The Pulverized Coal Boiler is a 20 MW_th unit focused on research demonstration and technological development. It can operate in air and oxy mode for different types of fuels (anthracite, bituminous, sub-bituminous, petcoke/anthracite mixture). It is a vertical water tube boiler with natural water circulation and balanced draft, equipped with different burners configurations.

The Circulating Fluidized Bed Boiler (30 MW_th) is a natural circulation, balanced draft, circulating fluidized bed boiler, designed to test CFB combustion under air and oxy-combustion conditions. Foster Wheeler is the technology supplier.

Design fuel is anthracite, but it is able to burn bituminous coal, sub-bituminous coal, pet coke, biomass and its blends, etc.

The oxidant stream required for oxy-combustion is obtained from mixing oxygen with recirculation gas in order to temper combustion. The O₂ concentration design parameters in the oxidant streams vary from 30% to 70%.

Coal and biomass combustion and oxy-combustion produces particulate matter and gaseous contaminants which need to be treated.

The flue gas cleaning system is aimed to treat flue gases emissions from the boilers to meet environmental legislative requirements and to reduce impurities to the maximum levels that can be treated at the CO₂ Compression and Purification Unit (CPU). The system includes the following main equipment: Cyclones, Selective catalytic reduction of NO_x (SCR) and Bag filter.

As result of the oxy-combustion process a flue gas highly concentrated in CO₂ is produced almost ready for transport and storage; nevertheless some contaminants need to be removed. Main contaminants form oxy-combustion for transport and storage are: water, O₂, NO_x, SO_x and particulates.

The Compression and Purification Unit (CPU) is aimed to treat oxy-combustion flue gases remaining impurities, which have not been removed at the Flue Gas Cleaning System, to get CO₂ ready for transport and storage.

Once CO₂ is captured, it needs to be transported to the geological storage site. One of the most suitable ways, from a technical and economical point of view, to transport high quantities of CO₂, is doing it by pipeline. The transport is usually performed in dense or supercritical phase.

The CO₂ Transport Experimental Facility at CIUDEN es.CO₂ is a first-of-its-kind facility aimed to test CO₂ behavior in transport by pipelines.

The core of the facility consists on ten coiled pipe racks, with a length of 300 m each (total length of 3000 m) and 2" diameter. Between these racks, there are six experimental areas: Depressurization, Leakage, Fracture, Corrosion, Instrumentation testing and Pressure Drop.

CIUDEN’s es.CO₂ Bubbling Bed Biomass Gasifier is an industrial scale facility that can achieve a gross power of 3 MW_th.

It is aimed to test biomass gasification and coal―biomass co-gasification by bubbling bed and atmospheric pressure gasifier.

Simplified process diagram of the full centre can be seen in Figure 2.

In post-combustion CO₂ capture, flue gases produced during combustion of primary fuel with air are treated to separate the CO₂. Post-combustion methods have been proposed to separate CO₂ from the flue gas stream in large point-sources, such as coal-fired power plants and energy intensive industries; in which CO₂ concentration varies from typically around 3% vol (dry) for a natural gas combined cycle plant to 35% vol (dry) in cement industry, and about 15% vol (dry) for a pulverized coal fired power plant.

These capture technologies can be applied to flue gases from all kinds of industrial processes, in particular power production from fossil fuels and biomass, cement, steel and aluminum production. Selection of the preferred capture technology depends on the flue gas properties (temperature, pressure, concentration and volume flow rate). These methods may be classified according separation principles into absorption, adsorption, cryogenics and membranes.

Among these technologies, amine chemical absorption is the most developed since there is great experience in the chemical and oil industries for the removal of CO₂ from gas streams. However, this process has a significant energy penalty due to the high energy requirements in the regeneration step. Large sorbent make-up flow is also required because of the chemical and oxidative degradation of the amines. The implementation of this technology for CO₂ capture from power plants is limited by its high costs derived given by the high energy penalties.

The accomplishment of lower energy penalties in the CO₂ capture process is crucial to the achievements of the goal of CCS deployment at industrial scale. CO₂ membrane separation is considered to be one of the most promising technologies to reduce energy penalty and cost of capture.

CO₂ membrane technology has been developed for removing CO₂ from mixtures with light gases such as CH₄, N₂ and H₂. Nevertheless, nowadays current membrane technology has to be developed to achieve good performance under a real post-combustion flue gas. The R & D needs are focused on the development of cheaper and more robust membrane modules with higher permeability and selectivity [3] .

Membrane structure and property guidelines have been extensively studying explored in an effort to improve the separation performances for gas separation (i.e. diffusivity and selectivity) [4] [5] . However, good selectivity has not been fully pursued as a route to enhance gas separation properties.

In an oxy-fuel power plant, pure oxygen is used in the combustion process instead of air which results in a flue gas highly concentrated in CO₂, more than 85% db, making easier the CO₂ separation and reducing the operating costs. To perform the combustion, very large volumes of oxygen are needed, which involves a large and costly air separation unit. Currently, the main commercial available technology to separate oxygen from air is cryogenic distillation (ASU). Nevertheless, cryogenic air distillation has the important inconvenient of the high energy demand needed, contributing to more than half of the energy penalty of the whole CCS processes. Another drawback is the fact that cryogenic distillation plants are only economically viable for very large oxygen productions, what makes more difficult its implantation in small applications. Typical ASU plants present a daily production between 50 - 4000 tons of oxygen, so, according to this, production rates below 50 ton/day are not viable.

For the 1st large scale demonstrations of oxy-fuel power plants, and the first commercial generations, cryogenic air separation will be the only viable air separation technology due to the large scale. In longer time perspectives, other air separation technologies based on membranes or adsorbents are seen as potential candidates.

Being O₂ production the main drawback of the oxy-combustion, one of the main purposes to develop cost efficiency oxy-combustion technology is to reduce specific energy consumption of today cryogenic processes, what is today in the range of 160 - 220 kWh/ton. A long term R & D target should be to reduce this to the range 120 - 140 kWh/ton for improved cryogenic processes. Other technologies could aim further, going down the range 90 - 120 kWh/ton such as membrane or sorbent based technologies.

Oxygen transport membrane (OTM) technology are then a substitutive way for O₂ production, reducing O₂ production costs and making in consequence, oxy-fuel combustion technology for CO₂ capture more economically attractive option than the others. Oxygen transport in these membranes consists on the oxygen diffusion through vacancies in the crystal lattice and simultaneous transport of electrons in the opposite direction, thus obviating the need for an external electrical short circuit. It can reach O₂ purity higher than 99% and can reduce efficiency drop to 5% - 6% in a power plant when CCS is implemented. The oxygen separation process follows three phases which can be identified:

・ Molecular oxygen in air is adsorbed, reduced and dissociated on surface of the membrane in the feed side to form oxygen-ions which are incorporated into the material lattice.

・ Oxygen ions (O₂-) diffuse selectively through the membrane under the driving force of a gradient in oxygen chemical potential. The flux of O₂- is charge compensated by a simultaneous flux of electrons or electron holes.

・ Lattice oxygen ions are desorbed and form oxygen atoms by oxidation reaction at the permeate side membrane surface.

System integration is one of the key factors to achieve the best efficiency high-temperature oxygen separating membranes. Another key challenge that nowadays technology present is the improvement of further materials with better flux, selectivity and upgraded performance at lower temperatures (below 700˚C).

Oxy-combustion produces a flue gas with higher concentration of pollutants, especially SO_x and NO_x, due to the absence of N₂ in all the system. It implies the development of new materials (or integrated ancillary equipment) able to tolerate these higher concentrations of pollutants. Membrane materials for oxy-fuel should be stable at sour conditions and to gas with high CO₂ concentrations.

For the implementation of OTM membranes in an oxy-fuel plant the technology should be scaled-up and knowledge on manufacturing a reactor design should be increased. Furthermore, for in any large scale application, industrial fabrication methods should be developed and, maintenance and reparation strategies defined precisely.

IGCC is based on the gasification of coal at elevated pressures (10 - 80 bar) and temperatures (950˚C - 2000˚C) with oxygen and steam as gasification agents [6] . The raw gas stream produced consists mainly of CO and H₂. It has to be cooled for the downstream process; the cooling can be done by using recirculated cold raw gas or by a wet quench injecting water into the gas stream. An intensive cleaning of the gas, eliminating dust and gaseous pollutants as HF, COS, H₂S, etc., in the gas conditioning part of the power plant finalizes the syngas production. The syngas produced is then fed to a combined cycle: to a combustion chamber of a gas turbine and the exhaust gases to a heat recovery steam generator.

Regarding membrane application, the IGCC process presents two advantageous operation conditions, high absolute pressure of the syngas and high partial pressure of the gas species of interest. Another advantage of the IGCC process is that between gasifier and the gas turbine island there are different temperature levels which allow implementing different membrane types with different working characteristics.

O₂, CO₂ and H₂ selective membrane can be applied to an IGCC system. H₂ selective membranes are the most specific to IGCC systems.

Process schemes may be obtained by integration of components and/or material development. Typically by combining a membrane into a catalytic process, to shift the equilibrium of the water-gas-shift reaction, H₂O + CO à H₂ + CO₂, in the direction of completion thus making following separation and/or purification steps redundant. The most challenging is to develop membranes with high flux, selectivity and stability at temperatures and elevated pressure.

The four main locations for H₂ membrane integration in an IGCC system were described and presented by Marano [7] . Integration options can be seen at scheme 2, these are:

・ Post WGS H₂ Recovery

・ CO₂ Compressor Interstage H₂ Recovery

・ WGS Interstage H₂ Recovery

・ WGS Membrane Reactor

Oxygen transport membrane reactors (OTM) may find applications in large scale processes for oxygen production, for chemical production (syngas produced from autothermal reforming―ATR or partial oxidation―POX) and for energy conversion (Coal to liquid, coal to gas, oxycombustion and IGCC processes). OTM membranes applied to IGCC systems will have similar approaches to the one performed in oxy-combustion. OTM membranes are seen as an alternative to reduce energy consumption for oxygen production. Main membranes R & D challenges are similar to oxy-combustion, but integration options and specific pollutants will be particular for IGCC systems. Locations for H₂ membrane integration in an IGCC system are studied also by Marano J.J. and Cifinero J.P [7] .

Oxygen production process is the main source of efficiency penalty in oxy-fuel plants and it has high capital cost. Dense electrolytic high temperature Oxygen Transport Membranes (OTM), based on mixed ions-electrons conducting materials, are a promising alternative to cryogenic ASUs, when integrated in power plants.

OTM are being developed to operate at high temperature, typically greater than 700˚C. High-temperature air separation process has better synergy with power generation systems. Commercial-scale OTM oxygen modules have been fabricated by Air Products (0.5 ton/day of oxygen); this technology requires 35% less capital (much simpler flow sheet) and 35% - 60% less energy (less compression energy associated with oxygen separation) than cryogenic air separation. [8] .

To reach satisfactory results in the scaling-up of the OTM technology, several stages of the whole process should be developed such as membrane materials manufacture, membrane module design and built, energetic system integration in an oxyfuel process, ancillary equipment design, etc. The membrane technology should go through several phases prior to the final development/demonstration at industrial scale between laboratory activities and pilot and demonstration.

Integrated OTM membranes in an oxy-combustion system will have to withstand several contaminants. The development of new membranes resistant to these contaminants is a major issue which is under being investigated. The most concentrated hazardous contaminants are SO₂, NO_x and CO₂. Other trace elements such as NH₃ can affect membrane stability too. Typical oxy-combustion contaminants concentrations, which have been analyzed at CIUDEN facility, can be seen in Table 1.

Testing of membrane materials under real conditions is a major issue for the scaling up of membrane and for the application to industrial environment. Membranes have to be adapted from laboratory ideal conditions to industrial hostile environments.

Considering all the aspects aforementioned, it will be necessary to ensure a suitable composition of the flue gas in contact with the membrane. Aiming to re-circulate a fraction of this stream to be used as sweep gas, ashes and other compounds like SO_x should be removed from the gas stream. Thus, it is necessary to consider the inclusion of ash filters and, depending on the membrane characteristics, sulphur oxides scavengers/filter too.

Due to the fact that it is very important to keep the gas in the operating temperature range, both the filter and the SO_x removal system must operate at these temperatures (typically above 1000˚C). For the case of ash removal, ceramic filters or the so-called ceramic candles are available for operation at such high temperatures (1000˚C - 900˚C), although only few commercial products fit these strong requirements. Consequently, it is envisaged that an important research and development task is necessary to address ash filtering.

Regarding to SO_X, the utilization of a variety of methods consisting mainly in wet and spray-dry scrubbing techniques, dry sorbent injection systems, and flue gas desulfurization using recycled sodium carbonate seem the most appropriate. Nevertheless, the applicability of these techniques might be limited by the maximum operation temperature since the membrane operation required maintaining the stream temperature as high as possible during the hot gas cleaning. Most common desulphurization processes operate in a low temperature range (e.g.

200˚C - 500˚C) due to either the lack of need to recycle the gas stream or process temperature. The use of CFB boilers, where SO_x removal can be done inside the boiler, will be more appropriate for membrane systems and their integration. SO_x removal in CFB boilers is accomplished by limestone injection in the furnace, showing the operation at temperatures of 900˚C - 1000˚C.

Ancillary equipment is considered a key player on the development of this technology. Due to the high stresses that will suffer the materials and the special needs of the membranes (dust removal at high temperatures, cleaning system control, fouling control, etc.), further study and development should be taken into account and the CIUDEN’s CCS facility can achieve important evaluations for the integration and testing of the system, both in materials performance and behavior, and distributed control system of the process.

Integration of membranes in the different boilers can have different special features. Fluidized bed boiler steam cycle plants, carry out desulfurization directly inside the bed, release an exhaust stream poor in SO_X, making easy the integration with high temperature membranes, while pulverized coal boiler steam cycle plants are at the moment the most widely adopted solution for power generation from coal.

Membrane integration can be done according to different module configurations:

・ Cross-flow (3 ends) module that minimizes the connection problems with the rest of the plant.

・ Counter-flow (4 ends) module (with a sweep stream) that introduces additional problems of integration but it allows the plant to achieve a higher efficiency.

The best thermodynamic performance configurations have to be identified. Such configurations may require different levels of R & D and of technological risk to be implemented in full-scale power plants. The maximum energy integration of oxygen separation system is the integration of the membrane system inside the boiler itself. However, this task would be very ambitious since it would entail strong modifications in the boiler design.

The integration of N₂ membranes inside the capture system is another option of application of membranes. The two possibilities for integration are the N₂/CO₂ separation after capture or N₂ separation from air to carry out partial oxy-combustion tests.

Although CIUDEN special feature is the developing of oxy-combustion capture, post-combustion and pre-combustion technological development can be carried out at CIUDEN facility.

Given that the installation can work under air combustion conditions, any kind of combustion flue gas can be reached to test post-combustion systems.

Pre-combustion needs a complete singular installation. Nevertheless some aspects of pre-combustion can be studied at CIUDEN through the use of the biomass/coal gasifier.