Jul-2008

CO2 infrastructure development: CCS options

How CCS options can be combined with linear programming to ensure optimum levels of CO2 reduction across a region

Tim Bullen and Michael Stockle, Foster Wheeler Energy Limited

Article Summary

There is mounting worldwide concern about the prospect of climate change due to anthropogenic CO2 emissions. The resulting political pressures provide opportunities for proactive energy and power generation stakeholders to reduce CO2 emissions by improving the efficiency of existing facilities, retrofitting them with CO2 capture and storage (CCS) facilities, or developing new projects that include CCS. Abu Dhabi has taken a lead in this area, and many others in the Middle East are reviewing their portfolios and developing long-term strategies for carbon management. The technologies for CO2 capture, transportation and sequestration are well known, as are the associated risks. With many existing sources of CO2, numerous technologies for CO2 capture, as well as various options for CO2 sequestration, choosing from a multitude of competing projects is a major issue.

CCS options
Carbon capture and storage, or carbon sequestration, is the process of capturing greenhouse gases emitted from process and power plants and injecting it into formations underground (Figure 1).

Emissions can be reduced by capturing CO2 and re-injecting it into oil fields, which leads to enhanced oil recovery (EOR). Oil companies benefit, since the CO2 injected into the field enables the recovery of additional reserves, thereby generating extra revenue. This can extend oil production by up to ten to 20 years for some fields that are approaching the end of their life using current conventional technology.

Carbon capture is best applied to large stationary sources such as power plants and industrial facilities, where CO2 can be separated at some stage of the process. There is a range of capture technologies at different stages of development. The most developed have been used in the petroleum industry for many years and have already been applied to a few small power plants producing CO2 for EOR or industrial uses. The concept of CO2 capture and storage consists of three main steps:
•    Separation and capture This involves capturing CO2 either before fuel combustion (pre-combustion) or by separating it from the flue gases generated by combustion (post-combustion)
•    Compression and transportation The captured CO2 is usually at low pressure and first is compressed (and pumped) before transportation by pipeline
•    Storage To prevent CO2 from entering the atmosphere, it must be stored over the long term. Geological formations such as depleted oil reservoirs, depleted natural gas fields, deep saline aquifers and unmineable coal seams appear to offer potential for the long-term security that is required. Direct injection into the oceans has also been the subject of discussion, although there are concerns over the long-term viability of such an approach due to uncertainties over the ecological impact and eventual equilibrium of the gas with the atmosphere.

Technology for CO2 capture
In the power, refinery and chemical sectors, both post-combustion and pre-combustion technologies are available. The technologies for CO2 removal from post-combustion exhaust gas streams are proprietary or licensed, as is much of the technology for the capture of CO2 pre-combustion. The technologies for large-scale CO2 removal in a CCS project that would be considered proven at the required scale (1–2 million tonnes per year of CO2) are:
•    Chemical solvent scrubbing
•    Physical solvent scrubbing.

There is much interest in emerging technology, particularly fuel cells, oxy-fuel combustion (in a concentrated CO2 working fluid) and integrated facilities, which as well as providing for gas pre-treatment, power generation and CO2 disposal can supply additional products such as desalinated water and co-produced hydrogen and carbon monoxide. The mainstream routes for CO2 capture are:
•    Post-combustion CO2 capture, where CO2 is removed after the fuel has been combusted and the energy generated, such as a conventional power station (Figure 2). Some projects are described as carbon capture ready. This means the facilities for carbon capture are designed and plot space set aside for these to be retrofitted, often in anticipation of changing legislation or more favourable carbon tax regimes
•    Pre-combustion CO2 capture, where the CO2 is removed from synthesis gas derived from the gasification of solid, liquid or gaseous fuel sources. The technologies used here include autothermal reforming, gasification, acid gas removal and combined cycle power generation using gas turbines that burn a hydrogen-rich fuel (Figure 3). Both of these main routes for CO2 capture have advantages and disadvantages, as shown in Table 1.

In the linear programming (LP) example contained in this article, both pre-combustion and post-combustion technologies have been considered.

CO2 disposal
There are different options for CO2 disposal. The first route, as shown in Figure 4, is that practised by BP and Sonatrach at the In Salah Carbon Capture Project in the Algerian desert. Rather than venting the CO2, which is the established practice on other projects of this type, the CO2 is compressed and injected into wells 1800m deep into a lower level of the gas reservoir, where the reservoir is filled with water.

The second route, illustrated in Figure 5, shows carbon capture by using CO2 in place of sour gas to facilitate EOR. Technology is available from existing proven EOR operations using CO2 miscible flood operations. This is an effective means of capturing CO2. In most cases, the CO2 is obtained from a subsurface formation rather than being a captured emission, and the economic incentive arises from the increased oil recovery, either directly or through reduced tax rates, or through the lifting of production rate restraints on “new” oil.

Basic linear programming
LP is a mathematical technique that can be used to optimise a complex system in which there are a number of interdependent variables. It has been used for some time to optimise the operation of a refinery and maximise the profit from the facility. LP will typically take account of the following factors: