Metal oxides in fluidized bed conversion systems

Several fluidized bed conversion concepts utilizing metal oxide particles have been proposed in the last decade. The use of metal oxide particles in fluidized bed conversion systems stems largely from the research around chemical-looping combustion (CLC). Here, a metal oxide is used as an oxygen carrier for transport of oxygen from combustion air to the fuel. Although the concept was proposed already in the 1950’s, most of the research has been carried out after the turn of the century. The research was sparked by the need for viable technologies for carbon capture, and CLC is here a breakthrough technology with low costs for capture and a high carbon capture efficiency. Today a large number of oxygen carrier particles have been developed and tested and around 30 continuous units up to 3 MW have been operated around the world. The research around CLC and oxygen carriers has sparked the development of several other fluidized bed processes where oxygen carriers or metal oxide particles can be used to enhance performance. These include fluidized bed processes for gas, solid and liquid fuels for heat, power and syngas production. Chalmers has been active within CLC research for over 15 years and also has activities within these related processes. In addition to CLC, this paper will provide an overview of these technologies: i) Oxygen carrier aided combustion, ii) Dual-bed fluidized bed gasification (DFBG), iii) Chemical-looping reforming (CLR) and iv) gasification gas conditioning (CLTR). Some important criteria for metal oxides will be discussed in relation to the application

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

An experimental study of a volatiles distributor for solid fuels chemical-looping combustion process

A novel concept called volatiles distributor (VD), with the purpose to achieve an even distribution of volatiles over the cross-section of a fluidized-bed and better contact between volatiles and bed materials, has been investigated. The concept could be useful for chemical- looping combustion, as well as other solid fuel conversion processes in fluidized-beds. An experimental study of the VD in a circulating fluidized-bed (CFB) cold-flow model was conducted under different fluidization velocities and flows of simulated volatiles. In the reference case without VD, a local plume of volatiles is formed and the maldistribution becomes more pronounced at higher fluidization velocity in the range from 1 m/s to 4 m/s. Conversely, higher fluidization velocity gives a more even volatiles distribution in the presence of VD. The relative standard deviation of volatiles horizontal distribution decreases from 131% in absence of VD to 22% in presence of VD at the fluidization velocity of 4 m/s. There is no significant effect of volatiles flow rate on VD performance at a fluidization velocity 1 m/s. As the fluidization velocity and volatiles flow rate increase, the bed level inside VD is lowered and the volatiles inside the VD become less diluted, because less air from the main fluidization passes through the VD

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

Steel Converter Slag as an Oxygen Carrier-Interaction with Sulfur Dioxide

Steel converter slag, also called Linz-Donawitz (LD) slag, has been considered as an oxygen carrier for biofuel chemical looping applications due to its high availability. In addition to its content of iron which contributes to its oxygen-carrying capacity, LD slag also contains a significant amount of calcium. Calcium, however, is known to interact with sulfur, which may affect the usability of LD slag. To get a better understanding of the interaction between sulfur and LD slag, batch scale experiments have been performed using solid and gaseous fuel with or without sulfur dioxide, together with LD slag as an oxygen carrier. The reactivity and sulfur interaction were compared to the benchmark oxygen carrier ilmenite. Sulfur increases the gasification rate of biofuel char and the conversion of CO for both LD slag and ilmenite. However, no effect of sulfur could be seen on the conversion of the model tar species benzene. The increased gasification rate of char was suspected to originate from both surface-active sulfur and gaseous sulfur, increasing the reactivity and oxygen transfer of the oxygen carrier. Sulfur was partly absorbed into the LD slag particles with calcium, forming CaS and/or CaSO4. This, in turn, blocks the catalytic effect of CaO towards the water gas shift reaction. When the SO2 vapor pressure was decreased, the absorbed sulfur was released as SO2. This indicates that sulfur may be released in loop-seals or in the air reactor in a continuous process

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

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

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

Study of the interaction between a Mn ore and alkali chlorides in chemical looping combustion

Chemical looping combustion (CLC) is a novel technology for heat and power generation with inherent CO2 capture. Using biomass in CLC (bio-CLC), negative CO2 emissions can be attained. Biomass usually contains high content of alkalis (mainly K and Na) which can be problematic in the process, such as potential alkali-bed interaction, and this is the focus of current work. This work uses charcoal with and without the impregnation with alkali chlorides, KCl and NaCl. The results are compared to previous data from samples impregnated with K2CO3 and Na2CO3. A low-alkali braunite manganese ore is used as bed material to study the oxygen carrier interaction with the alkalis in cyclic experiments at 950 \ub0C in a quartz batch fluidized-bed reactor. As compared to charcoal without alkali impregnation, the impregnation with KCl, NaCl, K2CO3, and Na2CO3 can improve the rate of gasification by a factor of 4, 3, 10, 8, respectively. Partial-defluidization of the braunite particles was found with all the alkali-fuels, although the extent differed, e.g., K2CO3 and KCl resulted in earlier onset of defluidization than Na2CO3 and NaCl. Further, indications of partial defluidization were earlier and more permanent with the carbonates than the chlorides. Partial agglomeration with soft agglomerates of the bed was observed, while hard agglomerations were never seen. Accumulation of K, Na, Si, and Ca was found in the agglomerates after cycles with K2CO3-charcoal and Na2CO3-charcoal, while little K and Na was detected in the bridges between particles after the KCl and NaCl cycles. A significant fraction of the alkali added was found in the oxygen carrier, with 80% or more being retained for the Na salts, and around 40% for the K salts. There was no clear difference between chlorides and carbonates with respect to retention. The fresh and used braunite have very similar reactivity with CH4 and H2, whereas some decrease in reactivity is noticed with CO

Techno-Economic Assessment of Chemical Looping Gasification of Biomass for Fischer-Tropsch Crude Production with Net-Negative CO2 Emissions: Part 2

This work presents a techno-economic analysis of a used as the primary gasification process for biofuel production through Fischer-Tropsch synthesis (FTS). Two different gas cleaning process configurations, cold-gas cleanup and hot-gas cleanup process trains, are explored, along with off-gas utilization possibilities, to study their influence on the process economics of an integrated CLG-FT process plant. Off-gas recirculation to increase Fischer-Tropsch (FT) crude production has a significant influence on reducing the levelized production costs for FT crude. The results indicate that the specific production cost estimated for a CLG-FT plant with a hot-gas cleanup train is roughly 10% lower than the case with a cold-gas cleanup train, while the total plant costs remain relatively the same for all plant configurations. In addition to this, the former has a considerably higher overall system energy efficiency of 63%, roughly 18% more than the latter, considering the co-production of FT crude, district heating, and electricity. The specific investment costs range from 1.5 to 1.7 M euro 2018/MWLHV, and the specific FT crude production cost ranges from 120 to 147 euro 2018/MWhFT. Roughly 60% of total carbon fed to the process is captured, enabling net-negative CO2 emissions. A CO2 price for negative emissions would significantly reduce the specific fuel production costs and would, hence, be competitive with fossil-based liquid fuels