What to do with CO 2 ?

The question that arises is what to do with the captured CO2. With the evolution and expansion of CO2 capture processes, the need to give a second life to carbon dioxide arises.

Several ideas are reported and described.

Emission source Description Share of CO2

emission (%)

CO2 concentration (vol.) in the off-gas flow (%)

Process furnaces Heat generation via combustion of fossil energy carriers for distillation columns and reactors

30-60 8-10

Steam generators Process steam generation via combustion of fossil energy carriers

20-50 4-15

Catalytic crackers

Burn-up of petroleum coke 20-50 10-20

Hydrogen production

Reforming of hydrocarbons to H2 and CO2

5-20 20-99

Chapter 1

1.4.1

CO

transport

First, it is important to study the transport of CO2. There are various ways. There are tanker trucks that can carry tens of tons of CO2. However, being a gas and being produced in large quantities, a pipeline is preferable while in some conditions it is also possible to transport by ship.

There are over 6000 km of CO2 pipelines installed in the United States to support enhanced oil recovery operations. To ensure that the fluid is maintained in a single phase, the duct must be kept at a pressure higher than the critical one which is for CO2 73.9 bar. Water is removed to prevent corrosion inside the pipes.

Let's now analyze the costs to compare the different transport technologies. As a base case we consider a pipeline that transports 10 MtCO2/year per year which corresponds to a cost equal to 1 $/tCO2/100 km. Using a truck, on the other hand, costs 7 $/tCO2/100 km. The ship, like the truck, has higher costs that are linked to the conditions of transport of CO2 that are −20°C and 20 bar. For intermediate transport, a pipeline is worthwhile while if you go over long distances, a ship can be competitive.

Transporting CO2 is however a very dangerous process as it is a gas heavier than air and accumulates on the ground. CO2, if inhaled in concentrations above 17%, can also lead to death.

1.4.2

Geological storage

In oil wells, daily production decreases over time as there is a decrease in pressure in the reservoir; moreover a large part of oil remains trapped in the pores of the rocks. The so-called EOR (Enhanced Oil Recovery) techniques are then used to increase the daily production. One of these techniques is the injection of CO2 into the reservoir with the aim of maintaining the pressure and releasing the oil from the rocky pores. It is important to underline that this is not a solution to fight climate change, but it is a way to use CO2 and make CCS profitable. Furthermore, these EOR projects help to spread pipelines for the transport of CO2. It has been proven by EOR studies that injecting CO2 into these rock formations is safe.

1.4.3

Target formation

In order to use a geological formation to store CO2, four criteria must be met.

1. The geological formation must have good permeability to let the water flow (for example bucket of sand).

2. The geological formation must be at least 800 m deep to allow the CO2 to remain in liquid form (the pressure must be above the critical pressure of CO2). In fact, at a depth of 800 meters, the hydrostatic pressure is about 80 bar which is greater than 73,9 bar.

3. The target formation must have an impermeable caprock to ensure that the CO2

remains trapped.

4. It is preferable to have a large and thick geological formation so that large quantities of CO2 can be stored there.

In practice, the formations that have these requirements can be two: the oil and gas fields as seen above or deep saline formations

1.4.4

Ocean storage

It is no longer used today but has been studied in the past. The aim is to put the CO2 at 800 meters in order to dissolve the CO2 in the water. The main problem remains the fact that CO2

does not remain in the seabed but interacts with the atmosphere. A major problem is related to the marine ecosystem that is being altered.

1.4.5

CO

utilization

Today, instead of storing CO2, we try to find ways to use it for useful purposes. This technique is called carbon capture and utilization (CCU). Unfortunately, this path is followed by only 1% of the total CO2 produced per year. CO2 is used in some fields such as carbonating beverages, flash freezing of foods (beverage industry), and as an expellant in fire extinguishers. This is since transporting and storing CO2 is expensive in small volumes.

A more commonly used way is to transform CO2 into a fuel. CO2 is transformed into fuel using renewable energies as the aim is precisely to avoid the production of new CO2 for combustion.

In Canada there is a CO2 capture process capable of capturing 30 tons per day and the captured CO2 is then reused to encourage the growth of plant species in greenhouses.

Furthermore, the company Saipem, leader in the oil and gas sector and in the CO2 capture technology sector, has patented a highly efficient and sustainable plant to produce urea starting from NH3 and CO2. The process is illustrated in the Figure 1.11.

This process requires compressors to increase the pressure of ammonia and carbon dioxide, then there is a reactor where urea is formed and a stripper to remove vapors such as ammonia and carbon dioxide that is not converted to urea from the stream. There is also the carbamate condenser which condenses these vapors and the ejector which recirculates ammonium carbamate solution to the reactor. The pressures are around 150 bar and the temperatures vary from 155 to 205 for the outgoing flows. The NH3/CO2 reactor inlet ratio is 3.2-3.4 molar and is also used at low watercarbon dioxide ratio (0.4-0.6 molar). The reactor contains many plates of simple design and guarantees a conversion of the CO2 input of 62-64%. In the HP section, there is a total CO2 conversion of 85-90% considering the loop. The steam consumed in the stripper is almost totally recovered in the condenser. Despite the severe working conditions, the equipment lasts for more than 20 years.

Chapter 1

Another idea of using CO2 is studied by Fernández-Dacosta and consists in the use of carbon dioxide in the synthesis of polyethercarbonate polyol. From his work it emerges that up to 16% can be saved compared to the conventional case, in fact CO2-based polyols cost around 1200 €/t which is 16% less than the cost of manufacturing conventional polyol. However, the demand for polyol is not sufficient to be able to use all the CO2 produced in Europe, in fact 10% of CO2 emitted would be enough for the European need for polyol.