Capturing the carbon created by electricity generation and storing it is one way to cut the amount of CO2 that human activity puts into the atmosphere. To integrate such a process into power plants requires consideration of its costs, risks and limitations, and the technologies available to carry it out.

 

Hans Christian Schröder, TÜV SÜD Industrie Service GmbH

Carbon dioxide at Vattenfall’s Schwarze Pumpe CCS demonstration plant in Germany is captured and then transported by trucks to Berlin

Energy generation accounts for a big fraction of the total emissions of carbon dioxide. In Germany, for example, the production of electricity is responsible for 41 per cent of the total output. The lion’s share of this is from coal fired power stations.

Carbon capture and storage (CCS) technology offers a way to make clear cuts in direct emissions of this greenhouse gas into the atmosphere but it also presents plant owners, plant operators and political bodies with special challenges. They have to ensure the implementation of CCS is technologically safe, cost-effective and legally-compliant.

Employing CCS impacts greatly on the operating strategy and the costs of power plants, and the technology has its limitations and risks. But to begin with, its potential for preventing CO2 getting into the atmosphere depends on two factors: the maturity and feasibility of the required technologies and, secondly, the availability of storage capacity.

According to initial, rough estimates by experts, global storage capacities amount to between 100 000 and 200 000 billion tonnes of CO2. If a reliable forecast of the potential for mitigating climate change is to be made, however, these capacities must be calculated more accurately. Competition over the use of the land where potential sites in Germany lie plays a part in determining the real extent to which theoretical storage capacities can be used for storing captured CO2. These geological formations including depleted natural gas reservoirs could store the CO2 produced by 40–130 years of operation of the country’s power stations but they could also be put to other uses, such as reservoirs for compressed air, seasonal reservoirs for natural gas or, in the case of saline aquifers, hydrothermal geothermal projects.

 

Manageable risks

 

The potential risks of integrating CCS technology into power station processes do not differ fundamentally from those in other industrial plants and are considered manageable. Within the scope of CCS, the transport and storage of CO2 and the safety of the geological reservoirs are the focus of interest. One of the major risks in CCS, and one that cannot be excluded, is that of leakage. CO2 could escape from the storage sites with adverse effects on the direct environment, such as injuries to persons and damage to property, and the climate.

Another conceivable risk involves geochemical processes, for example the dissolution of carbonate overburden by the CO2-water mixture which forms carbonic acid. These geochemical processes involve the risk of leakage and present a risk for the stability of the storage site. Decisions on whether a specific site is suitable for CO2 storage should depend on the type of storage and be based on a case-by-case risk assessment.

Installing CCS technology at power plants affects the operating strategy and the cost of generating electricity. To ensure cost-effectiveness CCS technologies must integrate seamlessly into the power station processes and the potential for optimizing operating processes must be fully exploited.

Depending on the type of technology used, CCS may cause loss in efficiency and higher investment costs – disadvantages that must be compensated for as efficiently as possible. The new generation of coal-fired power stations is one possibility.

By increasing operating pressures and temperatures and by applying new material concepts, the efficiency of a power plant can be improved to roughly offset the loss in efficiency caused by the use of CCS. Retrofitting an existing 800 MW power station with post-combustion CCS adds up to €300-€400 million ($404-$539 million) to costs, in other words, almost half of the investment spent on the power station itself.

These costs include: flue gas desulphurization, flue gas cooling, absorber or CO2 scrubbing, heat exchanger, desorber or CO2 scrubbing and CO2 compressor for transport. For efficient energy generation the process and operations engineering must be tailored to the specific technical processes and the subsequent operation of the respective plant. An integrated approach also embraces maintenance and inspection programmes.

These programmes must first be examined for their informative value and to ensure that they satisfy the integrated requirements of the complex system of a power station. Subsequently, plausibility and stresses occurring during operations are assessed. This customized approach ensures both safe and cost-effective operation.

 

Greater energy requirements

 

CCS has other limitations apart from the capital costs of the technology. These include the increased energy requirements and additional operating costs generated by the CCS system. Estimates assume that at an early commercial stage the costs of post-combustion CCS will amount to roughly €30 ($41) per tonne of CO2, a figure which could by 2030 have fallen to roughly €20 ($27).

In comparison the price at the European Climate Exchange in Amsterdam for the right to emit one tonne of CO2, in other words the price of one European Union Allowance (EUA), has ranged from a few euro cents to over €30 since the emissions trading scheme launched in 2005. As demand has dwindled in the wake of recession, EUAs are selling at just under €13 ($18). Among other factors, the cost-effectiveness of CCS will also depend critically on price trends for the EUAs saved by CCS.

The key to success, however, lies in balancing the technical and economic challenges of CCS technology for the processes at each individual power station.

When planning a retrofit or new installation of a CCS system, operating costs are critical in deciding for or against a particular form of technology. Operating costs include all future expenses for operation, servicing and maintenance and the degree of plant availability. They also include costs for the CO2 transport system and storage site. Maintaining financial reserves for unexpected events during CO2 transport or storage is sensible in this context.

The Oxyfuel process burns fuel in an atmosphere of almost pure oxygen to produce flue gas that is mainly CO2 and water vapour and contains almost no compounds of nitrogen and sulphur

An integrated cost-effectiveness analysis which considers the technological and geological parameters, the legal and corporate framework conditions and the total cost of ownership is crucial for ensuring a cost-effective integration of CCS in the long term.

Examples include warranty costs and the subsequent costing of service contracts. Reliable determination of these costs and their optimization is crucial for successful financial planning – from plant design, construction, installation and commissioning to actual operation.

Practical experience proves that the numerous interfaces between complex system and component solutions frequently cause problems which may impact significantly on later operating philosophy and even the quality of the systems.

To ensure the best possible operating philosophy and plant quality requires support by an experienced partner from design and construction to final approval before the system is taken into operation. Third-party organizations such as TÜV SÜD assist in the systematic prevention of interface problems.

Economic, technical, geological and legal risks must be assessed and controlled from the outset and problems solved in a way that leads to the desired results.

TÜV SÜD provides integrated system analysis and supports plant owners and plant operators in the implementation of CCS. TÜV SÜD provides these services through teams that combine technical, business, geological and legal knowhow and have long-standing experience in power station and plant engineering.

 

Fit for Carbon Capture

 

New power stations must be assessed for their potential to integrate suitable CCS technology and for their fitness for using it. No generally valid definitions and criteria have been established so far, so TÜV SÜD has developed its own standard, which forms the basis of the Fit for Carbon Capture certificate that TÜV SÜD’s power station specialists award. The certification procedure builds on the documentation of technical facts and measures, and offers improved planning certainty, higher investment security and wider public acceptance of plant-construction projects.

Plant owners and plant operators who consider the integration of future CCS technologies today will prevent competitive disadvantages and additional costs tomorrow. Choosing the right CCS technology for a power plant is paramount.

 

CCS technologies

 

Three technologies of choice are under development and should be readily available in the market in the next few years, although alternative techniques are under research.

 

Pre-combustion capture

 

This converts coal to coal to syngas at high temperature. The syngas consists mainly of carbon monoxide and hydrogen. Physical scrubbing of the the syngas captures the CO2, which is then compressed into liquid and stored. The process is based on the integrated gasification combined cycle. Focus here is on improving the already highly efficient hydrogen turbines. A commercial-scale demonstration CCS project with a gross output of 450 MW will test the feasibility of this CCS technology on a large scale as early as 2014. Advantages: small loss of plant efficiency. Disadvantages: complexity of the technology and of management of its operation.

 

Oxyfuel carbon capture

 

This process burns fuel in an atmosphere of almost pure oxygen to produce flue gas that is mainly CO2 and water vapour and contains almost no compounds of nitrogen and sulphur. Flue gas first passes through a tightly woven fabric to remove particulates and then recirculates. When the flue gas cools the water vapour condenses out to leave almost pure CO2 gas. This technology is still at the trial stage in electricity generation but other industries employ it. Advantages: significant cuts in total emissions; highly concentrated stream of CO2 emissions. Disadvantages: producing pure oxygen is expensive and consumes much energy.

 

Post-combustion capture

 

This involves the chemical scrubbing of flue gases. In a first stage nitrogen is removed from the flue gas. The flue gas then passes through a liquid sorbent that binds the CO2. Heating the sorbent makes the CO2 separate. The CO2 is then compressed into liquid for transportation. Advantages: the only process of the three that can be retrofitted; chemical flue-gas scrubbing is a fully mature technology. Disadvantages: requires a large amount of space; adds operating costs of up to €3000 ($4121) per hour for a flue gas volume of 3 million m3 per hour.

 

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