Regenerative Energie
As climate issues are gaining urgency worldwide, focus is set on reducing industrial carbon footprints through fossil fuel replacement and energetic optimization of processes. This paper contains results of a study on technical options to attain a higher energetic efficiency from Waste-to-Energy (WtE) boiler & steam cycles. The aim of the study is to determine the available margins for energetic optimization of WtE plants, taking into account the particular constraints of a waste combustion process. The impacts of distinct process variables are quantified and compared. These variables include a.o. temperatures, pressures, process ratios and recycling rates, as typically applied to flows of combustion air, flue gas, steam and condensate. A few selected cases are elaborated to illustrate the cumulative effect of technical choices during the design and the operation of WtE plants. Finally, the results also enable the knowledgeable reader to determine an indicative value for R1.
Within the EU, Municipal Solid Waste (MSW) is partially considered as a source of renewable energy. European waste policy now imposes energy recovery objectives, in addition to yet prevailing targets for landfill diversion. A so-called R1 value is to be determined for each waste combustion plant, according to the formula defined by the Waste Framework Directive (WFD). To obtain the status of recovery operation, a certain threshold value for R1 has to be met or exceeded: 0.60 for already existing plants and 0.65 for new-to-build plants (permitted in accordance with applicable EC legislation after December 31st 2008). Failure in surpassing these values results in a disposal status and a loss of right to claim renewable energy certificates.
For Waste-to-Energy (WtE) plants built as combined heat & power (CHP) sources, the presence of external steam/electricity consumers allows to exceed the requested R1 value rather easily. However, when neighbouring energy offtakers are lacking - i.e. a typical situation for public MSW incinerators due to historical evolution - a WtE plant is fully reliant on itself to meet the energy efficiency target. Nowadays, WtE plants are equipped with (basic) Rankine power cycles, driven by (superheated) boiler steam typically at forty to sixty bar and 400 to 430 °C. In order to maximize the electricity output from the steam turbine, one could consider boiler steam generation at higher pressure/temperature. Such a thermodynamically-inspired optimization builds on the assumption that WtE power cycles and conventional power cycles are alike. However, the intrinsic constraints of MSW when used as a fuel, the corrosive nature of the emerging flue gas and the associated maintenance costs limit the maximum allowable steam parameters in a WtE boiler. Furthermore, a boiler replacement or upgrade is a radical and very costly solution for existing WtE plants.
As MSW is a low-predictable and low-calorific fuel (compared to fossil fuels), the combustion process is heavily subjected to fluctuating conditions. Without state-of-the-art technology this results in sub-optimal plant operation. It explains why a number of (older) WtE plants on average do not attain the full load operation point they were originally designed for. De Greef et al. have illustrated that it is possible to reclaim the remaining potential through rather straightforward process optimizations. However, for a larger increase in (energy) efficiency of an existing WtE plant, extra investments remain mandatory for remediation of critical process spots and plant refurbishment/ revamping.
| Copyright: | © Thomé-Kozmiensky Verlag GmbH |
| Quelle: | Waste Management, Volume 4 (November 2014) |
| Seiten: | 12 |
| Preis: | € 0,00 |
| Autor: | Dr. ir. Johan De Greef Hans Van Belle Dr. ir. Kenneth Villani |
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