A novel approach for treatment of CO2 from fossil fired power plants,Part A: The integrated systems ITRPP

The environmental issues, due to the global warming caused by the rising concentration of greenhouse gases in the atmosphere, require new strategies aimed to increase power plants efficiencies and to reduce CO₂ emissions.This two-paper work focuses on a different approach for capture and reduction of CO₂ from flue gases of fossil fired power plant, with respect to conventional post-combustion technologies. This approach consists of flue gases utilization as co-reactants in a catalytic process, the tri-reforming process, to generate a synthesis gas suitable in chemical and energy industries (methanol, DME, etc.). In fact, the further conversion of syngas to a transportation fuel, such as methanol, is an attractive solution to introduce near zero-emission technologies (i.e. fuel cells) in vehicular applications.In this Part A, integrated systems for co-generation of electrical power and synthesis gas useful for methanol production have been defined and their performance has been investigated considering different flue gases compositions. In Part B, in order to verify the environmental advantages and energy suitability of these systems, their comparison with conventional technology for methanol production is carried out.The integrated systems (ITRPP, Integrated Tri-Reforming Power Plant) consist of a power island, based on a thermal power plant, and a methane tri-reforming island in which the power plants' exhausts react with methane to produce a synthesis gas used for methanol synthesis. As power island, a steam turbine power plant fuelled with coal and a gas turbine combined cycle fuelled with natural gas have been considered.The energy and environmental analysis of ITRPP systems (ITRPP-SC and ITRPP-CC) has been carried out by using thermochemical and thermodynamic models which have allowed to calculate the syngas composition, to define the energy and mass balances and to estimate the CO₂ emissions for each ITRPP configuration.The repowering of the base power plants (steam turbine power plant and gas turbine combine cycle) is very high because of the large amount of steam produced in the tri-reforming island (in the ITRPP-SC is about of 64%, while in the ITRPP-CC is about of 105%).The reduction in the CO₂ emissions has been estimated in 83% (15.4 vs. 93.4 kg/GJ_Fuelinput) and 84% (8.9 vs. 56.2 kg/GJ_Fuelinput) for the ITRPP-SC and ITRPP-CC respectively.     

O2, H2O or CO2 side-feeding policy in methane tri-reforming reactor: The role of influencing parameters

Methane tri-reforming is an efficient route to produce syngas. Distributing one component through a micro-porous membrane, namely side-feeding procedure, is an effective method for controlling reactions pathway and achieving the higher performance in membrane reactors. More recently, Alipour-Dehkordi and Khademi (2019) suggested a feasible and beneficial membrane multi-tubular reactor with O₂, H₂O or CO₂ side-feeding policy to describe the methane tri-reforming for producing a suitable syngas for the methanol and dimethyl ether direct synthesis processes. To complete the previous research, a theoretical study was presented to detect the role of effective parameters, including molar flow rate of feed components, membrane thickness, shell-side pressure, and inlet gas temperature on the H₂/CO ratio, CH₄ conversion, H₂ yield, and CO₂ conversion. Several results were observed, however one of the most attractive results was to achieve CO₂ conversion up to 40% in these configurations by controlling the influencing parameters (compared to CO₂ conversion in the conventional tri-reformer (i.e., 11.5%)); that would be favorable for the environment.     

Process simulation and economic feasibility assessment of the methanol production via tri-reforming using experimental kinetic equations

The purpose of this paper is to assess via techno-economic metrics the feasibility of a tri-reforming coupled methanol process. The simulation of the tri-reforming reactor considered empiric kinetic equations, developed by our group in previous studies. The flue gas coming from the furnace that provides the energy required by the reforming reactor was also used as feed, in order to reduce the CO₂ emissions. A sensitivity analysis was carried out to determine the influence of the feed composition and temperature in the tri-reforming process results, studying H₂O/CH₄ and O₂/CH₄ ratios (0.5–1.5 and 0.35–0.40, respectively), and varying the temperature between 850 and 1050 °C. The methanol plant was also simulated, and an economical study was carried out to know if the proposed process would be economically feasible. The most relevant economic parameters (including the Net Present Value, the Internal Rate of Return, the Payback Period and the break-even) were calculated, showing a quite robust process from an economical point of view.     

Optimal design of methane tri-reforming reactor to produce proper syngas for Fischer-Tropsch and methanol synthesis processes: A comparative analysis between different side-feeding strategies

Assessment of the recent research on the side-feeding strategy in the methane tri-reforming reactor, suggests that this procedure can be a beneficial method for producing syngas. In the present study, special attention is given to the length of methane tri-reformer due to its significant effect on the residence time of distributed components, reaction pathways, synthesis gas production, and reactor performance in side-feeding procedures. The optimal design of three types of membrane tri-reforming reactor, containing O-MTR, H-MTR, and C-MTR, in which O₂, H₂O, and CO₂ permeate as the distributed reactants through the micro-porous membrane, respectively, as well as the conventional tri-reformer (MTR) was carried out to produce proper syngas for methanol and gas-to-liquid (GTL) units. The results show that the O-MTR offers the most advantages in terms of CH₄ conversion (i.e., 99.98%), H₂ yield (i.e., 1.91), and catalyst lifetime due to no formation of hot spot temperature. Additionally, the CH₄ conversion and H₂ yield in the O-MTR increased by 5% compared to the MTR. However, the length of these reactor structures to produce appropriate syngas for Fischer-Tropsch and methanol synthesis processes was in the following order: MTR < C-MTR $?$ O-MTR < H-MTR.     

Process simulation and optimization of methanol production coupled to tri-reforming process

Tri-reforming, as a new approach for the treatment of CO₂ in flue stack gases, has been studied in this work. To determine the optimum operating conditions for the production of syngas with target ratio and maximum CO₂ conversion, the effects of temperature (400–1200 °C), CH₄/Flue gas ratio (0.4–1.0) and pressure (1–5 atm), on the compositions of syngas were investigated. Also, the methanol production from syngas has been rigorously simulated. An optimum heat exchange network was obtained with the objective of minimizing both utility and capital costs, which were calculated by General Algebraic Modeling system (GAMS). Furthermore, an economic analysis was carried out to substantiate the potential profits based on the conceptual results from heat integration. Results showed that the tri-reforming process, when integrated with methanol synthesis, is an economical approach for the treatment and utilization of CO₂ in flue gases.     

Hydrogen production and CO2 fixation by flue-gas treatment using methane tri-reforming or coke/coal gasification combined with lime carbonation

The production of hydrogen and the fixation of CO₂ can be achieved by treatment of flue gases derived from fossil fuel fired power plants via catalytic methane tri-reforming or by coal gasification in the presence of CaO. A two-step process is designed to be carried out in two reactors: a) a catalytic gasifier or steam-reformer, operating exothermally at 900–1000 K, with inputs of the flue gas, a carbonaceous source, steam and air, as well as CaO from the calciner, and outputs of H₂, and of “spent” CaCO₃ to the calciner; b) a calciner, operating endothermally at 1100–1300 K, with inputs of spent CaCO₃ from the gasifier, make-up fresh CaCO₃, and outputs of CO₂, as well as of CaO, partly recycled to the gasifier and partly processed in a cement plant. Thermochemical equilibrium calculations along with mass/energy balances indicate that for flue-gas treatment by tri-reforming, CO₂ emission avoidance of up to ∼59% and fossil fuel savings of up to ∼75% may be attained when concentrated solar energy is supplied as high-temperature process heat for the calcination step, all relative to conventional H₂ production by coal gasification. If instead fossil fuel would be used to drive the calcination step, the CO₂ emission avoidance and the fuel savings would be only 20% and 67%, respectively. Estimated annual H₂ production from a coal-fired 500 MWe burner by the proposed flue-gas treatment using either CH₄-tri-reforming or coal gasification would amount to 0.7 × 10⁶ or 0.6 × 10⁶ metric tons H₂, respectively. Estimated fossil fuel consumption for H₂ production by tri-reforming or coke gasification would be 149 or 143 GJ fuel/ton H₂.     

A novel approach for treatment of CO2 from fossil fired power plants. Part B: The energy suitability of integrated tri-reforming power plants (ITRPPs) for methanol production

In Part A of this two-paper work, a novel approach for treatment of CO₂ from fossil fired power plants was studied. This approach consists of flue gases utilization as co-reactants in a catalytic process, the tri-reforming process, to generate a synthesis gas suitable in chemical industries for production of chemicals (methanol, DME, ammonia and urea, etc.). In particular, the further conversion of syngas to a transportation fuel, such as methanol, is an attractive solution to introduce near zero-emission technologies (i.e. fuel cells) in vehicular applications. In fact, the methanol can be used in DMFC (Direct Methanol Fuel Cell) or as fuel for on-board reforming to produce hydrogen for PEMFC (Proton Exchange Membrane Fuel Cell).     

Assessment of CO2 capture options from various points in steam methane reforming for hydrogen production

Steam methane reforming (SMR) is currently the main hydrogen production process in industry, but it has high emissions of CO₂, at almost 7 kg CO₂/kg H₂ on average, and is responsible for about 3% of global industrial sector CO₂ emissions. Here, the results are reported of an investigation of the effect of steam-to-carbon ratio (S/C) on CO₂ capture criteria from various locations in the process, i.e. synthesis gas stream (location 1), pressure swing adsorber (PSA) tail gas (location 2), and furnace flue gases (location 3). The CO₂ capture criteria considered in this study are CO₂ partial pressure, CO₂ concentration, and CO₂ mass ratio compared to the final exhaust stream, which is furnace flue gases. The CO₂ capture number (N_cc) is proposed as measure of capture favourability, defined as the product of the three above capture criteria. A weighting of unity is used for each criterion. The best S/C ratio, in terms of providing better capture option, is determined. CO₂ removal from synthesis gas after the shift unit is found to be the best location for CO₂ capture due to its high partial pressure of CO₂. However, furnace flue gases, containing almost 50% of the CO₂ in produced in the process, are of great significance environmentally. Consequently, the effects of oxygen enrichment of the furnace feed are investigated, and it is found that this measure improves the CO₂ capture conditions for lower S/C ratios. Consequently, for an S/C ratio of 2.5, CO₂ capture from a flue gas stream is competitive with two other locations provided higher weighting factors are considered for the full presence of CO₂ in the flue gases stream. Considering carbon removal from flue gases, the ratio of hydrogen production rate and N_cc increases with rising reformer temperature.     

Tri-reforming of methane over Ni@SiO2 catalyst

A nickel-silica core@shell catalyst was applied for a methane tri-reforming process in a fixed-bed reactor. To determine the optimal condition of the tri-reforming process for production of syngas appropriate for methanol synthesis the effect of reaction temperature (550–750 °C), CH₄:H₂O molar ratio (1:0–3.0) and CH₄:O₂ molar ratio (1:0–0.5) in the feedstock was investigated. CH₄ conversion rate and H₂/CO ratio in the produced syngas were influenced by the feedstock composition. Increasing the amount of steam above the proportion of CH₄:H₂O 1:0.5 reduced the H₂:CO molar ratio in produced syngas to ∼1.5. Increasing oxygen partial pressure improved methane conversion to 90% at 750 °C. At low ∼550 °C reaction temperature the tri-reforming process was not effective with low hydrogen production (H₂ yield ∼20%) and very low <5% CO₂ conversion. Increasing reaction temperature increased hydrogen yield to ∼85% at 750 °C. From all the tested reaction conditions the optimal for tri-reforming over the 11%Ni@SiO₂ catalyst was: feed composition with molar ratio CH₄:CO₂:H₂O:O₂:He 1:0.5:0.5:0.1:0.4 at T = 750 °C. The results were explained in the context of characterisation of the catalysts used. The obtained results showed that the tri-reforming process can be applied for production of syngas with composition suitable for methanol synthesis.     

Modeling and simulation of an integrated power-to-methanol approach via high temperature electrolysis and partial oxy-combustion technology

The Power to methanol (PtMeOH) approach based on water electrolysis and CO₂ capture is studied. The novelties are the integration of solid oxide electrolysis, partial oxy-combustion capture technology and methanol synthesis process, and the development and validation with experimental data of a SOEC system model using Aspen Custom Modeler. Simultaneously, CO₂ capture bench-scale unit and methanol synthesis process have been modeled and experimentally validated using Aspen Plus. These three systems have been thermally integrated in a final model to assess its high-performance operation. As a result, a clean, synthetic methanol is produced, which can be used as fuel or energy storage. In this lab-scale, a SOEC system of 1.2 kW and a synthetic flue gas of 7 l/min (40% CO_2, 60% N₂) are considered to obtain a methanol flow of 0.16 kg/h, with an overall efficiency of 29% for the integrated scenario. The SOEC system with an optimized BoP has the highest energy consumption, mainly due to water electrolysis, with 44% of total required energy. The novel power-to-methanol integrated in this work achieves around 20% reduction of the energy penalties estimated for the base case and makes use of oxygen from electrolysis for partial oxy-combustion and the water by-product of methanol synthesis in water electrolysis. The integrated model of the overall process is considered a useful tool for future works focused on further scale-up of the process.     

A CO2 cryogenic capture system for flue gas of an LNG-fired power plant

In the present paper, a CO₂ cryogenic capture for flue gas of an LNG-fired power generation system is proposed, in which LNG cold energy can be fully utilized during the gasification process. First of all, the flue gas is compressed to facilitate the CO₂ solid formation and separation. Sequentially, the CO₂-removed flue gas expands to supply most of the cold energy needed for the cryogenic process. In comparison with traditional CO₂-capture systems in LNG-fired power generation cycle, the new system does not require gasifying excessive amount of LNG. Based on the HYSYS simulation, the CO₂ capture pressure and temperature are investigated as the key parameters to find the appropriate working conditions of the CO₂-capture system. The results show that the system can achieve a 90% CO₂ recovery rate or higher if the flue gas temperature can be lowered to less than ?140 °C.     

Tri-reformer with O2 side-stream distribution for syngas production

Methane tri-reforming combines steam reforming, dry reforming and partial oxidation of methane in a single reactor. The heat generated by the exothermic partial oxidation of methane can be used to supply the energy for the other two endothermic reactions (dry and steam reforming of methane). The thermoneutral condition allows the use of a tri-reformer with a simpler reactor structure since no external heat supply is necessary. Thermodynamic analysis of the thermoneutral reactor was performed using Gibbs free energy minimization approach. Conventional tri-reformers have heat and mass management problems. We developed a novel tri-reformer concept that utilizes proper distribution of O₂ gas to the reactor to address the problems. The optimization of the proposed reactor was performed with the objective function of minimizing total annual cost. Maintaining the peak temperatures by adjusting the O₂ flow rate at the distribution point along the reactor was shown to provide good load flexibility for the change in methane flow rate.     

Capturing CO2 in flue gas from fossil fuel-fired power plants using dry regenerable alkali metal-based sorbent

CO₂ capture and storage (CCS) has received significant attention recently and is recognized as an important option for reducing CO₂ emissions from fossil fuel combustion. A particularly promising option involves the use of dry alkali metal-based sorbents to capture CO₂ from flue gas. Here, alkali metal carbonates are used to capture CO₂ in the presence of H₂O to form either sodium or potassium bicarbonate at temperatures below 100 °C. A moderate temperature swing of 120–200 °C then causes the bicarbonate to decompose and release a mixture of CO₂/H₂O that can be converted into a “sequestration-ready” CO₂ stream by condensing the steam. This process can be readily used for retrofitting existing facilities and easily integrated with new power generation facilities. It is ideally suited for coal-fired power plants incorporating wet flue gas desulfurization, due to the associated cooling and saturation of the flue gas. It is expected to be both cost effective and energy efficient.     

Effect of ionic liquid in Ni/ZrO2 catalysts applied to syngas production by methane tri-reforming

This study investigates the influence of ionic liquid in morphology, acid-base properties, metal dispersion and performance of 5%Ni/ZrO₂ catalysts in the methane tri-reforming reaction. Zirconia was prepared by precipitation and the catalysts by wet impregnation. The ionic liquid modified the acid and basic character of the catalysts and positively influenced the methane tri-reforming reaction efficiency. The reaction was evaluated with synthetic biogas and with stoichiometric feed molar ratio (CH₄: CO₂: H₂O: O₂ = 1:0.5:0.5:0.1 and CH₄: CO₂: H₂O: O₂ = 1:0.33:0.33:0.16). The Ni/ZrO₂ prepared with ionic liquid exhibits promising catalytic activity and stability in methane tri-reforming at 800 °C in 4 h run, without coke formation. An increase in the reaction temperature results in an increase of hydrogen yield and the methane conversion, reaching ∼85% at 850 °C. The presented results demonstrate that the tri-reforming reaction could be used for production of syngas with H₂/CO ratio appropriate for methanol synthesis.     

Optimization of tri-reformer reactor to produce synthesis gas for methanol production using differential evolution (DE) method

This paper presents a study on optimization of a fixed bed tri-reformer reactor (TR). This reactor has been used instead of conventional steam reformer (CSR) and auto thermal reformer (CAR). A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methane conversion, hydrogen production and desired H₂/CO ratio as a synthesis gas for methanol production. A mathematical heterogeneous model has been used to simulate the reactor. The process performance under steady state conditions was analyzed with respect to key operational parameters (inlet temperature, O₂/CH₄, CO₂/CH₄ and steam/CH₄ ratios). The influence of these parameters on gas temperature, methane conversion, hydrogen production and H₂/CO ratio was investigated. Model validation was carried out by comparison of the reforming model results with industrial data of CSR. Differential evolution (DE) method was applied as a powerful method for optimization. Optimum feed temperature and reactant ratios (CH₄/CO₂/H₂O/O₂) are 1100 K and 1/1.3/2.46/0.47 respectively. The optimized TR has enhanced methane conversion by 3.8% relative to industrial reformers in a single reactor. Methane conversion, hydrogen yield and H₂/CO ratio in optimized TR are 97.9%, 1.84 and 1.7 respectively. The optimization results of tri-reformer were compared with the corresponding predictions from process simulation software operated at the same feed conditions.     

New process concepts for CO2 post-combustion capture process integrated with co-production of hydrogen

This work describes a study in advanced post-combustion based on CO₂-capture technologies to be integrated within the Hypogyny concept (electricity generation with co-hydrogen production). Two different Hypogen concepts based on integrating IGCC (Integrated Gasification Combined Cycle) and post-combusting CO₂ capture are proposed and investigated: the first concept, hydrogen production based on syngas shifting with high-pressure CO₂ capture, while the second concept, hydrogen is produced based on membrane separation from syngas.In the first concept, combining a high-pressure and an ambient-pressure CO₂ absorber in one flow sheet and one regeneration column is found to be feasible. However, the advantage of the high CO₂ partial pressure in the high-pressure absorber is more obvious if an advanced solvent like 2-amino-2-methyl-1-propanol (AMP) is used instead of monoethanolamine (MEA) solvent kind.The second concept of using polymeric membrane for hydrogen production is considered feasible and comparing to the first concept, cost competitive with around 10% higher overall capital cost. However, the membrane unit does not achieve high hydrogen purity because the investigated concept is limited to a maximum purity of around 95%. Therefore, hydrogen selective membrane technically requires an extra hydrogen purification step e.g. further membrane separations or a pressure swing adsorption (PSA).In addition to these two concepts, the influence of flue gas circulation, gasifier selection and an advanced solvent based on the sterically hindered amine AMP was investigated. Flue gas circulation (higher CO₂-concentrations) has no influence on the regeneration energy requirements when a high binding-energy solvent like MEA is used. The main benefit is that flue gas circulation results with more compact absorption equipment. For AMP type of solvents flue gas circulation results in a substantial reduction in regeneration energy and the overall cost of CO₂ avoided. 37% reduction in the avoided cost with a flue gas recycle ratio of 45% is achieved using AMP as a solvent comparing to 10% using MEA solvent.These Hypogen strategies appear to be feasible and the overall cost of these concepts is comparable with the conventional post-combustion capture process. However, there is a significant potential for further improvement by applying more developed solvents, processes, and membranes.     

Techno-economic-environmental evaluation of a combined tri and dry reforming of methane for methanol synthesis with a high efficiency CO2 utilization

Production of methanol, as a green energy, from syngas is coming into focus. However, natural gas based methanol plants, which are used steam reforming of methane for syngas production, have a high CO₂ emission resulting in the global warming. In this study, a novel process for methanol synthesis is proposed to reduce CO₂ emission. In this regard, natural gas and flue gas are fed to a parallel-series system with tri and dry reforming of methane for syngas production with the optimized stoichiometric number. Then, the produced syngas is converted to methanol in a reactor. Finally, the produced methanol is purified by two distillation towers. The proposed method is compared to a referenced method in the view of technological, economic and environmental metrics. The techno-economic-environmental analysis of the processes reveals that not only the proposed method, as compared to the referenced one, increases CO₂ conversion from 20.93% to 99.22%, but also it is more economical and environmentally friendly. In addition, the global warming potential of the proposed method is almost 60% lower than that for the referenced method due to the lower CO₂ emission. Therefore, the proposed method can save above MUS$ 8 a year by CO₂ capture.     

Post combustion CO2 capture by carbon fibre monolithic adsorbents

As generation of carbon dioxide (CO₂) greenhouse gas is inherent in the combustion of fossil fuels, effective capture of CO₂ from industrial and commercial operations is viewed as an important strategy which has the potential to achieve a significant reduction in atmospheric CO₂ levels. At present, there are three basic capture methods, i.e. post combustion capture, pre-combustion capture and oxy-fuel combustion. In pre-combustion, the fossil fuel is reacted with air or oxygen and is partially oxidized to form CO and H₂. Then it is reacted with steam to produce a mixture of CO₂ and more H₂. The H₂ can be used as fuel and the carbon dioxide is removed before combustion takes place. Oxy-combustion is when oxygen is used for combustion instead of air, which results in a flue gas that consists mainly of pure CO₂ and is potentially suitable for storage. In post combustion capture, CO₂ is captured from the flue gas obtained after the combustion of fossil fuel. The post combustion capture (PCC) method eliminates the need for substantial modifications to existing combustion processes and facilities; hence, it provides a means for near-term CO₂ capture for new and existing stationary fossil fuel-fired power plants.This paper briefly reviews CO₂ capture methods, classifies existing and emerging post combustion CO₂ capture technologies and compares their features. The paper goes on to investigate relevant studies on carbon fibre composite adsorbents for CO₂ capture, and discusses fabrication parameters of the adsorbents and their CO₂ adsorption performance in detail. The paper then addresses possible future system configurations of this process for commercial applications.Finally, while there are many inherent attractive features of flow-through channelled carbon fibre monolithic adsorbents with very high CO₂ adsorption capabilities, further work is required for them to be fully evaluated for their potential for large scale CO₂ capture from fossil fuel-fired power stations.     

Design and optimization of small-scale methanol production from sour natural gas by integrating reforming with hydrogenation

This paper presents the design and simulation-based optimization of a small-scale sour natural gas to methanol process from the view of maximizing the operating profit during operation. It fully integrates steam reforming and CO/CO₂ hydrogenation technologies, by which CH₄ and CO₂ in feeding gas are efficiently converted into methanol without considering CO/H₂ shift and CO₂ removal. In order to obtain the true performances and potential advantages, a simultaneous multi-variable optimization strategy with multi-start procedure is performed by using built-in sequential quadratic programming algorithm. Besides, four cases studies that correspond to distinct levels of CO₂ content are compared to investigate the effects of gas quality on the techno-economic performances. The optimization results show the proposed process has both economic and environmental benefits as it helps to achieve the valorization and carbon footprint reduction of CO₂-rich natural gas resources. In particular, the feeding gas with 20 mol% CO₂ concentration is beneficial for improving the operating profit of the process.     

Use of a micro-porous membrane multi-tubular fixed-bed reactor for tri-reforming of methane to syngas: CO2, H2O or O2 side-feeding

A one-dimensional heterogeneous model for four configurations of a reactor, three micro-porous membrane reactors with O₂ (O-MMTR), CO₂ (C-MMTR) or H₂O (H-MMTR) side-feeding strategy and one traditional reactor (i.e., multi-tubular fixed-bed reactor (MTR)), was developed to explain tri-reforming of methane to produce syngas. Effect of various side-feeding strategies on reactor performance containing CH₄ and CO₂ conversion, H₂/CO ratio, and H₂ yield was investigated under the same condition and then described by chemical species and temperature profiles. It was found that use of side-feeding strategies could be feasible, beneficial, and flexible in terms of change in membrane thickness and shell-side pressure for syngas production with H₂/CO = 2 which is proper for methanol and Fischer-Tropsch process, and = 1.2 which is suitable for DME direct synthesis. However, the syngas produced by the MTR is only appropriate for the methanol and Fischer-Tropsch synthesis under the base case conditions. Also, the results show that the micro-porous membrane reactors have higher CO₂ conversion, based on the H₂/CO = 1.2; so these strategies are more environmentally friendly compared to the traditional reactor.