Various technologies based on different chemical and physical principles can be used to reduce gaseous flue gas components. Physical separation of certain gas components from the flue gas stream can be achieved by adsorption or membrane technologies, among others. Other abatement technologies are based on chemical reactions of certain flue gas components, e.g. chemical absorption processes in a wet scrubber, or chemical conversion processes to reaction products without any environmental relevance. Some emission reduction technologies also use heterogeneous catalysts to facilitate the targeted reactions.
Catalytic reactions play a major role in numerous technical production processes in the chemical industry. In addition, the purification of flue gases is also an area of application that is becoming increasingly important. This includes not only emission control of car exhaust gases, but also the reduction of certain emissions from industrial stationary sources. In many cases, transition metals or even noble metals are used as catalyst components, which can involve significant costs for the catalysts. In addition, the catalysts can be sensitive to certain catalyst poisons which may be present in the flue gas stream at low concentrations or in trace amounts. This can lead to deactivation of the catalyst material and short service life. Therefore, the improvement of catalysts and the development of new catalyst types is a constant task for all catalyst manufacturers.
The SCR Process
The SCR (Selective Catalytic Reduction) process has so far been the most important catalytic process in the field of industrial flue gas cleaning. Extensive operating experience is now available from SCR projects in the cement industry. Different variants of the SCR process (high-dust, semi-dust, low-dust) have been installed in cement plants in Germany, Austria, Italy and the USA over the past 20 years. In most cases the HD (High-Dust) variant has been chosen, but there is also operational experience available with the LD variant. LD-SCR plants require higher investment costs, but as they are installed in the dedusted flue gas there is a lower risk of mechanical erosion and catalyst poisoning compared to the other variants, leading to longer service life and thus catalyst costs. Furthermore, experiences, especially from HD SCR installations, have shown that this is no “plug & play”-technology in the cement industry. Time for optimisation regarding the optimum temperature window, efficient dust cleaning, raw material impacts, poisoning etc. always has to be taken into account.
In the SCR process a so-called DeNOx catalyst is used. It is a ceramic catalyst (in most cases consisting of TiO₂) which contains V₂O₅ and WO₃ as catalytic materials. The catalyst is primarily designed to achieve a high NOₓ reduction rate accompanied by a very low NH₃ slip. Apart from this, operational experiences from different industrial sectors have shown that also organic compounds are decomposed and emission abatement rates of 50 % or more for TOC can be achieved.
Regenerative thermal oxidation
A specific technology for the abatement of organic compounds and of CO is the so-called thermal oxidation or regenerative thermal oxidation (RTO). The operation of an RTO plant is an appropriate measure to also eliminate odorous substances from the flue gas stream. Experiences with the operation of RTO’s are available from cement plants in Austria, Switzerland and in the US.
In principle, oxidation catalysts could also be used to convert CO and organic flue gas components. In most cases, oxides are used as support material and noble metals like platinum or palladium are used as catalytic components. While oxidation catalysts are state-of-the-art in other applications, this technology is in a very early stage of development for the clinker burning process. A few tests in the exhaust gas of the clinker burning process have shown very poor service life due to fast poisoning of the catalyst. It can therefore not be seen as an alternative to RTO plants today.
In the past years, integrated technologies have also been developed which can be seen as a combination of proven technologies for individual flue gas components. The so-called “DeCONOx” process is a combination of a low-dust SCR system with a regenerative thermal oxidation (RTO), which allows an efficient reduction of NOₓ, CO and TOC/VOC emissions. Operational experience is available from cement plants in Austria and Germany and an extensive demonstration project in a German cement plant (Fig. 1) will be completed by the end of the year. Nevertheless, the DeCONOx process should still be seen as an “emerging technology” and not as state-of-the-art in the cement industry.
Figure 1: DeCONOx plant in a German cement works
The further development of DeNOx catalysts could also be an option to extend the application possibilities of the SCR process. In the past, SCR catalysts have been designed to achieve a most efficient NOₓ reduction. The additional abatement of organic compounds (TOC/VOC) can be seen as a side effect, but associated with certain restrictions. For example, methane (CH4) and other C1/C2 components are not decomposed in relevant amounts.
Therefore, research work is being carried out to improve the characteristics and capabilities of DeNOx catalysts. For example, VDZ and other research partners are conducting a research project in which the catalytic reduction of short-chain organic substances in the exhaust gas of cement plants is being investigated - both with standard SCR catalysts and also new catalyst materials (Fig. 2).
Figure 2: Laboratory trial with honeycomb catalyst
The results which have been obtained so far from lab tests show that with optimised catalyst materials, components like propene and benzene could be decomposed to 85 – 99 % and ethane to 76 – 96 %. For methane (CH4), abatement rates between 7 and 22 % were achieved.
It can be expected that specific and integrated abatement technologies will also be available soon to meet the future emission limit values for the components addressed.