Mats Westermark

Potential and cost-effectiveness of CO2 reductions through energy measures in Swedish pulp and paper mills

Using the two criteria of potential CO2 reduction and cost of CO2 reduction, technical energy measures in Swedish pulp and paper mills are investigated. Principal CO2-reducing measures analysed are: decreased specific energy utilisation, fuel switch, and CO2 capture and sequestration. Among the investigated measures, conventional technologies for electricity conservation and improved electrical conversion efficiency in existing systems for cogeneration of heat and power are identified as the most cost-effective alternatives that also have large CO2 reduction potentials. For commercially available technologies, the results indicate an accumulated reduction potential of up to 8 MtCO2/y (14% of the Swedish net emissions). If emerging technologies for black liquor gasification (BLG) with pre-combustion CO2 capture and sequestration are considered, the CO2 reduction potential increases by up to 6 MtCO2/y (10% of the Swedish net emissions). Commercialised BLG, CO2 capture and reliable CO2 sequestration technologies are identified as important potential contributors to Swedish compliance with Kyoto Protocol targets, especially in a scenario of nuclear power closure.

A total energy system of fuel upgrading by drying biomass feedstock for cogeneration: a case study of Skellefteå bioenergy combine

Emissions of greenhouse gases, such as CO2, need to be greatly reduced to avoid the risk of harmful climate changes. One way to mitigate emissions is switching fuels, from fossil fuels to renewable energy, e.g., biomass. In this paper we investigate a new approach for improving the performance of biomass-based cogeneration plants, a bioenergy combine. The system is a conventional biomass-based combined heat and power (CHP) plant with integrated pellet production, where part of the CHP plants heat is used for drying biomass feedstock for producing pellets. This unique integration enables increased annual operational hours and an increased use of biomass because the upgraded pellets as an energy carrier can be economically and technically transported from regions with a surplus biofuel to regions with demand for biofuel. In the studied case of this paper, the produced pellets are transported to another CHP plant for substituting fossil fuels. The total energy system of the bioenergy combine and the linked CHP plant is analysed from a perspective of CO2 reduction and energy efficiency. The results show that the system has great potential for reducing CO2 and increasing the efficiency. Furthermore, the non-technical factors influencing the realisation of the project has also been studied through interviews, showing that the main criterion behind the investment was the potential for profitability. In addition, an important factor that facilitated the realisation was the co-operative environment between the municipality and Skelleftea Kraft. Environmental issues appeared not to be influencing direct, but indirect through government subsidies.

First Experiments on an Evaporative Gas Turbine Pilot Power Plant: Water Circuit Chemistry and Humidification Evaluation

The evaporative gas turbine (EvGT), also known as the humid air turbine (HAT) cycle, is a novel advanced gas turbine cycle that has attracted considerable interest for the last decade. This high-efficiency cycle shows the potential to be competitive with Diesel engines or combined cycles in small and intermediate scale plants for power production and/or cogeneration. A 0.6 MW natural gas-fired EvGT pilot plant has been constructed by a Swedish national research group in cooperation between universities and industry. The plant is located at the Lund Institute of Technology, Lund, Sweden. The pilot plant uses a humidification tower with metallic packing in which heated water from the flue gas economizer is brought into direct counter current contact with the pressurized air from the compressor This gives an efficient heat recovery and thereby a thermodynamically sound cycle. As the hot sections in high-temperature gas turbines are sensitive to particles and alkali compounds, water quality issues need to be carefully considered. As such, apart from evaluating the thermodynamic and part-load performance characteristics of the plant, and verifying the operation of the high-pressure humidifier, much attention is focused on the water chemistry issues associated with the recovery and reuse of condensate water from the flue gas. A water treatment system has been designed and integrated into the pilot plant. This paper presents the first water quality results from the plant. The experimental results show that the condensate contains low levels of alkali and calcium, around 2 mg/l Sigma(K,Na,Ca), probably originating from the unfiltered compressor intake, About 14 mg/l NO2- +NO3- comes from condensate absorption of flue gas NOx. Some Cu is noted, 16 mg/l, which originates from copper corrosion of the condenser tubes. After CO2 stripping, condensate filtration and a mixed bed ion exchanger the condensate is of suitable quality for reuse as humidification water The need,for large quantities of demineralized water has by manY authors been identified as a drawback for the evaporative cycle. However, by cooling the humid flue gas, the recovery, of condensed water cuts the need of water feed. A self-supporting water circuit can be achieved, with no need for any net addition of water to the system. In the pilot plant, this was achieved by cooling the flue gas to around 35degreesC.

Energy studies of different cogeneration systems for black liquor gasification

Abstract The prior goal for a pulp and paper mill is to achieve a chemical recovery system with high availability and good reliability. It is also important that the heat recovery system produces sufficient process heat for the heat-demanding processes in the mill. Black liquor gasification promises to be a future alternative or a complement to the conventional Tomlinson recovery boilers in the pulp industry. Black liquor gasification should offer several advantages from both a chemical recovery and an energy point of view. The gasification process produces a combustible gas which can be used for steam and power generation in many ways. Several studies have shown that the power output from a heat and power cycle integrated with black liquor gasification has potential to get a higher energy efficiency than todays heat recovery technology. Especially with pressurized gasification and a gas-turbine-based cogeneration system, the power output will be significantly improved. However, the gas turbine is very sensitive to corrosive elements. The gas cleaning equipment in the gasification system must therefore have a high removal efficiency and a high reliability. The ambition is to keep the gasification capacity constant during the year, but the fuel consumption for the gas turbine increases at low ambient temperatures. The seasonal variation will cause the gas turbine to operate at partial load and, consequently, there will be a decrease in the gas turbine efficiency. An alternative combined cycle with high power efficiency is given by a hybrid plant in which the gas turbine is fired with natural gas while the fuel gas from the pressurized gasifier is used as a supplementary fuel for the Rankine cycle. A major advantage of the hybrid system is the use of a high purity fuel gas in the gas turbine. The system also allows for variations in pulp capacity which is difficult in an integrated gasification combined cycle (IGCC). This study compares the effect of using an IGCC system and a hybrid combined plant instead of a conventional recovery system. The results indicate a potential to double the power output if the conventional system is replaced by an IGCC system. A hybrid combined cycle system has also a higher net power efficiency than the conventional recovery system. Furthermore, the hybrid system can make the mill self-supporting on power and allows an increase in the heat consumption.

A Study of Humidified Gas Turbines for Short-Term Realization in Midsized Power Generation—Part I: Nonintercooled Cycle Analysis

Humidified Gas Turbine (HGT) cycles are a group of advanced gas turbine cycles that use water-air mixtures as the working media. In this article, three known HGT configurations are examined in the context of short-term realization for small to midsized power generation: the Steam Injected Gas Turbine, the Full-flow Evaporative Gas Turbine, and the Part-flow Evaporative Gas Turbine. The heat recovery characteristics and performance potential of these three cycles are assessed, with and without intercooling, and a preliminary economic analysis is carried out for the most promising cycles.

A Study of Humidified Gas Turbines for Short-Term Realization in Midsized Power Generation—Part II: Intercooled Cycle Analysis and Final Economic Evaluation

Humidified gas turbine (HGT) cycles are a group of advanced gas turbine cycles that use water-air mixtures as the working media. In this article, three known HGT configurations are examined in the context of short-term realization for small to mid-sized power generation: the steam injected gas turbine, the full-flow evaporative gas turbine, and the part-flow evaporative gas turbine. The heat recovery characteristics and performance potential of these three cycles are assessed, with and without intercooling, and a preliminary economic analysis is carried out for the most promising cycles.

Experimental Results on Humidification of Compressed Air in a Tubular Humidifier for Evaporative Cycles

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.© 2003 ASME

The Impact of Thermodynamic Properties of Air-Water Vapor Mixtures on Design of Evaporative Gas Turbine Cycles

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.Copyright

An Integrated Gasification Zero Emission Plant Using Oxygen Produced in a Mixed Conducting Membrane Reactor

This thesis analyses two strategies for decreasing anthropogenic carbon dioxide (CO2) emissions: to capture and store CO2, and to increase the use of biomass. First, two concepts for CO2 capture with low capture penalties are evaluated. The concepts are an integrated gasification combined cycle where the oxygen is supplied by a membrane reactor, and a hybrid cycle where the CO2 is captured at elevated pressure. Although the cycles have comparatively high efficiencies and low penalties, they illustrate the inevitable fact that capturing CO2 will always induce significant efficiency penalties. Other strategies are also needed if CO2 emissions are to be forcefully decreased. An alternative is increased use of biomass, which partially could be used for production of motor fuels (biofuels). This work examines arguments for directing biomass to the transport sector, analyses how biofuels (and also some other means) may be used to reduce CO2 emissions and increase security of motor fuel supply. The thesis also explores the possibility of reducing CO2 emissions by comparatively easy and cost-efficient CO2 capture from concentrated CO2 streams available in some types of biofuel plants. Many conclusions of the thesis could be associated with either of three meanings of the word sense: First, there is reason in biofuel production – since it e.g. reduces oil dependence. From a climate change mitigation perspective, however, motor fuel production is often a CO2-inefficient use of biomass, but the thesis explores how biofuels’ climate change mitigation effects may be increased by introducing low-cost CO2 capture. Second, the Swedish promotion of biofuels appears to have been governed more by a feeling for attaining other goals than striving for curbing climate change. Third, it seems to have been the prevalent opinion among politicians that the advantages of biofuels – among them their climate change mitigation benefits – are far greater than the disadvantages and that they should be promoted. Another conclusion of the thesis is that biofuels alone are not enough to drastically decrease transport CO2 emissions; a variety of measures are needed such as fuels from renewable electricity and improvements of vehicle fuel economy.

Experimental Study on a Packed Bed Humidifier in an Evaporative Gas Turbine

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 m2 /m3 . 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.Copyright