Analysis and modelling of chemical looping combustion process with and without oxygen uncoupling

Effective control and limiting of carbon dioxide (CO₂) emissions in energy production are major challenges of science today. Current research activities include the development of new low-cost carbon capture technologies, and among the proposed concepts, chemical combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) have attracted significant attention allowing intrinsic separation of pure CO₂ from a hydrocarbon fuel combustion process with a comparatively small energy penalty. Both CLC and CLOU utilize the well-established fluidized bed technology, but several technical challenges need to be overcome in order to commercialize the processes. Therefore, development of proper modelling and simulation tools is essential for the design, optimization, and scale-up of chemical looping-based combustion systems. The main objective of this work was to analyze the technological feasibility of CLC and CLOU processes at different scales using a computational modelling approach. A onedimensional fluidized bed model frame was constructed and applied for simulations of CLC and CLOU systems consisting of interconnected fluidized bed reactors. The model is based on the conservation of mass and energy, and semi-empirical correlations are used to describe the hydrodynamics, chemical reactions, and transfer of heat in the reactors. Another objective was to evaluate the viability of chemical looping-based energy production, and a flow sheet model representing a CLC-integrated steam power plant was developed. The 1D model frame was succesfully validated based on the operation of a 150 kWth laboratory-sized CLC unit fed by methane. By following certain scale-up criteria, a conceptual design for a CLC reactor system at a pre-commercial scale of 100 MWth was created, after which the validated model was used to predict the performance of the system. As a result, further understanding of the parameters affecting the operation of a large-scale CLC process was acquired, which will be useful for the practical design work in the future. The integration of the reactor system and steam turbine cycle for power production was studied resulting in a suggested plant layout including a CLC boiler system, a simple heat recovery setup, and an integrated steam cycle with a three pressure level steam turbine. Possible operational regions of a CLOU reactor system fed by bituminous coal were determined via mass, energy, and exergy balance analysis. Finally, the 1D fluidized bed model was modified suitable for CLOU, and the performance of a hypothetical 500 MWth CLOU fuel reactor was evaluated by extensive case simulations.

Separation of lignin in Pulp Mill process and its effect on sodium sulphur balance

The aim of this thesis is to define effects of lignin separation process on Pulp mill chemical balance especially on sodium/sulphur-balance. The objective is to develop a simulation model with WinGEMS Process Simulator and use that model to simulate the chemical balances and process changes. The literature part explains what lignin is and how kraft pulp is produced. It also introduces to the methods that can be used to extract lignin from black liquor stream and how those methods affect the pulping process. In experimental part seven different cases are simulated with the created simulation model. The simulations are based on selected reference mill that produces 500 000 tons of bleached air-dried (90 %) pulp per year. The simulations include the chemical balance calculation and the estimated production increase. Based on the simulations the heat load of the recovery boiler can be reduced and the pulp production increased when lignin is extracted. The simulations showed that decreasing the waste acid stream intake from the chlorine dioxide plant is an effective method to control the sulphidity level when about 10 % of lignin is extracted. With higher lignin removal rates the in-mill sulphuric acid production has been discovered to be a better alternative to the sulphidity control.

The feasibility of torrefaction for the co-firing of wood in pulverised-fuel boilers

Torrefaction is the partial pyrolysis of wood characterised by thermal degradation of predominantly hemicellulose under inert atmosphere. Torrefaction can be likened to coffee roasting but with wood in place of beans. This relatively new process concept makes wood more like coal. Torrefaction has attracted interest because it potentially enables higher rates of co-firing in existing pulverised-coal power plants and hence greater net CO2 emission reductions. Academic and entrepreneurial interest in torrefaction has sky rocketed in the last decade. Research output has focused on the many aspects of torrefaction – from detailed chemical changes in feedstock to globally-optimised production and supply scenarios with which to sustain EU emission-cutting directives. However, despite its seemingly simple concept, torrefaction has retained a somewhat mysterious standing. Why hasn’t torrefied pellet production become fully commercialised? The question is one of feasibility. This thesis addresses this question. Herein, the feasibility of torrefaction in co-firing applications is approached from three directions. Firstly, the natural limitations imposed by the structure of wood are assessed. Secondly, the environmental impact of production and use of torrefied fuel is evaluated and thirdly, economic feasibility is assessed based on the state of the art of pellet making. The conclusions reached in these domains are as follows. Modification of wood’s chemical structure is limited by its naturally existing constituents. Consequently, key properties of wood with regards to its potential as a co-firing fuel have a finite range. The most ideal benefits gained from wood torrefaction cannot all be realised simultaneously in a single process or product. Although torrefaction at elevated pressure may enhance some properties of torrefied wood, high-energy torrefaction yields are achieved at the expense of other key properties such as heating value, grindability, equilibrium moisture content and the ability to pelletise torrefied wood. Moreover, pelletisation of even moderately torrefied fuels is challenging and achieving a standard level of pellet durability, as required by international standards, is not trivial. Despite a reduced moisture content, brief exposure of torrefied pellets to water from rainfall or emersion results in a high level of moisture retention. Based on the above findings, torrefied pellets are an optimised product. Assessment of energy and CO2-equivalent emission balance indicates that there is no environmental barrier to production and use of torrefied pellets in co-firing. A long product transport distance, however, is necessary in order for emission benefits to exceed those of conventional pellets. Substantial CO2 emission reductions appear possible with this fuel if laboratory milling results carry over to industrial scales for direct co-firing. From demonstrated state-of-the-art pellet properties, however, the economic feasibility of torrefied pellet production falls short of conventional pellets primarily due to the larger capital investment required for production. If the capital investment for torrefied pellet production can be reduced significantly or if the pellet-making issues can be resolved, the two production processes could be economically comparable. In this scenario, however, transatlantic shipping distances and a dry fuel are likely necessary for production to be viable. Based on demonstrated pellet properties to date, environmental aspects and production economics, it is concluded that torrefied pellets do not warrant investment at this time. However, from the presented results, the course of future research in this field is clear.

Muistipotilaan tarkoituksenmukaisen hoitopaikan määrittäminen Balance of Care -menetelmällä

Tutkimuksen tarkoituksena oli kuvata Balance of Care (BoC) -menetelmän käyttöä ikääntyneen muistipotilaan tarkoituksenmukaisen hoitopaikan määrittelyssä. Tutkimuksen avulla pyrittiin selvittämään miten yhtenäinen on asiantuntijoiden muodostama käsitys ikääntyneen muistipotilaan tarkoituksenmukaisesta hoitopaikasta, sekä mitkä tekijät vaikuttavat asiantuntijoiden päätöksentekoon muodostettaessa tarkoituksenmukaisia hoitopaikkoja. Tutkimus on osa kansainvälistä EU:n rahoittamaa RightTimePlaceCare -projektia (RTPC), jonka tavoitteena on kehittää muistisairaiden ikääntyneiden hoitokäytänteitä tilanteisiin, joissa pohditaan tarkoituksenmukaisinta hoitopaikkaa. Tutkimuksessa tarkasteltiin asiantuntijoiden (N = 27) päätöksentekoa käytettäessä BoC -menetelmää. Tutkimus toteutettiin viidessä kunnassa, jossa kussakin kokoontui yksi asiantuntijaryhmä pohtimaan BoC -menetelmän avulla tarkoituksenmukaista hoitopaikkaa 14 esimerkkitapaukselle. Esimerkkitapaukset kuvasivat tyypillisiä ikääntyneitä muistipotilaita. Kukin asiantuntijaryhmä tutustui seitsemään esimerkkitapaukseen. Jokaista esimerkkitapausta arvioi kaksi tai kolme asiantuntijaryhmää. Asiantuntijat valitsivat ja kirjasivat ensin itsenäisesti hoitopaikan esimerkkitapaukselle, tämän jälkeen asiantuntijat keskustelivat ryhmässä ja pyrkivät muodostamaan konsensuksen tarkoituksenmukaisesta hoitopaikasta sekä mahdollisesti tarvittavista palveluista. Myös nämä kirjattiin. Aineisto analysoitiin kvantifioimalla ja induktiivisesti teemoittelemalla. Asiantuntijoiden itsenäisesti muodostamissa hoitopaikkapäätöksissä oli paljon eroja. Ainoastaan yhden esimerkkitapauksen (7 %) kohdalla asiantuntijoiden päätös oli yksimielinen. Osalle asiantuntijoista tuotti vaikeuksia valita vaihtoehdoista vain yksi hoitopaikka ikääntyneelle muistipotilaalle. Asiantuntijaryhmien päätökset olivat yhtenäisempiä, kuin yksittäisten asiantuntijoiden. Ryhmät päätyivät täysin yhteneviin hoitopaikkasuosituksiin seitsemän esimerkkitapauksen (50 %) kohdalla. Tulosten mukaan ikääntyneen muistipotilaan hoitopaikkaa koskevat päätökset olisi järkevää toteuttaa moniammatillisissa asiantuntijaryhmissä, näin saavutetaan yhdenmukaisempia hoitopäätöksiä. Sekä asiantuntijoiden että asiantuntijaryhmien päätöksistä suurin osa painottui kotihoitoon, joten suurinta osaa tässä tutkimuksessa mukana olleista ikääntyneistä muistipotilasta voidaan hoitaa palveluiden turvin kotona. Asiantuntijoiden päätöksentekoon vaikuttavat tekijät muodostuivat muistipotilaan hoidon tukipalveluista, fyysisestä hoitoympäristöstä, muistipotilaan läheisistä ja ikääntyneestä muistipotilaasta. Tutkimuksen avulla saatua tietoa voidaan hyödyntää päätöksenteon tukena määritettäessä tarkoituksenmukaista hoitopaikkaa ikääntyneelle muistipotilaalle sekä suunniteltaessa menetelmää muistipotilaan hoidon koordinointiin.