HomeCoal FiredClean Coal TechnologiesCapturing CO2 through oxy-combustion

Capturing CO2 through oxy-combustion

Aerial view of CIUDEN’s es.CO2 Centre
CCS technology

Spanish foundation CIUDEN has achieved a groundbreaking capture of carbon dioxide through oxy-combustion in circulating fluidised bed technology, opening a route to commercial scale carbon capture and storage, write Monica Lupion, Ruth Diego and Lionel Loubeau

Unless policies change, the coming years will bring heavy CO2 emissions. While fossil fuels are expected to contribute a falling share of the future energy mix, they are still set to dominate global power generation for many decades.

Meanwhile, world energy demand is set to grow according to similar projections by the World Energy Council, the International Energy Agency (IEA), and the US Energy Information Administration.

But carbon capture and storage (CCS) technologies could help contain the threat. In fact, by 2050 they could deliver up to 20% of the reduction in CO2 emissions needed to combat climate change. Current European initiatives on CCS and Clean Coal technologies (CCTs) include the Technology Development Centre for CO2 Capture, or es.CO2 Centre, supported by the Spanish government through CIUDEN (the Foundation Ciudad de la Energia).

Investment of $157m

CIUDEN’s Technology Development Centre for CO2 Capture Centre (Figure 1) has been set up with a €120 million ($157 million) investment to host advanced equipment for developing oxy-combustion CCS. Located in Cubillos del Sil in Spain’s northwest province of Leon, CIUDEN is supported by European R&D initiatives in partnership with industry, small and medium-sized enterprises (SMEs), universities and R&D centres.

Oxy-combustion ranks among the most promising CO2 capture technologies now under development, offering advantages such as near zero emissions and the flexibility of using different fuels. In place of air, fuel is combusted with a mixture of pure oxygen and recirculated flue gases, which results in a concentrated CO2 flow ideal for capture projects. CIUDEN’s es.CO2 Centre incorporates:

  • a fuel preparation unit;
  • a 20 MWth pulverised coal boiler (PC) that can operate in air-mode and full oxy-combustion-mode;
  • a circulating fluidised bed boiler (CFB), operating at 15 MWth in air-mode and 30 MWth in full oxy-mode;
  • a 3 MWth biomass gasifier;
  • a comburent preparation system;
  • a flue gas cleaning train to remove dust, nitrogen oxides (NOx) and sulphur oxides (SOx);
  • a CO2 compression and purification unit;
  • a CO2 transport test rig.

The two boilers are designed to burn a wide range of coals, biomass and pet coke under conventional combustion and oxy-combustion conditions.

Oxycombustion facility

The oxy-combustion facility’s main units include an oxidant preparation system, a pulverised coal boiler, a circulating fluidised bed boiler, a gas cleaning system and a CO2 processing unit.

Independent oxidant preparation systems enable the effects of oxygen concentration, humidity and temperature in each oxidant stream to be studied independently.

Three configurations are possible: (1) air (conventional combustion mode); (2) air, oxygen and recirculated flue gases (partial oxy-combustion); (3) oxygen and recirculated flue gases (total oxy-combustion). The oxygen concentration compatible with the materials used ranges from 21 per cent to 40 per cent.

Three oxidant streams have been considered in the design of the PC boiler. Primary oxidant (CB1) is used to inject pulverised coal into the furnace through the burners and to supply part of the oxygen necessary for combustion. Secondary (CB2) and tertiary (CB3) oxidants are introduced to keep combustion progressing.

The CFB boiler has four oxidant streams. Primary oxidant is used to fluidise the bed and also distributes the solid fuel along the bed for proper combustion. Secondary oxidant (CB2) supplies the rest of the oxygen required for combustion. Additional oxidant streams supply high pressure oxidant for loop seal and for INTREX fluidisation as well as oxidant for conveying solids. Each stream has a dedicated oxygen mixer for oxygen control.

Flue gases are recirculated under oxy-mode (partial/total) through two streams:

  • Stream FGR1: taken downstream from the FGD unit (‘wet recirculation’);
  • Stream FGR2: taken upstream from the FGD unit (‘dry recirculation’).

Maximum variability between the ratios of primary/secondary/tertiary streams enables stratification effects to be investigated. In PC technology, it is well established that varying the ratio between secondary and tertiary oxidant streams alters the fluid dynamics of the combustion process and affects associated phenomena such as the distribution of temperature profiles and the generation of NOx and unburnt coal. For CFB, the primary/secondary oxidant streams form the major flows, and the high pressure and solids conveying oxidant streams are less significant.

The PC boiler features four 5 MWth ‘turbulent’ horizontal/wall burners, installed in two pairs facing each other across the furnace. These low NOx burners can modify the fluid dynamic characteristics of primary and secondary streams so different flame configurations can be studied.

The burner system incorporates pulverised fuel receptacles, ignition electrodes, gas valves, BMS control and monitoring trains. Each has a high voltage electric lighter and a gas lance to ignite the primary fuel. The burner control system includes the latest generation sensors for individual flame monitoring.

oxy-CFB boiler
Main components of the oxy-CFB boiler
(1) combustion chamber
(2) solid separator
(3) ash sealing-direction device
(3a) ash duct to the furnace
(3b) ash duct to the cooler
(4) furnace cooler – INTREXà¢â€ž¢
(5) heat recovery zone
(6) steam cooled walls
(7) economiser

Pulverised coal and primary oxidant mixture (CB1) are injected through a central nozzle in the burner. Secondary oxidant outlets (CB2) around this central outlet envelop the central flame and complete the initiated combustion. The PC system’s extensive instrumentation includes standard systems such as flame detectors and optical pyrometers as well as specific instrumentation such as laser doppler and acoustic sensors. Sampling points have been arranged in the combustion chamber and sections of the boiler, uniformly distributed along the flame path.

The 30 MWth CFB unit from Foster Wheeler was first fired with coal in September 2011. As part of commissioning, the first functionality tests were carried out with anthracite coal from Spain’s El Bierzo region in a conventional combustion mode using air. Oxy-CFB conditions were first tested in December 2011.

Functionality test runs were carried out during the first half of 2012. Four types of fuels and blends (anthracite, petcoke, sub-bit, biomass) with different compositions have been already tested during more than 1300 hours of operation, of which 920 hours were under oxy-combustion conditions. Two types of limestone were also tested as sulphur sorbent.

PC boiler system configuration
PC boiler system configuration

The CFB unit design enables operation under either conventional combustion with air or under oxy-combustion conditions (Flexi-Burn concept). The experimental boiler is sized to give results that can be scaled to commercial units, while maintaining relatively low investment costs and operating expenses. Fuels and operating conditions can therefore be economically tested.

The CFB boiler has been designed as a testing unit, including many measurement points and offering varied operating conditions, to meet the boundary conditions. Maintenance and inspection procedures have been optimised for all its components. Extra instrumentation has been installed to gather additional operating data.

The CFB boiler’s main features and components are: the combustion chamber, solids separator, loop seal and the INTREX superheaters, in a separate fluidised bed chamber. The CFB also make provision for SNCR for NOx reduction and fly-ash reinjection.

Solid material is circulated through the cyclone downcomer to the loop seal. Part of this circulated solid material is returned directly to the furnace and the rest is cooled in a final superheater box before entering the furnace for combustion temperature control.

The cyclone provides gas mixing for injecting ammonia at the correct reaction temperature to reduce NOx emissions. Limestone injection keeps SO2 emissions within the permitted environmental limits and make-up sand is used as needed to maintain the proper bed characteristics.

Fine solids (fly ash) and flue gas exit at the top of the cyclone and are directed to the boiler’s convection section. Part of the flue gas is recirculated to the boiler by the respective oxidant fans. Recycled fly ash is fed to the lower part of the furnace to increase retention time and reduce unburned material. Bottom ash is removed from the bottom of the furnace through an ash drain to maintain bed inventory and bed material quality.

Gas cleaning system

The flue gas cleaning train consists of a cyclone system, a selective catalytic reduction (SCR) unit, a bag filter unit and a flue gas desulphurisation (FGD) unit. All the units can be by-passed to adjust flue gas composition and conditions according to the combustion technology and testing operating conditions.

The SCR unit is downstream of the cyclone system, with the main purpose of reducing nitrogen monoxide (NO) and nitrogen dioxide (NO2) in the combustion gases. Once the gases leave the SCR unit, they go through a feed water pre-heater, from which they enter in the bag filter unit at about 200à‚ºC. Finally, flue gases pass through a wet FGD unit to remove SO2 and reduce loss of efficiency in the CO2 absorption system downstream.

Schematic process diagram of CIUDEN’s technology centre for CO2 capture and transport

The second-generation CO2 compression and purification unit (CPU) has been designed as a scaled-up unit where CO2 is generated under proper conditions for transport and storage. Heavy metals such as mercury and acid contaminants are removed. Integrated operational tests with the oxy-CFB system and the CO2 CPU were carried out in October 2012 after Isolux Corsan commissioned a CPU with Air Liquide technology.

In achieving this milestone, researchers and technologists at CIUDEN’s centre have opened up a new stage of collaboration with business, pushing forward developments that will make Spanish and European companies more competitive. This, in turn, can boost CO2 emitting sectors and promote the creation of new industries through transfer and technological dissemination.

design fuel

The es.CO2 Centre has also established itself as an outstanding reference on CCS as a transition technology for mitigating climate change. The promotion of these technologies through international scientific and technological consensus, promoted by governments and industries in the more advanced countries, offers one tool to move towards low-carbon energy scenarios.

CIUDEN’s 30 MWth CFB has been designed, supplied and manufactured by Foster Wheeler SL in Spain and is financed under the European Economic Recovery Program of the EU. CIUDEN, jointly with Endesa Generacion and Foster Wheeler Oy, are cooperating in this project, whose objective is the validation of CFB technology and the subsequent development of a 300 MWe commercial size demonstration power plant.


Part of the work presented is co-financed under the FP7 Programme and the European Union’s European Energy Programme for Recovery. The sole responsibility of this article lies with the author. The European Union is not responsible for any use that may be made of the information contained therein.

Monica Lupion is CIUDEN’s International Affairs Director, Ruth Diego is Head of R&D Projects Department and Lionel Loubeau is Head of Innovation and Business Development Department