Ca-ADD

  Copyright: © TEER Figure 1: Schematic of the Raman in-situ analysis and its integration into the existing fluidized-bed reactor

Flue gas cleaning systems in waste-to-energy plants, conventional power plants and other combustion plants, such as biomass cogeneration plants, use state-of-the-art lime-derived sorbents for the separation of acidic flue gas components. Conditioned dry sorption is the most established process: Lime hydrate is injected as a sorbent into entrained-flow, baffle, or fluidized-bed reactors; beforehand, the flue gas is preconditioned to a defined humidity and temperature. The conditioning accelerates mass transport and reaction processes. Significant limitations of the process regarding its applicability result at low flue gas humidities, temperatures above 200 °C, and a molar chlorine-sulfur ratio in the flue gas of less than 1.3. The use of lime hydrate is limited in plants in which these conditions occur, for example sewage sludge mono-incineration plants.

The sewage sludge regulation stipulates that the phosphorus contained in sewage sludge must be recovered. Recovery can be done directly from the sewage sludge or after incineration from the sewage sludge ash. Efficient recovery requires high phosphorus contents in the ash, which can only be achieved by monoincineration of the sewage sludge. Consequently, there is an increased demand for planning and construction of such plants throughout Germany and thus the need for efficient, economical processes for the purification of waste gases from sewage sludge incineration.

The aim of the Ca-ADD project is to optimize industrial flue gas purification by means of lime-based sorbents using additives. Initial investigations have shown that additives such as fly ash or mineral substances significantly improve the physical as well as chemical interaction of the lime hydrate and thus the separation effect against SO₂. The systematic investigation of the reactions taking place on the lime particle and the dominant mechanisms for the reduction of acidic flue gas components should enable the targeted adaptation of processes based on lime hydrate to plants with unfavorable flue gas conditions. This results in a positive benefit for environmental protection since, among other things, the potential for phosphorus recovery can be utilized while maintaining high standards in air pollution control.

The work of TEER initially concentrates on material selection and characterization of suitable substance groups for the additivation of lime hydrate. In addition to optimizing the reduction efficiency, economic and ecological aspects are relevant criteria for selecting additives. This criteria ensures that the research results can later be applied in practice. In the TEER laboratory-scale test stand synthetic flue gases are produced and conditioned. In experimental test series, the influence of the selected additives on the reduction efficiency with hydrated lime in a fluidized-bed reactor is investigated under different conditions. With the project partner LTTa method is established to carry out in-situ chemical characterization of intermediates on the sorbent–surface during SO₂ conversion using Raman spectroscopy, compare Figure 1. In this way, the predominant mechanisms in the reduction process can be identified and fundamental findings can be obtained for optimizing acid flue gas purification using lime-derived sorbents through additivation. The optimized process will be tested in continuous trials under practical conditions at a pilot plant operated with sewage sludge on a technical scale (innovative fluidization furnace).

The AiF e. V. project Ca-ADD (22821 BG) is funded by the BMWK through the Industrial Cooperative Research (IGF) program for two years.

The project consortium is composed as follows:

• Chair of Energy Process Engineering (EVT), TU Dresden (coordination)
• Unit of Technology of Fuels (TEER), RWTH Aachen University
• Chair of Technical Thermodynamics (LTT), RWTH Aachen University
• Forschungsgemeinschaft Kalk und Mörtel e. V.