing philosophy (i.e., 2 x 50% + 1)

 


The actual process conditions vary considerably with the type of CCS system employed. Some of the principal features of the various designs are generalized below.

Amine systems
 - Suction pressures – 15; 22; 25; 30 psia
 - Regeneration heat required
    • Conventional amines – 1550 Btu/lbm-CO2
    • Advanced amines – 1200 Btu/lbm-CO2
    • Really advanced amines – 800 Btu/lbm-CO2
 - 8% parasitic power
 - Post combustion - New & Retrofit

Ammonia-based systems
 - Suction pressures – ~ 30-300 psia
 - Regeneration heat required
    • Aqueous ammonia – 493 Btu/lbm-CO2
    • Chilled ammonia – TBD
 - 4% parasitic power
 - Post combustion - New & Retrofit

Selexol/Rectisol
 - Suction pressures 50, 150 & 300 psia with sidestreams
 - Regeneration heat required for the Claus Plant
 - 5% parasitic power
 - IGCC (new) only

Oxy-fuel systems
 - Raw gas feed – 15 to 500 psia
 - Twin purified suction streams – ~150 & 300 psia
 - 12-13% parasitic power
 - New & Retrofit

Discharge pressures – 1200; 1600; 2000; 2215; 2500; 2700; 2900 psia

Of particular concern in the specification is the accurate specification of inlet pressure which has a very large influence on the compressor selection. All too often the term “atmospheric pressure” is used to define what really requires a number with at least one significant digit.

The other factor with typically little or no definition is the cooling system. The first stage and likely the 4th stage of a conventional 8-stage geared compressor are affected by the CCS process. The other stages are affected by the cooling system, yet very little attention is paid to its impact in the majority of compressor specifications that are prepared.

The following table offers some insight into these sensitivities. A simple change in water temperature and intercooler approach temperature from 60°F/9°F to 85°F /15°F can change stage inlet temperatures from 69°F to 100°F and increase the power 8-10%.

Specifying 23.5 psig, instead of 23.5 psia can reduce the power by 16%.

Compressor Power Sensitivity

Another interesting specification issue and one generally not appreciated, even in the industry, is that using a Ramgen CO2 compressor to reach full 2215 psia discharge pressure actually results in small compressor size and considerably less first cost.

Conventional geared designs are fixed-speed units and therefore limited to a relatively constant inlet capacity. To change compression ratio, integral-geared designs add stages. The inlet capacities are relatively fixed within a frame size, because the gearing and gearing center distances are fixed, and therefore the speeds are fixed. Frame development is an expensive proposition and these frames need to cover as wide a capacity range as possible, without sacrificing efficiency.

Ramgen, as a single-stage offering, changes speed to achieve design pressure or head, following a line of constant specific speed. As the compressor speed is increased to achieve higher pressure ratios, the capacity of the unit will also increase. The power will go up, of course, but the unit can be one, or even two sizes smaller.

Frame Sizes vs Compressor Specific Speed

As a case in point, going from a 1200 psia, or a 1600 psia compressor discharge pressure to the full 2200 psia level will reduce the compressor size by 2 and 1 full frame size, respectively, and eliminate the pump thought to be a cost savings!

The simplicity of the compressor design and this single-stage approach also allows Ramgen to consider smaller frame size increments to more closely match the project and process requirements.

Copyright © 2008 Ramgen Power Systems, LLC