CO2 : Bone of Contention

Interest in recovery of carbon dioxide (CO2) from flue gases / any other gas is being propelled by multiple factors, which may vary from merchant CO2 market, Enhanced oil recovery (EOR), and pressure of GHG emissions. In fact, it has always been a problem to all.

Issues

1. Low CO2 Partial Pressure Flue gases have very low CO2 partial pressures because they are typically available at or near atmospheric pressure with CO2 concentrations of typically 5 to 15 vol% except other gases e.g. Distillery Biogas or Waste gas from reactors where it can go as high as 50-60% also.

The overall energy consumption invariably results in unattractive economics due to compression requirement which is higher than the cost of recovered CO2. The only commercial absorbents active enough for recovery of dilute CO2 from atmospheric pressure gas are monoethanolamine (MEA) and other primary amines including the newly developed hindered amines.

2. High Flue Gas Temperature Hot flue gases can cause solvent degradation and decrease absorber efficiency. The flue gas must be cooled to a water dew point of 50 °C prior to entering the absorber. This is accomplished through an scrubber which may require SO2 OR H2S scrubbing also if need be, or in a direct contact water cooler. This DCC can be used as a good resources for low grade waste heat using heat pump.

3. Regeneration Energy. Absorbents that are effective at low pressure requires higher regeneration energy due to higher chemical activity. The design challenges are to minimize regeneration energy by selecting a solvent with a relatively low reaction energy, and to use low-value heat sources to provide this energy.

4. Other Components. Other gaseous components e.g. SOx, NOx, H2S etc. are the major problematic constituents which complicate the overall design efficiency of the CO2 recovery system. They add to many additional steps e.g. removal & pre-treatment. Also cause corrosion & therefore metallurgical demands also goes up for the system design adding to capital requirement.

5. Foriegn Material. Flue gases generally carry ash, soot etc hence require filtration/separation of them as they tend to foul solvents/absorbents on permanent basis. Since, solvents are generally very costly manufacturers can't afford to loose them.

6. Value or Uses. This is the biggest factor which controls the entire business strategy for CO2 recovery plants. The major uses are, as mentioned above, merchant market which is mainly dry ice market and EOR market where full potential is stil not tapped and technology is also in the intital phase.

7. Nitrogen. A gas which does nothing but compel to do everything. In most of the cases, the nitrogen content is so high which occupies space, reduces effective partial pressure of CO2, carries heat & solvent and can cause up to 50% higher capital requirement compared to the case if nitrogen is not there. BUT, the removal of nitrogen itself is a matter of great deal, so better to leave it unless volumes are large enough.

8. Mode. System design also depend on the final objective of CO2 removal system. The costs are significantly different if CO2 is to be recovered in comparison to the case when the purpose is removal only.

Technologies

Different technologies which are available currently vary widely as can be seen below.

1. Chemical Absorption. As discussed earlier, the low pressure system can work only on chemical solvents other technologies are out of context when partial pressure of CO2 is less. The probable pressure range may vary from 5 to 10 Atm of partial pressure below which only chemical absorbents are feasible such as Potassium Carbonate based systems, MEA based systems or in the latest developments hindered amines.

The operating costs are higher in this case due to higher reactivity of the solvent with CO2 & hence, need more heat input.

2. Physical Absorption. When CO2 partial pressure is more than either physical or compbination of chemical & physical solvents can be used to minimize the operating costs depending on the requirement of final level of CO2 in feed gas i.e. total CO2 recovery. Water is the cheapest solvent out of them. Others such solvents may be Selexol etc.

3. Membrane Process. Membranes suffer from both the cost of compression and heat exchange to obtain a high pressure feed and also they produce an impure CO2. The pressure range may start at 25 Atm level. There are currently no commercial applications of membranes for recovery of CO2 from flue gases, though they have been used in large EOR projects to recycle CO2 from the associated gas. The presence of fly ash and the effects of trace components such as SOx, & NOx are also potential complications.

The most likely applications for membranes are in small skid-mounted plants where an impure CO2 product is acceptable and offshore applications that can take advantage of their compact size and low weight. Membrane-amine and membrane-cryogenic separation hybrids have been considered for special applications such as offshore locations where again their compact size and low weight are beneficial.

4. Cryogenic Separation. Cryogenic separation of CO2 is still at conceptual stage due to very low concentration & hence, cryo level cooling requirement. For more info on this you can look at my article at http://cheresources.com/ammonia.shtml.

5. PSA Separation. Carbon dioxide separation through PSA is offered in the Low Cost Ammonia Process (LCA). PSA is scalable and may be more economical because of efficient carbon dioxide recovery at higher pressures. However, further development in this direction is essential for the recovery of high purity carbon dioxide as desired in many cases. However it is comparatively better than Membrane process in terms of tolerability limits for feed gas for membranes.

Costs

A. Fixed COST FACTORS

1. Equipment Size

Cost of CO2 removal or recovery vary depending on many factors.

The first & most important is the total gas flow rate which directly affects the equipment size. Interestingly, the CO2 concentration in the feed gas does not affect the absorber column sizing. The absorber column dia is directly proportional to square root of gas volume flows in (Nm3/hr) & can be approximated with a mutiplying factor of 0.8 to 1.5, thus can be represented as below.

Absorber Column Dia (Meter) = A x (Feed Gas Flow)^0.5

Where,

A = 0.8 to 1.5

Feed Gas flow is in Nm3/Hr

Clearly, the CO2 concentration is not in the role which is indirectly included in total gas flow.

However, it affects the height of the column directly. A factor is a function of absorbent properties. Currently availabe best solvents can offer A = 0.8 or more.

Similarly Stripper can be approximated by the following equation.

Stripper Column Dia (Meter) = S x Da x (Vol% CO2 in Feed)^0.5
Where,
S = 0.25 to 0.5
Da = Absorber Dia calculated above

Please note that these are the approximations only for preliminary cost estimates for carrying out design feasibility studies.

3. Solvent Circulation

The solvent circulation rate can be correlated to CO2 partial pressure which is the governing factor depending on the nature of solvent. For physical solvents, circulation will be more due to higher vapor pressure of CO2 over solution whereas it will be less for chemical solvents.

Circulation rate (Te/hr) = C x CO2 partial pressure in feed gas (Atm.)

Where,

C = solvent factor which is,

= 200 for chemical solvents &

= 200 - 300 for physical solvents.

B. VARIABLE COST FACTORS

4. Energy Costs

Steam consumption in totality varies widely among different type of solvents. It ranges from 500 Kcal/Te of CO2 to 1000 Kcal/Te of CO2. The best part is that all the heat required is low level heat upto 3-4 Atm pressure.

5. Power Costs

Major power consumers are flue gas blowers and solvent circulation pumps. Based on above equations power consumption comes close to the following correlation.

Power Load (Pumps) kW = P x CO2 partial pressure in Atm.

Where,

P = 120 to 160

For flue gas blower, it varies according to the CO2 concentration in feed gas.

Blower Power kW = B x Feed gas flow Nm3/hr.

Where,

B = 0.040

Other costs include various utilities including cooling water, Solvent losses etc out of which solvent losses are the major portion & may vary from 5 - 10 % of the total operating cost.

Conclusion

The technology to recover CO2 from flue gases is commercially available and, though mature, is being significantly improved. The process economics greatly depends on the size of the plant. This paper provides approximates for establishing preliminary estimates of fixed & operating costs. A 1000 te/d plant of conservative design can produce CO2 for $30/tonne or from coal-fired flue gas. However, 4600 te/d single-train plants are possible. Economy of scale, together with the ongoing development in improving the solvent properties (e.g. MHI has developed an hindered amine based solvent KS-1 which is supposed to reduce the production cost for low pressure systems such as recovery from flue gases), have the potential of delivering CO2 at a price much lesse than that of the reference 1000 te/d plant.