Colloquia

Samenvatting

At the Waste to Energy (WtE) company Twence, municipal waste is combusted in order to produce high pressure steam. This steam is used to generate electricity in a turbine and generator. At the same time, a fluctuating amount of steam is also be delivered to the district heating network and local industry. The amount of steam that is used for electricity production depends on the heat demand of district heating and industry. The steam-condensate cycle is described by an adapted Rankine cycle.

Twence has built a carbon capture plant in order to reduce the CO2 emissions from its flue gases. At the plant, the CO2 is separated from the rest of the flue gases using an amine-based absorption-desorption cycle. The CO2 is then liquefied using multiple compression and cooling steps. The goal of this thesis is to analyse and model the energy and exergy efficiency of the steam-condensate cycle in combination with the carbon capture plant, taking both the fluctuating flue gas conditions and heat demand into account. Additionally, the goal was to look for possibilities of heat integration between the carbon capture plant and the rest of the steam-condensate cycle. This analysis was done by constructing a MATLAB model with a heat and mass balance of every step in the steam-condensate system and the carbon capture plant.

Four additional heat integration concepts were found. All of these options lead to a larger electricity production, a lower cooling water consumption and a higher energy and exergy efficiency. The largest improvement would be to add a heat exchanger between the hot flue gases entering the carbon capture plant and the cold flue gases exiting the carbon capture plant. This would lead to approximately a 1.1% increase of the electricity production and 9.5% reduction of cooling load. The second possibility is to use the heat from the hot desorber cooling water from the desorber to reheat the condensate of the main steam-condensate cycle. This would lead to an electricity production increase of 0.8-1.0% and cooling load reduction of 10%. The final two heat integration options, namely using the hot flue gases at the desorber and using the heat that is released at the CO2 compression, lead to smaller energetic and exergetic improvements.

Furthermore, some research into feasibility was done. This led to the conclusion that the hot desorber cooling water is the most feasible in terms of size, investment costs and possible additional income for Twence.