Oxyfuel Process for Hard Coal with CO2 Removal

Project Leader: Professor Dr-Ing Alfons Kather
Research AssistantDipl-Ing C Hermsdorf, Dipl-Ing M Klostermann, Dipl-Ing K Mieske
Duration: 01.10.2004 - 28.02.2009

This research project is part of the Joint Project ADECOS I&II  (Weiterentwicklung des Oxyfuel-Prozesses für Braun- und Steinkohle mit CO2-Abscheidung = Further Development of the Oxyfuel-Process for Lignite and Hard Coal with CO2 Removal), performed under the auspices of the COORETEC Programme.

Apart from reducing the CO2 emissions by increasing efficiency there is at present intensive discussion on achieving the same goal through separating the CO2 from the exhaust and storing it. In coal-fuelled steam power plants it is possible to use an end of pipe-solution by washing the CO2 out of the unpressurised exhaust gas by means of liquid Monoethanolamine (MEA). For regenerating the MEA solution heat from the LP-stages of the Turbine has to be used. This reduces the overall efficiency of the power station by approximatels 10-14 percentage points. Due to this high loss and the consecutive increased consumption in resources, this process is for the time being considered only in EOR applications seeking to increase recovery from oil reserves.

By comparison it is preferable to use the so-called Oxyfuel-Process and this was the reason that this process was included in the Framework of the COORETEC Programme. The Oxyfuel process is based on the classical coal steam power plant. Here coal combustion takes place in the boiler not with air but in an atmosphere containing Oxygen, supplied by an Air Separation Unit (ASU), and recirculated exhaust gas. The main components of the exhaust gas are CO2 and Water. The CO2 is now so highly concentrated that it can be separated and liquefied with a relatively small energy penalty. The volumetric concentration of CO2 in the dry exhaust increases from 15 %, in the usual steam power plant, to approximately 90 % in the Oxyfuel power plant.

The process consists in principle of three main components: the ASU, the Steam Power Plant and the Exhaust Gas Liquefaction Plant. The ASU, which is based on the established cryogenic Linde principle, delivers large quantities of oxygen with a purity above 95% at ambient temperature and pressure conditions. The remaining impurities consist essentially of Argon. The Oxygen stream is mixed with recirculated exhaust gas before being fed into the combustion chamber, so that combustion temperature and the resulting thermal stresses on the heat exchange surfaces can be kept within safe limits. The actual substances in the combustion gas and the associated overall sizing of the exhaust recirculation pipework depend on the temperature and the heat capacity at the point of extraction of the exhaust gas. This is, therefore, an issue where further optimisation is necessary. In principle, however, it appears that approximately 2/3 of the exhaust stream downstream of the boilier must be recycled.

The boiler itself and the water/steam side of the power plant cycle resemble essentially those in conventional modern steam plants with the now usual overcritical fresh steam parameters and intermediate superheating.

The necessity to cool the exhaust gas down to ambient temperature to condense the water and to also cool the air and exhaust gas compressors, combined with the fact that now most likely an air preheater is no more needed, leads to an oversupply in low temperature heat. The feeding of this heat back into the process contributes to the regenerative preheating of the feedwater and benefits in this way the overall power plant efficiency.

The exhaust is cooled in the exhaust gas dryer down to ambient temperature. In this manner most of the water in the exhaust condenses and so CO2 concentration increases. The rest water is removed by using suitable adsorbents downstream. The volumetric concentration of CO2 in the dried exhaust gas reaches almost 90%. The remainder is mostly excess oxygen - necessary to ensure satisfactory coal combustion in the combustion chamber - along with Argon and small amounts of nitrogen, CO, NOx and SOx. While the pollutants are partially soluble in the condensate as well as in the CO2, for increasing the concentration of CO2 in the exhaust further necessitates the removal of most Ar and O2 through liquefaction of the CO2 below ca. -45°C. This yields CO2 with a purity of over 95 % which, at a transportation pressure above 100 bar, remains also under ambient temperature conditions liquid.

In a first study at TUHH the Oxyfuel concept was considered for the case of an existing hard coal power plant. The currently most modern hard coal power plant in Germany, the Rostock Power Station, was used as basis for simulations with the Flowsheet Programm Aspen Plus®. Following this, the same simulations tools were used to model the Oxyfuel process for hard coal, keeping as much as possible unaltered the water/steam side of the power station. The ASU and the exhaust gas liquefaction plant were modelled in detail. By comparing the differences in the results between the Oxyfuel and the Reference power plants yields an efficiency loss of some 8 percentage points. The excess oxygen ratio was assumed to have been the same in both case and equal to 19 %. The CO2 retention factor was equal to 83%.

Currently the Oxyfuel process is evaluated on the basis of the Nordrhein-Westfalen Reference Power Plant, which corresponds to the current state of the art in hard coal-fuelled power stations and achieves with air operation a net efficiency of 45,9 %. In this work not only the integration of the ASU and the exhaust gas processing are incorporated in simulation tools such as Aspen Plus® und Ebsilon®, but also the influence of the altered combustion conditions on the heat equilibrium of the boiler are calculated in detail using the boiler simulation software PPSD®. The effect of the modified Oxyfuel conditions on the combustion process and the creation of pollutants in the combustion chamber are in parallel investigated experimentally, using the Drop Tube Furnace that the Institute possesses.

Emphasis is placed in this work in applying realistic boundary and operating conditions, to replicate as much as possible the conditions applicable in the actual power station praxis.