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Abstract
This paper is giving an overview of advanced and proven technologies for CO2 compression. These include reciprocating and centrifugal compression - with focus on modern integrally geared designs for CO2 applications. A short overview about the processes that require high-pressure CO2, Enhanced Oil Recovery (EOR), Carbon Capture & Storage (CCS) and Coal Gasification (IGCC, Oxyfuel), will be given. Traditional and advanced compression technologies will be compared and benefits of integrally geared compressors will be discussed. Design challenges concerning thermodynamical and mechanical design will be pointed out with special regard to topics arising when compressing CO2 in the supercritical region. These topics include the pronounced real gas behavior of supercritical CO2 and the close-coupled design process of integrally geared compressors.

Introduction
CO2 has a long tradition in modern industrial processes and furthermore plays an increasing role in the present discussion of the world wide climate change. It is used in refinery, chemical and food industry applications as well as for fire extinguishers or as a solvent. Today many industrial processes require CO2 not in a gaseous but in a compressed state at a specific pressure and temperature.

The traditional approach for compressing CO2 is by using high-speed reciprocating compressors. Nevertheless this technology is limited in several ways – e.g. the flow capacity has strong restrictions due to the mechanical design. For this reason centrifugal type compressor systems are now state of the art and a promising solution for future CO2 projects. Centrifugal compressors generally can be split into two major types which are distinguished by their design namely single-shaft (inline, between bearings) centrifugals and multi-shaft integrally geared centrifugals. This paper will focus mainly on integrally geared centrifugal compressors.

MAN Diesel & Turbo has manufactured reciprocating compressors in the past and still is manufacturing both types of centrifugal technologies (inline and integrally geared type, according to API 617 as stated by MACEYKA, PICKEREL (2007)) for CO2-services. We therefore feel confident and in a position to give a comprehensive overview and comparison of all technologies.

Overview of CO2 related applications and processes
Before we step into compression technology some explanations concerning CCS and EOR are given:

Carbon Capture and Storage (CCS)
A steeply rising demand for energy world-wide, combined with increasingly strict environmental regulations have created the need for new technologies for the clean and sustainable generation of power. Carbon Capture and Storage labels a group of technologies which all have one aim: To separate carbon dioxide in the process of conventional thermal power generation.

Whether the method by which CO2 is separated is IGCC, Oxyfuel or Post-Combustion: Every one of these processes requires the compression of CO2 for transport and storage for future use. The process paths of different approaches can be seen in Figure 1.