Trocknung von Braunkohlen mittels hochfrequenter-elektromagnetischer Strahlung unter besonderer Berücksichtigung der Braunkohlen des rheinischen Reviers

Lignite is one of the three columns of Germany’s energy mix and has the potential to affect the entire national economy. It accounted for approximately 40% of Germany’s primary energy production in 2007 equivalent to 180 million tons. Almost all of this fuelled conventional power plants while a small portion was converted to pulverised lignite or briquettes. In order to use lignite in the existing boiler systems efficiently, it is dried prior to combustion. This creates a higher gross calorific value and shifts the water molecules within the lignite matrix or vaporises them. To achieve a combustible product several processes are employed. The power plants in the Rhineland use either mill drying or the fluidised bed dryer with internal heat recovery. This thesis examines an alternative pre-combustion drying procedure which utilises microwaves at frequency of 2,45 GHz. These heat the water in the lignite above the boiling point and generates, by the nature of the process, a volumetric drying instead of a convection heat transfer. Due to the excess energy in such a system, local overheating can occur that may lead to degradation of the carbon structure within the coal molecules. Taking this into consideration the volatile matter of each sample was surveyed. The microwave drying process was conducted in two steps. The first focussed on the fundamentals of the process while the second addressed the up-scaling feasibility. The initial research was carried out on a laboratory scale utilising a modified household microwave. The industrial up-scaling tests were conducted in an industrial microwave oven. The drying tests in the laboratory were accompanied by an ultimate analysis to check for any alteration in coal properties due to microwave radiation. The microwave was set to 900 W for a maximum of 7.5 minutes at intervals of 30 seconds. Within 6.6 minutes the moisture content of a 100 g sample dropped to the targeted 14%. The ultimate analysis proved that the only hydrogen removed from the samples was that due to the evaporation of water, not from the carbon structure. Furthermore the volatile matter slightly decreased but this was not mirrored by a decrease in carbon content or calorific value. The experiments on a pilot plant scale confirmed the results from the laboratory and further experience was gained about the drying behaviour of lignite treated at 5 kW and 10 kW. As expected the process speeds up but without the loss of volatile matter from the coal. As well as a drying effect the microwaves change the porosity of the coal matrix that shows up as enlarged pore sizes and radii. The macro pores could be significantly enlarged by the radiation. The energy balance is considered to be a crucial criterion when assessing any drying technology. The general requirements for an industrial scale microwave drying plant were calculated using data gathered from an operating lignite fuelled power plant in the Rhineland. Employing this information the capital and operating costs of a microwave drying plant under variable conditions were deduced with reference to the energy balance and the laboratory data. The energy content of a hard coal equivalent served as the economic benchmark for the process. Real and idealised microwave lignite drying processes were compared to hard coal in order to evaluate their economical viability. The results showed that to derive an equal amount of energy the real microwave process costs 2.39 times more than the hard coal alternative and the idealised one 2.13 times more. Even if the investment cost were ignored the cost for supplying the same amount of hard coal equivalent would be 1,84 times higher. The process becomes economically viable if the price for the hard coal equivalent exceeds 181 €/t and the price for installed microwave radiation is below 50€/kW. Lignite fuelled power plants will come under increasing pressure because they have higher CO2 emissions than hard coal plants. The cost of the necessary Carbon Emission Certificates worsens the economic case for the microwave drying process. Based on the work carried out in this thesis the conclusion is that the drying of lignite from the Rhenish Basin by microwave methods is not currently economical nor is it likely to be in the foreseeable future