Making Industrial Processes More Cost Efficient


Development and characterisation of slaked lime granules for chemisorption in desulphurisation reactors

Recently imposed limitations of sulphur emission of marine vessels have increased the interest in development of efficient solutions for sulphur removal from flue gases. Chemisorption reactors (dry scrubbers) with slaked lime granules have the advantage that they do not produce liquid waste which is expensive to treat on the vessels. Slaked lime granules remain solid and the spent granules can be unloaded at ports and used for other purposes. Besides, the reaction can be carried out in compact reactors.

Performance of such granules is a multidimensional issue. However, it becomes necessary to find methods to characterise the granules for comparing granules of different suppliers, different sizes, different porosities, different prices, etc. so that their performance and economics in full scale applications can be guaranteed.

In case of chemisorption of sulphur dioxide by slaked lime granules, it is highly desirable that they have a large chemisorption capacity, and that the rate of chemisorption is fast enough. Chemisorption capacity at infinite time is one important theoretical measure, but for industrial purposes, more realistic measures are needed. Similarly, the initial chemisorption rate is not a sufficient measure of how fast the uptake of sulphur dioxide is.

In this work, experiments were carried out with a variety of granules of different sizes with different incoming sulphur dioxide concentrations at different temperatures with a pilot scale reactor, and a few practical performance measures of chemisorption capacity and rate, including breakthrough time, were calculated from the experimental results. Nonlinear models were then developed for predicting those measures.

The tests also relate the granulation process variables with the performance characteristics of slaked lime granules from experimental data.

Desulphurisation is a necessary environmental protection measure in several industries including power generation and mineral processing. It has recently been in the limelight because of a new European commission directive 2005/33/EC, which essentially states that sulphur content of the flue gas from marine vessels in sulphur emission control areas (most of northern parts of Europe) must not exceed a limit corresponding to 0.1% sulphur content in the fuel starting from 1 January 2015. Shipping lines can either use more expensive fuel with low sulphur content or install desulphurisation equipment on board.

There are several desulphurisation processes in use in industries, including wet, semi-wet, semi-dry and dry processes, which generally make sulphur oxides react with slaked lime or in rare cases, caustic soda. The wet processes tend to have a higher efficiency, but also have higher pressure drops and energy consumption. Besides, they produce liquid effluent which cannot be discharged into the seas untreated. Wet scrubbers, including semi-dry and semi-wet reactors are still the most commonly used equipment. Dry processes have the benefit of producing solid waste which is easy to store on the ship and unload on ports. The chemisorption process can be carried out in fairly compact fixed bed column reactors like the one developed in Nordkalk. It has several benefits, and can be the preferred equipment in future if efficient granules are available, and if there is sufficient deep knowledge about the operation of the reactor so that maximum mileage is derived from limited space.