CNG und LNG aus biogenen Reststoffen – ein Konzept zur ressourcenschonenden Kraftstoffproduktion

Es wurde ein Verfahren entwickelt, das die Umwandlung von Reststoffen zu methanbasierten Kraftstoffen unter höchstmöglichem Erhalt des biogenen Kohlenstoffs mithilfe von elektrischer Energie aus erneuerbaren Quellen ermöglicht. Waldrestholz, Stroh und Klärschlamm wurden als besonders relevante Einsatzstoffe identifiziert. Durch die hochintegrierte Kopplung von Vergasung, Hochtemperaturelektrolyse und Methanisierung wird der biogene Kohlenstoff aus den Edukten nahezu vollständig in das Produkt Methan überführt. Die Gestehungskosten sind dabei mit denen gegenwärtig eingesetzter Technologien vergleichbar und liegen bei Großanlagen im Bereich üblicher Werte der Biomethanerzeugung

Behavior of sulfur oxides in air and oxy-fuel combustion

This thesis evaluates the behavior of sulfur oxides in pulverized fuel (PF) fired air and oxy-fuel systems. Sulfur oxides are responsible for certain operational problems and considerable gas cleaning requirements in air as well as oxy-fuel firing. A better understanding of the related issues will allow for a technical and economical optimization of the oxy-fuel combustion technology. A range of experimental investigations studying the stability and retention of sulfur oxides in ashes and deposits, acid gas (SO2, SO3, and HCl) control in air and oxy-fuel combustion by dry sorbent injection, and SO3 formation were conducted. The experimental work is in parts supported by theoretical considerations and thermodynamic equilibrium simulation. Studies for different coals and lignites showed that in practically relevant oxy-fuel configurations the exclusion of airborne N2 from combustion leads to an increase of the SO2 concentrations in oxy-fuel, compared to air firing, by a factor of about 3.4 to 4.2, referring to dry, and of about 2.9 to 3.5, when referring to wet flue gas conditions. The increased SO2 levels in oxy-fuel combustion are responsible for an increased stability of sulfates in oxy-fuel power boiler systems so that for example the decomposition temperature CaSO4 rises by about 50 to 80 °C, depending on flue gas atmospheres. The enhanced stability of sulfates in deposits at high temperatures when operating with increased SO2 levels was experimentally demonstrated. Compared to air firing, a considerable increase of the sulfur retention in the ash by 10 to 12 percentage points has been observed for oxy-fuel recycle combustion of Lusatian lignites. This leads to lower SO2 emissions and higher SO3 levels in process ashes and deposits. The results indicate that for fuels, such as the used lignites, the temperature level at which fouling by sulfatic deposits is problematic may be shifted to higher temperatures in oxy-fuel combustion and that the sintering of deposits by sulfation may be more pronounced. In contrast, in air and oxy-fuel combustion experiments with a hard coal with a low sulfur retention potential differences in the SO3 contents and degrees of sulfation of ashes and deposits were small. Besides higher SO3 contents and sulfation degrees, no other significant changes between the deposit samples from air and oxy-fuel combustion were identified. Experiments on dry sorbent injection in air and oxy-fuel mode showed that an increase of the average flue gas residence time in the furnace by flue gas recirculation and, to a lesser extent, the higher sulfate stability enhance the desulfurization efficiency in oxy-fuel recycle combustion considerably. SO2 capture efficiencies in oxy-fuel recycle combustion of 50 % to more than 80 % at moderate molar sulfur to calcium ratios between 1.7 and 2.9 were reached, when injecting CaCO3 and Ca(OH)2 together with the fuel or directly to the furnace. Under comparable injection conditions, the oxy-fuel performance was by as much as 29 percentage points higher than in air firing. Also an efficient SO3 and HCl control by DSI could be demonstrated. Experiments on formation of SO3 show that higher SO2 levels in oxy-fuel firing are the most important parameter responsible for the observed increase of the SO3 concentrations.Diese Arbeit untersucht das Verhalten von Schwefeloxiden in Luft- und Oxy-Fuel-Staubfeuerungen. Bei der Luft- wie auch der Oxy-Fuel-Verbrennung sind Schwefeloxide für bestimmte Betriebsprobleme sowie einen erheblichen Rauchgasreinigungsbedarf verantwortlich. Ein besseres Verständnis von deren Verhalten ermöglicht eine weitergehende technische und wirtschaftliche Optimierung des Oxy-Fuel-Verbrennungsverfahrens. Im Rahmen der Arbeit wurden eine Reihe von experimentellen Untersuchungen zur Stabilität und zur Einbindung von Schwefeloxiden in Aschen und Belägen, zur Bildung von SO3 und zur Abscheidung der sauren Gase (SO2, SO3 und HCl) im Luft- und Oxy-Fuel-Betrieb durchgeführt. Die experimentellen Arbeiten werden teilweise auch durch theoretische Überlegungen und thermodynamische Gleichgewichtssimulationen ergänzt. Die durchgeführten Untersuchungen mit unterschiedlichen Stein- und Braunkohlen ergaben, dass die SO2-Konzentrationen in praktisch relevanten Oxy-Fuel-Systemen, aufgrund des bei der Verbrennung fehlenden Luftstickstoffs, um einen Faktor von ca. 3,4 bis 4,2, bezogen auf trockenes und von ca. 2,9 bis 3,5, bezogen auf feuchtes Rauchgas, ansteigen. Die erhöhten SO2-Gehalte bei der Oxy-Fuel-Verbrennung bewirken je nach Gasatmosphäre eine erhöhte Stabilität von Sulfaten, sodass beispielsweise die Zersetzungstemperatur von CaSO4 um ca. 50 bis 80 °C ansteigt. Die Erhöhung der Sulfatstabilität in Belägen bei hohen Temperaturen, durch erhöhte SO2-Konzentrationen, wurde experimentell demonstriert. Im Vergleich zur Luftfeuerung, wurde bei der Oxy-Fuel-Verbrennung mit Rauchgasrezirkulation von Lausitzer Braunkohle, eine Erhöhung der Schwefeleinbindung in die Asche um 10 bis 12 Prozentpunkte beobachtet. Die bessere Schwefeleinbindung ist für niedrigere SO2-Emissionen und höhere SO3-Gehalte in Aschen und Belägen verantwortlich. Die Ergebnisse deuten darauf hin, dass im Oxy-Fuel-Betrieb bei Brennstoffen wie den verwendeten Braunkohlen das Temperaturniveau, bei welchem die Verschmutzung durch sulfatische Beläge problematisch ist, hin zu höheren Temperaturen verschoben und, dass das Versintern von Belägen durch Sulfatisierung verstärkt ist. Im Gegensatz dazu, waren die Unterschiede, in Bezug auf SO3-Gehalte und den Grad der Sulfatisierung von Aschen und Belägen aus Luft- und Oxy-Fuel-Verbrennungsversuchen mit einer Steinkohle mit niedrigem Potential zur Schwefeleinbindung, klein. Neben den erhöhten SO3-Gehalten und Sulfatisierungsgraden, wurden keine weiteren signifikanten Änderungen zwischen den Belagsproben aus Luft- und Oxy-Fuel-Verbrennung beobachtet. Versuche zur Eindüsung trockener Sorbentien im Luft- und Oxy-Fuel-Betrieb zeigten, dass die Erhöhung der mittleren Rauchgasverweilzeit in der Brennkammer durch die Rauchgasrezirkulation und, zu einem geringeren Anteil, die erhöhte Sulfatstabilität, die Entschwefelungseffizienz im Oxy-Fuel-Betrieb mit Rauchgasrückführung deutlich verbessern. Bei der Eindüsung von CaCO3 und Ca(OH)2 zusammen mit dem Brennstoff oder direkt in die Brennkammer konnten im Oxy-Fuel-Betrieb mit Rauchgasrezirkulation und moderaten molaren Schwefel-zu-Kalzium-Verhältnissen zwischen 1,7 und 2,9, SO2-Abscheideeffizienzen von 50 % bis zu mehr als 80 % erzielt werden. Dabei war die Abscheideleistung bei vergleichbarer Sorbenseindüsung im Oxy-Fuel-Betrieb um bis zu 29 Prozentpunkte besser als bei konventioneller Luftverbrennung. Auch eine effiziente SO3- und HCl-Abscheidung mittels Trockensorbenseindüsung konnte demonstriert werden. Versuche zur Bildung von SO3 zeigen, dass die im Oxy-Fuel-Betrieb erhöhte SO2-Konzentration der wichtigste Parameter hinsichtlich der Erhöhung der SO3-Konzentrationen ist

Mercury and SO3 emissions in oxy-fuel combustion

This paper presents results on experiments carried out at a 20 kW combustion rig simulating different extents of oxy-fuel recycle gas cleaning by impurities injection to the oxidant gas of the once-through combustion reactor. A comprehensive set of total (Hg^tot), elemental (Hg⁰) and oxidized (Hg<sup2+></sup>) mercury as well as SO₃ concentrations was obtained before and after the combustion rig's baghouse filter for in total 14 air and oxy-fuel experiments with 3 Australian coals. Based on this data, an assessment in respect to Hg oxidation, SO₂/SO₃ conversion and Hg and SO₃ capture on the test rig's filter was performed. The air and the oxy-fuel experiments with different extents of recycle gas cleaning, revealed differences in the Hg and SO₃ formation and capture behavior: the Hg<sup2+></sup>/Hg^tot ratios in the flue gas are higher during oxy-fuel combustion compared to air-firing. This effect is even more pronounced at the filter outlet, after flue gas has passed through the filter ash. In some experiments, even a net oxidation of Hg⁰ entering the filter to Hg<sup2+></sup> was observed. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. However, the Hg capture was altered by the different oxy-fuel recycle configurations, leading to decreased Hg capture efficiencies on the filter for one of the coals. A coal-specific trend of increased SO₂/SO₃ conversion ratios with increased flue gas SO₂ levels was observed that could be related to the ash composition of the three different coals. This and the higher SO₂ concentrations in the flue gas lead to considerably higher SO₃ levels in oxy-fuel combustion with SO₂ recycling. During the experiments, also a considerable capture of SO₃ in the baghouse filter was observed (up to 80% under air- and up to 66% under oxy-fired conditions). A reduction of the SO₃ capture on the filter under oxy-fuel conditions may be related to the higher SO₃ levels in this process

CEMCAP - making CO2 capture retrofittable to cement plants

Paper providing a general overview of the H2020 CEMCAP Project, from the GHGT13 Conference, Nov 14-18, Lausanne, Switzerlan

Process simulation and techno-economic assessment of SER steam gasification for hydrogen production

In the SER (sorption enhanced reforming) gasification process a nitrogen-free, high calorific product gas can be produced. In addition, due to low gasification temperatures of 600 e750 �C and the use of limestone as bed material, in-situ CO2 capture is possible, leading to a hydrogen-rich and carbon-lean product gas. In this paper, results from a bubbling fluidised bed gasification model are compared to results of process demonstration tests in a 200 kWth pilot plant. Based upon that, a concept for the hydrogen production via biomass SER gasification is studied in terms of efficiency and feasibility. Capital and operational expenditures as well as hydrogen production costs are calculated in a techno-economic assessment study. Furthermore, market framework conditions are discussed under which an economic hydrogen production via SER gasification is possible

SO₃ emissions and removal by ash in coal-fired oxy-fuel combustion

The sulfur oxide (SOₓ) concentrations during oxy-fuel combustion are generally higher compared to conventional air firing. The higher SOₓ concentrations, particularly sulfur trioxide (SO₃) in combination with high concentration of water in the recycled flue gas, increase the sulfuric acid dew point temperature in oxy-fuel fired systems, thereby increasing allowable flue gas temperatures and reducing the thermal efficiency of a power plant. This paper presents results of experiments carried out at a 20 kW once-through combustion rig of the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart simulating different extents of oxy-fuel recycle gas cleaning by impurities injection to the oxidant gas of a once-through combustion reactor. Three Australian coals that have previously been tested under air and oxy-fuel conditions at the Aioi furnace of IHI in Japan were used in the experiments. The SOₓ emissions were measured, conversion ratios of sulfur dioxide (SO₂) to SO₃ were calculated, and results were compared with existing literature, finding good agreement. The experiments with different extents of recycle gas cleaning and therefore different SO₂ levels in the system, revealed differences in the SO₃ generation behavior: A coal-specific trend of increasing conversion ratios of SO₂to SO₃ with increased flue gas SO₂ levels was observed that could be related to the ash composition of the three different coals. The capture of SOₓ in a baghouse filter was also evaluated. Acid dew point temperatures (ADPs) for the flue gas were calculated for the various firing conditions. Acid dew point (ADP) temperatures increased by up to 50°C when changing from air to oxy-firing with recycling of H₂O and SO₂. Considerable differences in the ADPs were found for different extents of oxy-fuel recycle gas treatment and were evaluated in respect to power plant efficiency implications

Mercury emissions and removal by ash in coal-fired oxy-fuel combustion

This paper presents results of experiments performed at a 20 kW once-through combustion rig of the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart. A methodology to investigate oxy-fuel process configurations was used in which impurities were injected to the oxidant gas of the once-through reactor to simulate different extents of oxy-fuel recycle gas treatment. Three Australian coals, which had previously been tested in the Aioi furnace of IHI in Japan, were used in the experiments. A comprehensive set of total (Hg^tot), elemental (Hg⁰), and oxidized (Hg²⁺) mercury concentrations was measured for various air and oxy-fuel combustion conditions. These data enable an evaluation of process parameters that influence the Hg emissions of an oxy-fuel combustion process. A theoretical mass balance between Hg fed to the process (fuel and Hg⁰ injection) and Hg measured before the filter matched well, indicating that no mercury was captured by fly ash at high temperatures. The capture of Hg⁰ and oxidized Hg²⁺ by ash in a baghouse filter has been determined for all experiments. Measured Hg concentrations show an increase when switching from air to oxy-fuel operation for all investigated coals and oxy-fuel settings, even when no additional Hg⁰ is injected to the oxidant gas. Moreover, the Hg²⁺/Hg^tot ratios in the flue gas are higher during oxy-fuel combustion. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. Reduction rates in a range between 18 and 51% for air and between 11 and 29% for oxy-fuel combustion were observed

Sulfur capture by fly ash in air and oxy-fuel pulverized fuel combustion

Ash produced during oxy-fuel combustion is expected to differ from ash produced during air combustion because of the higher CO₂ and SO₂ atmospheres in which it is generated. For a quantitative understanding of the sulfation behavior of fly ash in oxy-fuel combustion, fly ash from three commercial Australian sub-bituminous coals was tested and decomposed under an inert atmosphere. Thermal evolved gas analysis was completed for ash produced in both air and oxy-fuel environments. Pure salts were also tested under the same conditions to allow for identification of the species in the ash that capture sulfur, along with thermodynamic modeling using FactSage 6.3. Sulfur evolved during the decomposition of air and oxy-fuel fly ash was compared to the total sulfur in the ash to close the sulfur balance. Both total sulfur captured by the ash and sulfur evolved during decomposition were higher for oxy-fuel fly ash than their air counterparts. Correlations of capture with ash chemistry are presented