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Carbon Capture and Storage
Amine is used to remove CO2 in various areas ranging from natural gas production to the food and beverage industry, and have been for over sixty years. This technology can be used for carbon capture from pre- or post-combustion gases as well.

There are multiple classifications of amines, each of which has different characteristics relevant to CO2 capture. For example, Monoethanolamine (MEA) reacts strongly with acid gases like CO2 and has a fast reaction time and an ability to remove high percentages of CO2, even at the low CO2 concentrations. Typically, Monoethanolamine (MEA) can capture 85% to 90% of the CO2 from the flue gas of a coal-fired plant, which is one of the most effective solvent to capture CO2.

Challenges of carbon capture using amine include:
 * Low pressure gas increases difficulty of transferring CO2 from the gas into amine
 * Oxygen content of the gas can cause amine degradation and acid formation
 * CO2 degradation of primary (and secondary) amines
 * High energy consumption
 * Very large facilities
 * Finding suitable location for the removed CO2

The partial pressure is the driving force to transfer CO2 into the liquid phase. Under the low pressure, this transfer is hard to achieve without increasing the reboiler’s heat duty, which will result in higher cost.

Primary and secondary amines, for example, MEA and DEA, will react with CO2 and form degradation products. O2 from the inlet gas will cause degradation as well. The degraded amine is not able to capture CO2 any more, which will decrease the overall carbon capture efficiency.

Currently, variety of amine mixtures are being synthesized and tested to achieve a more desirable set of overall properties for use in CO2 capture systems. One major focus is on lowering the energy required for solvent regeneration, which has a major impact on process costs. However, there are tradeoffs to consider. For example, the energy required for regeneration is typically related to the driving forces for achieving high capture capacities. Thus, reducing the regeneration energy can lower the driving force and thereby increase the amount of solvent and size of absorber needed to capture a given amount of CO2, thus, increasing the capital cost.

MEA and DEA
MEA and DEA are primary and secondary amines. They are very reactive and can effectively remove a high volume of gas removal due to a high reaction rate. However, due to stoichiometry, the loading capacity is limited to 0.5 mol CO2 per mole of amine. MEA and DEA also require a large amount of energy to strip the CO2 to during regeneration, which can be up to 70% of total operating costs. They are also more corrosive and chemically unstable compared to other amines.

MDEA Blends
MDEA is less reactive towards CO2, but has an equilibrium loading capacity approaching 1 mole CO2 per mole amine. It also requires less energy to regenerate. To combine the advantages of MDEA and the smaller amines, MDEA is usually mixed with a catalytic promoter such as piperazine, PZ, or a fast reacting amine such as MEA to retain reactivity, but lower regeneration costs. Activated MDEA or aMDEA uses piperazine as a catalyst to increase the speed of the reaction with CO2. It has been commercially successful. .Many tests have been conducted on the performance of MDEA/MEA or MDEA/Piperazine mixtures compared to single amines. CO2 production rates were higher than MEA for the same heat duty and total molar concentration when experiments were performed in the University of Regina pilot plant, which is a modeled after a natural gas plant. There were also insignificant trace amounts of degradation products detected. However, when the same control variables and tests were conducted at the Boundary Dam plant, the CO2 production rate for the mixed solvent was lower than MEA. This was a result of the reduction in the capacity of the solvent to absorb CO2 after degradation. Because the Boundary Dam plant is a coal-fired power plant, it operates under harsher environments and produces an impure flue gas containing, fly ash, SO2, and NO2 that are fed into carbon capture. Even with flue gas pretreatment, there is still enough to produce degradation products such as straight chain amines and sulfur compounds, which accumulate so it is no longer possible to regenerate MEA and MDEA. For these blends to be successful in reducing heat duty, their chemical stabilities must be maintained.

Degradation
Major oxidative degradation products of MDEA include monoethanol amine (MEA), methyl-aminoethanol (MAE), diethanolamine (DEA), amino acids bicine, glycine and hydroxyethyl sarcosine (HES), formyl amides of MAE and DEA, ammonia, and stable salts formate, glycolate, acetate, and oxalate. Higher temperatures and higher CO2 loading accelerate the rate of degradation, resulting in an increase of alkalinity loss as well as total formate production. While MDEA is more resistant to degradation as a standalone compared to MEA, MDEA is preferentially degraded when in an MDEA/MEA blend. Because of the formation of DEA and MAE, which could form nitroso-compounds or diethylnitrosamine and diethylnitraine, the blend could potentially have an adverse impact in terms of atmospheric admissions. In the Boundary Dam plant, emissions increased when CO2 leading of lean amine increased for the blend and MEA. However, decreasing the lean loading increases the rebiller heat duty, which results in an obvious tradeoff between emissions and heat duty or energy costs.

Carbon Capture and Storage


Amine blends that are activated by concentrated piperazine are used extensively in commercial CO2 removal for carbon capture and storage (CCS) because piperazine advantageously allows for protection from significant thermal and oxidative degradation at typical coal flue gas conditions. The thermal degradation rates for methyl diethanolamine (MDEA) and piperazine (PZ) are negligible, and PZ, unlike other metals, protect MDEA from oxidative degradation. This increased stability of the MDEA/PZ solvent blend over MDEA and other amine solvents provides for greater capacity for and requires less work to capture a given amount of CO2.

Piperazine's solubility is low, so it is often used in relatively small amounts to supplement another amine solvent. One or more of piperazine's performance advantages are often compromised in practice due to its low concentration; nonetheless, the CO2 absorption rate, heat of absorption, and solvent capacity are increased through the addition of piperazine to amine gas treating solvents, the most common of which is MDEA due to its unmatched high rate and capacity efficiency. For example, a 5 m PZ/5 m MDEA blend yields an 11% larger difference in CO2 concentration than 8 m PZ between the lean (inlet absorbent) and rich (outlet absorbent) amine solvent streams, or in other words, more CO2 is removed from the sour (flue) gas stream per unit mass of solvent, and an almost 100% larger concentration difference than 7 m MEA.

Given that typical amine-based absorption processes run at temperatures from 45°C to 55°C, the capabilities of piperazine are well within the bounds of and thus favored for carbon capture. Piperazine can be thermally regenerated through multi-stage flash distillation and other methods after being used in operating temperatures up to 150°C and recycled back into the absorption process, providing for higher overall energy performance in amine gas treating processes.

The advantages to using concentrated piperazine (CPZ) as an additive had been confirmed through, for example, three pilot plants in Australia that are operated by CSIRO. This program was launched to explore remedies to the high costs of post-combustion carbon capture, and the results were positive. Using CPZ, which is more reactive and thermally stable than standard MEA solutions, capital and compression (energy) costs were lowered through size reductions in absorber columns and solvent regeneration at higher temperatures.

Chemistry
The amine groups on piperazine react readily with carbon dioxide to produce PZ carbamate at a low loading (mol CO2/equiv PZ) range and PZ bicarbamate at an operating range of 0.31-0.41 mol CO2/equiv PZ, enhancing the rate of overall CO2 absorbed under operating conditions (refer to Figure 1 below). Due to these reactions, there is limited free piperazine present in the solvent, resulting in its low volatility and rates of precipitation as PZ-6H2O.