Using steam reforming to produce hydrogen with carbon dioxide capture by chemical-looping combustion

In this paper, a novel process for hydrogen production by steam reforming of natural gas with inherent capture of carbon dioxide by chemical-looping combustion is proposed. The process resembles a conventional circulating fluidized bed combustor with reforming taking place in reactor tubes located inside a bubbling fluidized bed. Energy for the endothermic reforming reactions is provided by indirect combustion that takes place in two separate reactors: one for air and one for fuel. Oxygen is transferred between the reactors by a metal oxide. There is no mixing of fuel and air so carbon dioxide for sequestration is easily obtained. Process layout and expected performance are evaluated and a preliminary reactor design is proposed. It is found that the process should be feasible. It is also found that it has potential to achieve better selectivity towards hydrogen than conventional steam reforming plants due to low reactor temperatures and favorable heat-transfer conditions

Synthesis gas generation by chemical-looping reforming in a continuously operating laboratory reactor

Chemical-looping reforming is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. This paper describes continuous chemical-looping reforming of natural gas in a laboratory reactor consisting of two interconnected fluidized beds. Particles composed of 60 wt% NiO and 40 wt% MgAl2O4 are used as bed material, oxygen carrier and reformer catalyst. There is a continuous circulation of particles between the reactors. In the fuel reactor, the particles are reduced by the fuel, which in turn is partially oxidized to H-2, CO, CO2 and H2O. In the air reactor the reduced oxygen h of reforming were recorded. Formation of solid carbon was noticed for some cases. Adding 25 vol% steam to the natural gas reduced or eliminated the carbon formation

Testing of minerals and industrial by-products as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300W laboratory reactor

Chemical-looping combustion (CLC) is a promising technology for future energy production with inherent CO2 separation. One approach is to use minerals or industrial by-products as oxygen carriers to reduce the costs of the process. This study focuses on the investigation of two iron-based oxygen carriers, which were examined under continuous operation in a 300 W laboratory reactor. Ilmenite is an iron–titanium oxide mineral, whereas iron oxide scale (IOS) is obtained as a by-product from the rolling of sheet steel. Syngas was used as a fuel – pure and with steam addition to suppress the formation of solid carbon. During the experiments the variables reactor temperature, fuel flow and air flow were changed. Furthermore the effect of steam addition to the fuel was investigated. Particle properties were compared over the span of 85 h of continuous operation for ilmenite and 37 h for IOS. The analysis is based on gas measurements from the actual CLC operation, but also on scanning electron microscopy, X-ray powder diffractometry and measurements of BET surface area and density. With ilmenite oxygen carrier it was possible to achieve full conversion of syngas up to about 190 Wth fuel equivalent at 900 °C. With design fuel flow of about 300 Wth at 900 °C the combustion efficiency was above 98%. There was almost no visible difference in reactivity of fresh activated particles and those used for 85 h. Combustion efficiency up to 99% was achieved with IOS oxygen carrier at 900 °C and about 100 Wth fuel equivalent. At 300 Wth fuel equivalent and 900 °C a combustion efficiency of only 90% could be reached. Both oxygen carriers were operated for tens of hours, which allowed for a better understanding of lifetime behavior and other basic characteristics. Whereas ilmenite oxygen-carrier particles were mostly stable over the course of 85 h of experiments, a large fraction of IOS oxygen-carrier particles had disintegrated to fines after only 37 h of experiments. The gathered data indicates that both oxygen carriers could be an alternative to synthesized particles, though with more drawbacks for IOS than for ilmenite

Chemical-Looping Combustion and Chemical-Looping Reforming in a Circulating Fluidized-Bed Reactor Using Ni-Based Oxygen Carriers

Three oxygen carriers for chemical-looping combustion and chemical-looping reforming have been investigated in a small circulating fluidized-bed reactor. N2AM1400 was produced by freeze granulation with MgAl2O4 as a support material and had a NiO content of 20%. Ni18-αAl was produced by impregnation onto α-Al2O3 and had a NiO content of 18%. Ni21-γAl was produced by impregnation onto γ-Al2O3 and had a NiO content of 21%. Over 160 h of operation has been recorded. The conversion of natural gas into products was 96−100% depending on oxygen carrier and experimental conditions. For chemical-looping combustion, N2AM1400 and Ni21-γAl provided poor selectivity toward CO2 and H2O while Ni18-αAl initially showed very high selectivity, which declined as a function of time. For chemical-looping reforming, operating the reactor at the desired process parameters, which was a fuel reactor temperature of 950 °C and an air factor of 0.30, was possible with all of the tested oxygen-carrier materials. When only natural gas was used as fuel, there was significant formation of solid carbon in the fuel reactor for Ni18-αAl and Ni21-γAl. Adding 30% steam or CO2 to the fuel removed or decreased the carbon formation. During the course of the experiments, N2AM1400 and Ni18-αAl retained their physical and chemical structure, while Ni21-γAl displayed a significant reduction in porosity but remained highly reactive

Chemical-looping combustion and chemical-looping reforming of kerosene in a circulating fluidized-bed 300W laboratory reactor

The reaction between a nickel-based oxygen carrier and a liquid fuel has been demonstrated in a chemical-looping reactor with continuous particle circulating. An injection system was constructed, in which sulfur-free kerosene was evaporated, mixed with superheated steam and fed directly into the lab scale chemical-looping reactor. A nickel-based oxygen carrier composed of 40 wt% NiO and 60 wt% MgO-ZrO2 was used for both chemical-looping combustion (CLC) and chemical-looping reforming (CLR) experiments, which were performed for about 34 h and 20 h, respectively. For the CLC experiments, 95-99% of the fuel carbon was converted to CO2 and only a minute amount of hydrocarbons was detected in the off-gas. For the CLR experiments, synthesis gas was produced with concentrations of hydrocarbons as low as 0.01%. The particles were analyzed before and after the experiments using XRD, SEM, BET surface area and particle size distribution. It was shown that it is possible to use liquid fuel in a continuous chemical-looping process and also achieve nearly complete fuel conversion. With a nickel-based oxygen carrier virtually all hydrocarbon could be fully oxidized

Combined oxides as oxygen-carrier material for chemical-looping with oxygen uncoupling

Oxygen-carrier materials for chemical-looping with oxygen uncoupling (CLOU) must be capable of taking up and releasing gas-phase O2 at conditions relevant for generation of heat and power. In principle, the capability of a certain material to do so is determined by its thermodynamic properties. This paper provides an overview of the possibility to design feasible oxygen carrier materials from combined oxides, i.e. oxides with crystal structures that include several different cations. Relevant literature is reviewed and the thermodynamic properties and key characteristics of a few selected combined oxide systems are calculated and compared to experimental data. The general challenges and opportunities of the combined oxide concept are discussed. The focus is on materials with manganese as one of its components and the following families of compounds and solid solutions have been considered: (MnyFe1-y)Ox, (MnySi1-y)Ox, CaMnO3-δ,(NiyMn1-y)Ox, (MnyCu1-y)Ox and (MnyMg1-y)Ox. In addition to showing promise from a thermodynamic point of view, reactivity data from experimental investigations suggests that the rate of O2 release can be high for all systems. Thus these combined oxides could also be very suitable for practical application

Waste products from the steel industry with NiO as additive as oxygen carrier for chemical-looping combustion

Fe2O3-containing waste materials from the steel industry are proposed as oxygen carrier for chemical-looping combustion. Three such materials, red iron oxide, brown iron oxide and iron oxide scales, have been examined by oxidation and reduction experiments in a batch fluidized-bed reactor at temperatures between 800 and 950°C. NiO-based particles have been used as additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H2. The experiments indicated modest reactivity between the waste materials and CH4, which was used as reducing gas. Adding small amounts of NiO-based particles to the sample increased the yield of CO2 in a standard experiment, typically by a factor of 1.5-3.5. The fraction of unconverted fuel typically was reduced by 70-90%. The conversion of CH4 to CO2 was 94% at best, corresponding to a combustion efficiency of 96%. This was achieved using a bed mass corresponding to 57 kg oxygen carrier per MW fuel, of which only 5 wt% was NiO-based synthetic particles. The different materials fared differently well during the experiments. Red iron oxide was fairly stable, while brown iron oxide was soft and subject to considerable erosion. Iron oxide scales experienced increased reactivity and porosity as function of the numbers of reduction cycles

Oxygen Carrier Aided Combustion (OCAC) of Wood Chips in a Semi-Commercial Circulating Fluidized Bed Boiler Using Manganese Ore as Bed Material

Oxygen Carrier Aided Combustion (OCAC) is realized by using an active oxygen-carrying bed material in fluidized bed boilers. The active material is reduced in fuel rich parts of the boiler and oxidized in air rich parts. Advantages could be achieved such as new mechanisms for oxygen transport in space and time. Here calcined manganese ore has been used as active bed material in a 12 MWth circulating fluidized bed boiler. The fuel was wood chips and the campaign lasted more than two weeks. From an operational point of view, manganese ore worked excellently. From the temperature profile of the boiler it can be concluded that fuel conversion was facilitated, especially in the dense bottom bed. The effect did not always translate to reduced emissions, which suggests that final combustion in the cyclone outlet was also influenced. Substituting 10% of the sand bed with manganese ore made it possible to reduce the air to fuel ratio without generating large amounts of CO. The use of 100% manganese ore resulted in higher emissions of CO than the sand reference, but, when combined sulphur feeding, dramatic reductions in CO emissions, up to 90% compared to sand reference, was achieved

Combined Cu/Mn Oxides as an Oxygen Carrier in Chemical Looping with Oxygen Uncoupling (CLOU)

This study investigates the O2 uncoupling properties of five different oxygen carrier particles, consisting of combined oxides of CuO and Mn3O4. The oxygen carriers were produced by freeze granulation followed by calcination at 950 °C for 6 h. Particles with 5, 10, 20, 31, and 61 wt % CuO were examined in both an inert (pure N2) atmosphere and in the presence of solid fuel (wood char) at 750 °C. At this relatively low temperature during fluidization with N2, the samples were capable of releasing gas-phase O2 in concentrations up to 1%. During reduction with wood char in 15 g of oxygen carriers, some materials could release gaseous O2 equal to 1.4% of their total mass. When the crushing strength and attrition index were measured with a customized jet cup, the mechanical stability of these samples was compared. These measurements showed that, in general, samples with a higher CuO content were more mechanically stable. On the basis of XRD analysis of the oxygen carriers, the major phase transitions were Mn2O3 ↔ Mn3O4 and combined spinel (Cu,Mn)3O4 ↔ CuMnO2. These transitions both provide a considerable amount of O2. It is concluded that the Cu–Mn–O system has considerable potential to be used as a oxygen carrier in chemical-looping applications at lower temperatures, perhaps interesting for biofuel combustion