The current and predicted global massive dependency on fossil fuels calls for the exploration of new options to limit the future carbon dioxide (CO 2 ) emissions. One such option that has been studied within the IEA Greenhouse Gas Implementing Agreement, is the capture and disposal of CO 2 from combustion gases. Such options for Sweden have been examined in a system study financed by NUTEK (The Swedish National Board for Industrial and Technical Development). Aquifers that should be suitable for disposal of CO 2 , exist in the South of Sweden - Denmark and in the Baltic Sea close to Lithuania. Based on commercially available technologies, CO 2 can be captured from NGCC (natural gas combined cycle) and coal based IGCC (integrated gasification combined cycle) power plants. Most of the energy needed for the CO 2 capture could then be recovered as district heating, thus maintaining the total energy efficiencies. At approximately 300 MW power production capacities, the heat quantities produced (55–85 MW heat) are likely to be suitable for a reasonable number of the Swedish district heating networks. CO 2 neutral production and utilisation of methanol as an automative fuel for the transport sector integrated with production of electric power and district heat, could be achieved with biomass combined with natural gas or coal as a raw material. An amount of CO 2 corresponding to the carbon in the fossil fuel then has to be captured and disposed. Examples of possible process concepts have been examined. The resulting estimated total costs for capture, transport and disposal of CO 2 , are in the same order of magnitude as the current Swedish carbon dioxide tax (365 SEK/ton CO 2 ). Plant owners have to be credited for the captured and disposed CO 2 in order to make this option economically justifiable and interesting for them. It will be important for the total economy to find favourable combinations of energy conversion, CO 2 capture and recovery, transport and disposal. There is also a need to reduce todays uncertainties in the available basis for estimation of costs for large scale transport, injection and disposal of CO 2 into aquifers.

In the evaporative gas turbine (EvGT) cycle, also referred to as the humid air turbine (HAT) cycle, the compressed air is humidified with water to increase the mass flow through the expander, resulting in high power output and high exhaust heat recovery potential. This paper presents a design methodology for tubular humidifiers, in which pressurized air is led inside of smooth metallic tubes and is brought into contact with a falling water film. The heat required for humidification is mainly taken from exhaust gas from the gas turbine on the shell side and also by recirculating water through the intercooler and the aftercooler. The most important parameters for designing tubular humidifiers are: heat transfer coefficient on the flue gas side; and mass transfer coefficient for water vapor on the air side. Important design aspects include: proper wetting of the tubes; how to avoid flooding of the tubes; entrainment of water droplets into the air stream; and boiling in the water film. All calculations in this paper are based on an evaporative gas turbine cycle applied for combined heat and power generation with a partial-flow humidification circuit, where a fraction of the compressed air is humidified while a major part is by-passed directly to the recuperator. It is concluded that the required heat exchanging surface can be reduced if humidification is carried out for only a fraction of the air (20–30 percent). Finned tubes are recommended to enhance the heat transfer per unit tube length.

Reliable thermodynamic property data for air-water vapor mixtures are lacking for the design of evaporative gas turbine cycles (EvGT). Due to high working pressures and temperatures of gas turbines, considerable error would occur when applying the ideal models instead of the real gas mixture models. This paper presents an extensive literature study regarding models for computing thermodynamic property data of gas mixtures. The Hyland and Wexler model is found to be the best available despite the limited temperature range. However, experimental data are needed to verify the extrapolation. Furthermore, this paper evaluates the impact of thermodynamic properties of air-water vapor mixtures on the design of EvGT cycles. A suggested EvGT configuration, with results based on ideal gas mixture model and steam tables, is selected as a reference. The real properties of the working fluid mixture are recalculated by the means of the Hyland and Wexler model and applied in the cycle calculation. The results based on real data are compared to those based on ideal. The results show that the real gas model predicts higher saturation humidity at a given temperature. The higher volatility of water improves the humidification performance. In the case studied here, the flue gas temperature is lowered by about 3°C and the cycle efficiency is improved only marginally. The real gas model predicts higher heat duty for superheating of moist air by about 10 percent, or 2 MW. Finally, it can be concluded that thermodynamic property data mainly affect component sizing, especially the humid air superheater and to some extent the boiler.

This paper examines the performance of gas turbine cycles operating with a mixture of air and water vapor. Special attention is paid to the humidification tower, where the water vapor is added to the air. The experiments in this study have been carried out in the first evaporative gas turbine pilot plant located at Lund Institute of Technology in the southern part of Sweden. This pilot plant is based on a Volvo VT600 gas turbine with a design load of 600 kW. The compressor pressure is just above 8 bars and the intake air-flow is 3.4 kg/s. Roughly 70 percent of the compressed air is humidified in the humidification tower, which is the only humidifying device. The tower diameter is 0.7 m and the total flexible packing height is 0.9 m of a stainless steel structured packing with a specific surface area of 240 m 2 /m 3 . The number of mass transfer units in the humidifier was experimentally determined to about 3 for a packing height of 0.45 m. The height of a transfer unit from the literature data for the packing is predicted to be 0.24 m. With a packing height of 0.45 m, only about 2 transfer units are expected from the packing. However, the droplet zones above and below the packing contribute about 1 transfer unit. Thus, it is concluded that the mass transfer performance of the packing is adequately predicted by literature data. Equations are provided to adjust the height of a transfer unit for other pressures and temperatures. For full-scale plants operating at higher pressures and temperatures it is suggested that the high quality exhaust heat, (temperatures above the boiling point) is recovered in a boiler and injected as steam. The remaining part of the exhaust heat, (temperatures below the boiling point) is used to produce hot water for a relatively small humidification tower using only a portion of the compressed air flow.

The evaporative gas turbine (EvGT), also known as the humid air turbine (HAT) has the potential to compete with diesel engines and combined cycles in small and intermediate sizes. Most EvGT concepts include a packed bed tower for humidification; however, an alternative is a tubular humidifier unit. A tubular humidifier test rig was designed and constructed at the Royal Institute of Technology in Stockholm. A thorough investigation of the humidifier’s performance and characteristics at different pressures was conducted in 1998. The humidifier consists of a single vertical surface-extended tube and shell. The water and the compressed air are brought into countercurrent contact inside the tube, resulting in evaporation of the water film in the compressed air. The heat required for the evaporation comes mainly from the exhaust gas cooling on the shell side. Experimental results show that the tubular humidifier operates satisfactorily. The exhaust gas heat is recovered to significantly low temperatures. The temperature and the humidity ratio of the compressed air are promising for EvGT applications. The flooding velocity and the wetting limit were also examined. In most cases an inner tube diameter of approximately 50 mm and a tube length of 9 m are considered suitable for this type of application.

In striving for sustainable energy systems, the development of advanced technologies for combined heat and power generation is a critical factor. In many regions of the world there is a demand for heating only during a small part of the year and the yearly peak of power demand often occurs during the cooling season. Hence, the concept of trigeneration, i.e. the combined generation of power, heat and/or cooling is of great interest when it comes to obtaining a high yearly overall efficiency. In this paper, system studies are used to evaluate different types of trigeneration systems and the potential for an increasing electrical yield. The trigeneration systems consist of different types of gas engines coupled to different types and numbers of absorption chillers. The concepts are compared with regards to: the potential for increasing the overall electrical yield for a plant; cost-effectiveness; and environmental impact in terms of avoided CO 2 . Results indicate that the use of a humidified gas engine coupled to absorption chillers is a cost-effective and environmentally promising method to increase the electricity yield of a power cycle.