Carbon Dioxide Recovery Skid

Nowadays more and more plants walk away from building various process systems on-site and rely on suppliers for ready to connect modules. Suppliers who are usually the manufacturers (or a proxy between a manufacturer and a contractor) deliver these process systems that are built under off-site conditions. As a result of that, modular constructions have been adopted for ease of handling in commercial use. Such a mobile construction of a process system is called “skid” in English or “anlage” in German. Skids are usually built within a frame. Depending on the dimension of a skid and logistical planning, it can fit, for example, on the back of a truck or in the shipping container [1,2].

Most factories emit a certain volume of carbon dioxide (CO₂). For example power plants, oil rigs, refineries, waste incinerators, sewage treatment plants, etc. Recent climate restrictions imposed a reduction of CO₂ emissions to the environment. As result, a feasible way to capture the CO₂ was searched. Depending on the costs and size of a plant, some have decided to build CO₂ capture systems onsite, while others (usually smaller) have made decision to pay emission fees. CO₂ capture systems built onsite are usually large and may cause additional operational problems. Currently, another rare possibility is to add a compact CO₂ capture skid, that solution can benefit both medium and smaller factories. Demand for building reliable systems in the market caused skid manufacturers to build dedicated CO₂ recovery systems. Before delivery to a plant, a commercial CO₂ skid should be factory acceptance tested (FAT) and certified for its operational use on a manufacturer site, which is an advantage in comparison to these built on plant site without a guarantee [2].

An example of a compact module including CO₂ capture process is presented in articles [1,3,4]. The proposed power plant module is composed of a gas-steam cycle (with CO₂ separation) coupled to a CO₂ capture system as shown in Figure 1. The compactness of the proposed gas-steam power plant skid is achieved due to a novel small size designs of the water combustion chamber (WCC) and the spray-ejector condenser (SEC), both based on enhancements of the energy conversion. Conventionally, the size of a combustion chamber and a condenser would be about 30-fold and 32-fold greater than the WCC and the SEC, respectively.

Starting with the WCC in the gas-steam cycle part, the oxy-combustion process takes place, where supplied methane is burnt in the presence of clean oxygen supplied from the ASU through the compressor CO₂. The ASU separates the high-purity oxygen from atmospheric air basing on the cryogenic method. The waste gas from the ASU is then directed to the GTN2 expander which is connected to the main shaft. Due to a high temperature of the oxycombustion in the WCC, the chamber walls are refrigerated by the water injection. Injected water acting as an inert medium, evaporates at the nano level to the combustion chamber. The working fluid exiting the WCC, which consists of 10-20% CO₂ vapor and 80-90% steam is directed to expand in the vapor turbine GT and the vacuum vapor turbine GTbap. In the literature there is also the name of the gas-steam turbine in relation to the vapor turbine (GT + GTbap) [1,3,4]. Both turbines are connected to the main shaft with the generator G, the oxygen compressor CO₂ and the expander GTN2. The exhaust vapor leaving the GTbap is directed to the SEC after cooling in the regenerative heat exchanger HE. The vacuum in the SEC is generated in the result of properly sprayed nano-droplets of water through the nozzle longitudinally to the main jet, while the working fluid is sucked perpendicularly to the main jet and then immediately mixed in the path of sprayed water. In the main jet, the steam fraction is partially condensed and CO₂ compressed simultaneously. The mixture of water steam, liquid water, and compressed CO₂ leaves the SEC and raises its pressure to 1bar entering the condensate-cooler exchanger with the separator CHE+S. In the CHE+S liquid water is cooled, while the remaining mixture of steam water and CO₂ is separated in the result of the main jet action in the SEC. Some of cooled water exiting CHE+S is pumped back to the SEC spraying nozzle. The rest of water is pumped back to the WCC after being preheated in the HE, while an excess of water is being removed (which is produced during combustion) [1,3,4].

Figure 1: The compact cycle with oxy-combustion and CO₂ capture, where WCC - wet combustion chamber, GT+GTbap - vapor turbine divided into two parts, SEC - spray ejector condenser, ASU - air separation unit, CO₂ - compressor, HE - regenerative heat exchanger, CHE+S - condensate-cooler heat exchanger and separator, CHE - cooling heat exchanger, M - motor, G - generator, P1 - supply water pump, P2 - water pump for cooling combustion chamber, GTN2 - expander N2, CCO₂ - CO₂ compressor

In the CO₂ recovery part, the mixture of compressed CO₂ and steam from the CHE+S separator from the gas-steam cycle skid is directed to series of CO₂ compressors C_CO2 powered by a motor, between compressors water is condensed from the steam in the cooling heat exchangers CHE. As the result, clean compressed CO₂ is produced from the outlet of the CO₂ recovery skid.

In conclusion, the feasible way to capture CO₂ was presented within the compact build of power plant skid. Wide commercial application of such skids including CO₂ capture is the matter of certain factors, like the cost of a CO₂ capture skid unit, the climate regulations, the CO₂ emission fees, and the size of a plant. While the cost of manufacturing of a compact CO₂ uptake skid will possibly reduce, both the CO₂ fees are likely to be increased and the climate regulations be tightened with time. It can be assumed that the balance of these factors will shift toward the cost-effective application of CO₂ reuptake skid units. Especially smaller factories would benefit from this. From the technical view, the development progress of viable CO₂ capture skids is demanded to be done to maintain this trend.

This article has been prepared within the frame of the project “Negative CO₂ emission gas power plant” - NOR/POLNORCCS/ NEGATIVE-CO₂-PP/0009/2019 the project is co-financed by POLNOR 2019 under the Norwegian Financial Mechanism 2014- 2021.

1. Ziółkowski P (2018) A thermodynamic analysis of a gas-steam turbine incorporating a full model of a spray-ejector condenser. Trans IFFM 139: 63-96.
2. Pentair PLC (2016) A compact enclosed skid CO₂ recovery system for biogas upgraders, Italy.
3. Ziółkowski P, Badur J, Kruczek HP, Nedźwiecki Ł, Kowal M, et al. (2019) A novel concept of negative CO₂ emission power plant, based on combustion the gas from sewage sludge gasification in a gas turbine with spray-ejector condenser. XXIV International Symposium on Combustion Processes, Australia, pp. 144-145.
4. Ziółkowski P, Badur J (2019) A study of a compact high-efficiency zero-emission power plant with oxy-fuel combustion. In: Stanek W, Gładysz P, Werle S, Adamczyk W (Eds.), ECOS, Proceedings of the 32^nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Wroclaw, Poland, pp. 1557-1568.