Technical assessment of high-temperature heat exchangers for the thermal integration of O$_{2}$-transport membranes in the OXYCOAL-AC process

  • Technische Bewertung von Hochtemperatur-Wärmeübertragern für die Wärmeintegration einer O$_{2}$-Transport-Membran in dem OXYCOAL-AC-Prozess

Verbaere, Vincent; Kneer, Reinhold (Thesis advisor); Beckmann, Michael (Thesis advisor)

Aachen (2015, 2016)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2015

Abstract

Oxyfuel combustion processes require an air separation unit as an integral part of the power plant process. Such an air separation unit can be based either on the commercially available cryogenic process or an ion transport membrane (ITM), which has to be operated at typical temperatures in a range from 800 to 900°C. For efficiency reasons, the OXYCOAL-AC project conducted at RWTH Aachen is focused on such a membrane-based coal combustion process. To achieve a membrane operation temperature higher than 800°C, heat has to be transferred to the feed air, advantageously by integrating heat exchangers positioned either in the (cleaned) recycled flue gas pipe or in the boiler. These two alternatives are labelled four-end concept and three-end concept respectively. Due to severe process temperatures and flue gas composition, the design and realization of such heat exchangers have to be addressed. This is the subject of the present study.The study can be summarized as follows: • Since the net plant efficiency of the oxyfuel process is influenced by the membrane and heat exchanger characteristics, a process simulation was carried out to identify the optimum utilization of these components and provide boundary conditions for the present study. This is addressed in Chapter 3. • In view of preparing for the sizing of heat exchangers (Chapter 10), approaches to design heat exchangers and analyze their heat transfer performance (heat transfer in relation to pressure drop) are presented in some details in Chapter 4. •Materials used in heat exchangers exposed to the mentioned conditions deserve particular attention. In Chapter 5, a literature review was undertaken to identify suitable materials; this reveals that - expensive - nickel alloys can still be employed up to 800°C. This means that ceramics are not necessary. • For the purpose of establishing what kind of heat exchanger geometry would be the most appropriate for high pressure and temperature conditions, it appeared necessary to gather empirical data from patents and manufacturer and research reports. In situations where fouling is not an issue (four-end concept), it was found that tubular exchangers as well as some plate exchangers, such as dimpled-plate and cross-corrugated plate types can be applied. Otherwise (three-end concept), only a conventional tubular exchanger based on the model of evaporators and superheaters customary in coal power plants should be considered. • Chapter 7 examines the minimum wall thickness for the selected heat exchanging geometries. It allows for example to exclude oval tubes from this application. • Sizing of heat exchangers can only be achieved by prior knowing their friction and heat transfer coefficients. Since tubular and cross-corrugated surfaces are widely addressed in open literature, a presentation in given in Chapter 8. Based on the knowledge of these coefficients, it could be determined how some geometrical parameters affect the heat transfer performance. • Dimpled-plate surfaces incorporate two three-dimensional channels dissimilar in nature, namely the wavy channel on one side and the tubular channel on the other side. The lack of data in relation to their friction and heat transfer coefficients required a specific CFD analysis. Heat transfer and friction coefficients have been determined locally and globally in Chapter 9. The results are discussed on the basis of the flow fields calculated as well as public data obtained for two-dimensional channels bearing strong similarities with dimpled-plate channels. • Based on data prepared beforehand, sizes of tubular, cross-corrugated-plate and dimpled-plate heat exchangers have been assessed in the OXYCOAL-AC conditions and have subsequently been compared to one another. This is presented at the end of the study (Chapter 10). The following points emerged for the four-end concept: A tubular exchanger remains competitive with plate-type exchangers; its surface area can be further abated by the addition of wire coil inserts and - under certain conditions - longitudinal fins and an increase in pressure drop on the feedair. The latter has the advantage of not affecting the process efficiency too much. Above 700°C, it has been shown that dimpled plates achieve significant volume savings, but require more material than most tubular geometries. A promising achievement in terms of material and volume savings is identified for cross-corrugated heat exchangers. An estimation of the dimensions of the tubular heat exchanger was also done for the three-end concept. It is also shown that by using an extern recuperator this could reduce the heat exchanging surface area within the boiler by a factor of three. However, it is an unfortunate fact that this measure is accompanied by a fall in the net plant efficiency from 1.5 up to 3.0% points.