Exploiting the potential of chemical looping processes for industrial decarbonization and waste to energy conversion. Process design and experimental evaluations

The impact of anthropogenic activities on the environment is leading to climate changes and exceptional meteorological phenomena all over the world. To address this negative trend, the scientific community agrees that the environmental impact from fossil fuels-based power production must be mitigated by the integration with alternative and sustainable technologies, such as renewable energy. However, the time required for the complete development and diffusion of such technology poses the urgency of finding a midterm solution to significantly reduce CO2 emissions. Carbon capture, utilization, and storage (CCUS) technologies represent an interesting option to mitigate CO2 emissions. CCUS involves (among other possible applications) the separation of the CO2 content from industrial off-gases, its transport and storage or its reconversion to a chemical/fuel. Chemical looping can be considered as an oxyfuel combustion where the oxygen supply comes from the lattice oxygen atoms of a solid. It is based on gas-solid reactions where a solid also known as oxygen carrier, generally a metal oxide, undergoes successive reduction and oxidation steps. In the reduction step, normally occurring at high temperatures (700-1000 °C), the oxygen carrier interacts with a reducing agent, such as coal, natural gas, syngas etc. and loses part of its oxygen atoms. By controlling the degree of reduction of the oxygen carrier is thus possible to achieve a complete oxidation of the reducing agent (the fuel) to CO2 and H2O (chemical looping combustion) or a partial oxidation to a syngas (chemical looping reforming and gasification). In these latter case, the introduction of external CO2 and H2O can be of help to support the reforming or gasification processes. The oxygen carrier in the reduced phase is then sent to an air reactor, where it reacquires the oxygen atoms by an exothermic reaction with air. This process presents several advantages according to the specific application. In chemical looping combustion, intrinsic separation of N2 and CO2 is achieved, because the two streams are involved in two different reaction steps. This largely simplifies the CO2 separation effort for storage or utilization purposes. On the other hand, in chemical looping reforming it is possible to achieve autothermal operation thanks to the exothermicity of the oxidation step in the air reactor, as well as high reforming efficiencies. Similarly, in chemical looping gasification the resulting syngas is characterized by no N2 dilution, lower tar release and possibility of autothermal operation. These benefits enhance the energy efficiency of the process, leading to a better energy utilisation. In this work, strategies for the decarbonisation and circularity of the industrial and power sector are proposed based on the synthesis of hydrogen and hydrogen-derived fuels. In particular, the potential of chemical looping technology is deeply studied aiming at exploiting its ability to reconvert or valorise CO2 or waste streams to a syngas and then to a liquid fuel/chemical, such as methanol or ammonia. This task is carried out through modelling and experimental evaluations. The modelling activities mainly concern design of process schemes involving the chemical looping section for waste or CO2 reconversion and the liquid fuel synthesis section. The experimental evaluations are focused on two crucial that have been limitedly discussed in the literature: the thermochemical syngas production step by oxidation with CO2 and H2O streams, the effect of high-pressure operation on the redox abilities of a typical iron and nickel-based oxygen carrier. In Chapter 1, a general overview on the main research developments on chemical looping technology is provided. A section is reserved for each chemical looping variant, i.e. combustion, reforming and gasification, and a general description of each process is provided along with the summary of the main research achievements. Subsequently, the technology is divided by application in power production and chemicals production. Main findings from techno-economic assessment and process designs are discussed in comparison with benchmark technologies and other clean pathways. In Chapter 2 steel mills are taken as an example of the hard-to-abate industry. A H2-based decarbonization strategy is proposed and assessed by Aspen Plus simulation. The strategy starts from an initial configuration that is characterized by a typical blast furnace-basic oxygen furnace steel mill and consider the introduction of direct reduction – electric arc furnace lines, that are more efficient and involve natural gas as reducing agent rather than coke. Sensitivity analyses are carried out to assess the effect of the introduction of H2/CH4 blendings in the direct reduction plant and of the utilization of scrap material in the electric arc furnace. The impact of each configuration on the CO2 emissions and the energy flows of the plant is assessed by mass and energy balances. The results indicate a promising decarbonization potential of the introduced technologies but require large investments to increase the renewable sources penetration in the energy mix and large availability of H2. Therefore, alternative pathways for an earlier decarbonization of hard-to-abate industries and for large scale syngas/H2 production need to be considered. In Chapter 3, a novel process scheme is proposed involving chemical looping for syngas production. The CO2 content in blast furnace gases is separated with a calcium looping cycle and subsequently injected with H2O into the oxidation reactor of a chemical looping cycle. Assuming an inlet stream of pure CO2, mass balances on the chemical looping plant are carried out to compare the performance of nickel ferrites and iron oxides in terms of required oxygen carrier flow rate to process 1 t/h of CO2. Computational fluid dynamics simulations with integrated reaction kinetics are then carried out to validate the assumptions on the oxygen carrier conversion and syngas compositions. In Chapter 4 and 5, experimental evaluations are carried out on two crucial aspects for the successful operation of a chemical looping plant aiming at syngas production. In Chapter 4, the syngas productivity by CO2 and H2O splitting over a Fe bed is investigated. This is a very important step, and the effect of various parameters was considered. Firstly, the CO2 splitting is analysed for different temperatures with an inlet flow rate of 1 NL/min to ensure a substantial dissociation of the CO2. Subsequently, combined streams of CO2 and H2O are evolved in the reactor. The effect of the total flow rate, reactants molar ratio and bed height is investigated and from the results, the optimal syngas composition is identified. SEM and XRD are used to assess the morphological evolution and the phase changes of the material during the test. On the contrary, in Chapter 5 the effect of high-pressure operation on the redox abilities of two NiFe aluminates is assessed. The aluminates present similar Fe loadings, but different Ni loadings. High pressure operation is crucial for the development of this technology because it facilitates downstream processing of the syngas to liquid fuels. For a comparative analysis, preliminary tests at low pressure are carried out at three temperatures. Subsequently, the effect of reactants flow rate, temperature, total pressure, gas composition is analysed at high pressure conditions. Finally, long term tests are performed both at ambient and high-pressure conditions. Material characterization by SEM, XRD and H2-TPR is used to support the comparative analysis. In Chapter 6, a techno-economic analysis on a process scheme encompassing methanol and ammonia production from chemical looping gases is carried out. Chemical looping hydrogen production is a very versatile technology and allows for the combined production of power and H2 or syngas. With proper calibration of the flow rates, a stream of high purity N2 can also be obtained at the air reactor outlet and used for ammonia synthesis. Back up with an alkaline electrolyser is considered for the supply of the required amount of hydrogen. Sensitivity analyses are carried out on the chemical looping plant to evaluate the effect of fuel flow rate, steam flow rate, and oxygen carrier inlet temperature to the fuel reactor. Subsequently, a techno-economic analysis is carried out evaluating several parameters among which: the specific CO2 emissions, the energy intensity, and the levelized cost of methanol and ammonia. Finally, a comparison with benchmark technologies and other clean alternatives is presented. In this way, the benefits as well as the drawbacks of chemical looping in terms of environmental and economic parameters are assessed and the missing elements to reach industrial competitivity are clarified

Syngas quality in fluidized bed gasification of biomass: comparison between olivine and K-feldspar as bed materials

The relevance of selecting an appropriate bed material in fluidized bed gasification is a crucial aspect that is often underestimated. The ideal material should be economical, resistant to high temperatures and have small chemical interaction with biomass. However, often only the first of such three aspects is considered, neglecting the biomass–bed interaction effects that develop at high temperatures. In this work, olivine and K-feldspar were upscale-tested in a prototype fluidized bed gasifier (FBG) using arboreal biomass (almond shells). The produced syngas in the two different tests was characterized and compared in terms of composition (H2, CH4, CO, CO2, O2) and fate of contaminants such as volatile organic compounds (VOCs), tar and metals.. Moreover, the composition of olivine and K-feldspar before and after the biomass gasification process has been characterized. The aim of this work is to show which advantages and disadvantages there are in choosing the most suitable material and to optimize the biomass gasification process by reducing the undesirable effects, such as heavy metal production, bed agglomeration and tar production, which are harmful when syngas is used in internal combustion engines (ICE). It has been observed that metals, such as Ni, Cu, Zn, Cd, Sn, Ba and Pb, have higher concentrations in the syngas produced by using olivine as bed material rather than K-feldspar. In particular, heavy metals, such as Pb, Cu, Cd, Ni and Zn, show concentrations of 61.06 mg/Nm3, 15.29 mg/Nm3, 17.97 mg/Nm3, 37.29 mg/Nm3 and 116.39 mg/Nm3, respectively, compared to 23.26 mg/Nm3, 11.82 mg/Nm3, 2.76 mg/Nm3, 24.46 mg/Nm3 and 53.07 mg/Nm3 detected with K-feldspar. Moreover, a more hydrogen-rich syngas when using K-feldspar was produced (46% compared to 39% with olivine)

Analysis of the NOx Emissions Deriving From Hydrogen/Air Combustion in a Swirling Non-Premixed Annular Micro-Combustor

The objective of this paper is to analyse the thermal performances and emission levels of a swirling non-premixed annular micro-combustor. A three-dimensional simulation of the internal flow in an annular combustion chamber fuelled by hydrogen/air in non-premixed conditions was carried out through Ansys Fluent 2020 R2. A detailed mechanism involving many species and reactions was required to represent the complexity of the combustion process. A 19-reactions and 9 species mechanism was deemed to be appropriate to predict the combustion characteristics with great accuracy and limited processing resources. The reaction mechanism was implemented in Fluent through Chemkin. This model was then compared with the built-in model of the chemical kinetics, which involves just one reaction. The results showed that the 19-reactions model is more accurate to reproduce the centerline temperature distribution than the default simpler model: the 1-reaction model differs by 230 K compared to the experimental data, while the 19-reactions one only by 80 K. The comparison between the velocity of the mixture inside the combustor with and without combustion was also carried out, providing an estimate of the influence of temperature on the flow field. Finally, to meet the emission limits established by the European Commission in 2015, the thermal NOx and NOx derived by NNH were examined on both the outlet and the centerline of the combustor. The study reveals great potential of hydrogen mixtures in reducing emissions and provides insights on the cooling requirements to guarantee wall materials reliability

Synthesis of methanol from a chemical looping syngas for the decarbonization of the power sector

One promising pathway for carbon capture and utilization is represented by the coupling of chemical looping cycles with liquid fuel synthesis processes. Methanol is an interesting fuel for gas turbines engines, due to its potential reduction of NOX and particulate emissions along with the absence of SO2 emissions. In this work, methanol production from the syngas generated by a three-reactors chemical looping process is investigated by mass and energy balances. The cycle is composed by a reducer reactor, where Fe2O3 is reduced to FeO by the injection of a reducing agent; an oxidizer reactor, where FeO reacts with CO2 and H2O to produce a syngas; an air reactor, where Fe3O4 is regenerated to Fe2O3 by ambient air. The produced syngas is then sent to a methanol synthesis plant. Several syngas compositions deriving from different CO2/(H2O+ CO2) molar fractions (1–3) at the oxidizer inlet are taken into account. The resulting methanol flow rates are almost equal in all investigated configurations (about 0.35 t/h). From an energy standpoint, the required electric power is greater for higher hydrogen mole fractions in the syngas. However, the case with 75% H2 content is characterized by the greatest methanol yield (⁠12.6%⁠), carbon efficiency (⁠23%⁠) and a high feed/recirculation ratio (⁠0.80⁠), thus representing the most indicated configuration among the investigated ones. Finally, by burning methanol in a gas turbine, the total CO2 emissions are halved with respect to the case without the system (if the CO2 associated with biogenic carbon in the reducer reactor is considered as net-zero)

On the reduction of NiFe/Al2O3 oxygen carrier in high-pressure chemical looping applications

15 figures.Chemical looping represents a promising technology with various applications ranging from clean power production to alternative syngas production. In this work, two oxygen carriers with different Ni loadings (4.3% wt. and 12% wt.) and similar Fe loadings (9.9% wt. and 8.5% wt.) are synthesized through a co-precipitation/impregnation route and tested in two thermogravimetric analyzers. Firstly, the effect of temperature (700–900 °C) on the oxygen transport capacity and reduction conversion of both materials is assessed at ambient pressure (0.5 nl/min with 20% H2/N2). The influence of material loading is also studied, and it is shown that higher Ni loadings provide a significant improvement in material activity. A complete reduction conversion is achieved at 900 °C and ambient pressure. At high pressure (10–20 bar), tests are carried out in a temperature range of 700–850 °C. The effect of flow rate (2 nl/min to 6 nl/min with 50% H2/N2) is first assessed to prevent external mass transfer limitations. Higher total pressures have a negative effect on reduction kinetics, while higher Ni loadings demonstrate increased final reduction conversion also at high pressure, reaching about 75% conversion after 20 min. The long-term cyclability of the material is also investigated both at low (100 cycles) and high pressure (80 cycles) conditions and a conversion gain is observed throughout the cycles in both cases. No changes in the material microstructure are observed after 80 high-pressure cycles.This project has received funding from the European Union’s Horizon 2020 Research and Innovation Program under grant agreement No 884197 (GLAMOUR).Peer reviewe