Combined NOx and SOx reduction at lower cost16 November 2001
Lextran Ltd has developed a catalytic process promising a low operational cost solution for combined NOx and SOx reduction
Lextran Ltd is a privately owned R&D 'start-up' company. It was established in 1997 and is operating within the framework of the Katzrin Technological Incubator in the northern part of Israel.
It is Lextran that provides the chemical technology for the new 'deSNOx' process together with the necessary absorption technologies for the discharge gases. Ludan Engineering is leading the project in collaboration with utility IEC (Israel Electric Company). The company intends to build an industrial pilot/demonstration plant, then in due time an industrial plant to produce the reagent. They plan also to offer a turnkey service including all engineering, procurement, construction and management tasks and full operation of the plant.
If it all works as advertised the benefits to the user aside from reduced emissions are that installation is simpler and less costly than an FGD/deNOx system, with a significant saving in plant footprint and operating costs compared to conventional installations. It is also possible to convert FGD installations to remove SOx and NOx simultaneously, with a significantly smaller investment than the cost of building a deNOx plant. The system should also enable users to burn less expensive, high sulphur content fuels, while still complying with environmental standards and providing the opportunity for emissions trading.
The heart of the process (Figures 1, 2) is an organic catalyst named Lextran, a high boiling point organic liquid produced from petroleum by an undisclosed process. Lextran and water in the form of a dispersed emulsion acts as a SOx and NOx scrubbing medium by conventional contact absorption techniques. It also promotes the formation of an NO/NO2 complex (N2O3) and the further oxidation of the complex into nitric acid.
The fluid has a further catalytic effect in promoting oxidation of the SOx/NOx into sulphate/nitrate forms, which dissolve easily in water. This aqueous phase is separated and bled from the system; it contains a sulphuric/nitric acid mixture at 20-30 per cent, a valuable source material for liquid fertiliser.
The following analysis relates to NOx scrubbing which is more problematic than the SOx case. A model of the NOx emissions is represented by a mixture of NO (insoluble) and NO2 (soluble). The chemical behaviour of the mixture is as follows:
NO + NO2 fi N2O3
N2O3 + H2O fi 2HNO2
2HNO2 + 2NO2 fi 2HNO3 + 2NO
NO + 3NO2 + H2O fi 2HNO3 + 2NO
Since NO is created, the reaction leads to inherently inefficient scrubbing, which is why conventional technologies use relatively expensive catalytic reduction techniques.
The Lextran catalyst enhances oxidation of the N2O3 complex with the available air:
NO + NO2 fi N2O3
N2O3 + O2 + H2O fi 2HNO3
NO + NO2 +O2 + H2O fi 2HNO3
The resulting nitric acid is easily extracted by aqueous scrubbing.
At the Hadera power station demonstration plant 500 m3/h of flue exhaust is fed to the column base and passes through the ceramic packing by fan assisted forced flow. The Lextran emulsion is introduced at the top of the column and flows downwards to be recirculated. To raise the NO2/NO ratio, which is well below 50 per cent in this oil fired station, the plant is equipped with an ozone generator injecting the gas at 56 ppm to increase the NO2 proportion. The addition of ammonia allows pH control.
Field tests using a bench scale column on flue gases have been carried out at Hadera power station and at Haifa Chemicals works, in the latter case for NOx removal only. The process has been run with and without ozone injection. During 1000 hours of running without ozone injection (600 continuously), 99 per cent deSOx efficiency was achieved (from 500 ppm to 1 ppm) together with 99 per cent H2S absorption. With ozone injection simultaneous deNOx and deSOx efficiencies of 80 per cent were achieved, (from 126 ppm to 28 ppm for the NOx). Analysis of the by-product showed that some heavy metals had also been removed, while consumption of the emulsion was very low. The results are shown in Tables 1 and 2 and Figure 3.
TablesTable 1 Hadera power station results Table 2 Haifa Chemicals results (NOx only)