Patent Application
20240278174
The present invention relates to the use of hydroxide ions as a heat source in a CO2 absorption process. Specifically, the invention relates to the use of hydroxide ions which have been generated via a process using energy which has not produced CO2 emissions.
The increasingly serious climate situation caused by large CO2 emissions from fossil fuel-derived energy production has become a critical societal challenge and has caused serious public concern in recent years. As a result, innovative solutions for CO2 capture techniques are constantly sought and widely studied.
There are various technologies for reducing CO2 emissions from small and/or mobile sources (e.g. waste incineration, boilers, industrial turbines, industrial engines, cars, trains, trucks, ships, ferries). Such technologies include the use of: energy carriers that do not produce CO2 during energy conversion, such as H2, NH3 and electricity; battery-combustion hybrids; biofuels; synthetic fuels, including hydrocarbons; and various post-combustion CO2 capture technologies, including membranes and compact/modularized amine-based post-combustion, rotating absorption/desorption techniques.
However, such existing technologies have significant disadvantages. All existing technologies for reducing CO2 emissions from small and/or mobile sources are significantly more expensive than using current hydrocarbon fuels in conventional combustion engines. The use of synthetic fuels, H2, NH3 and electricity instead of hydrocarbons are not direct CO2 emission reduction technologies, but only reduce emissions if renewable or carbon capture and storage (CCS) is used for producing them. It is expensive to keep their lifecycle CO2 emissions very low. The use of electricity requires batteries that are heavy, low capacity/range, and use scarce metals. Battery manufacture can have significant lifecycle CO2 emissions. H2/NH3 fuel cells are heavy and use scarce metals. H2 and NH3 have demanding safety issues and are not easily storable in contrast to petrol and diesel. The use of battery-combustion hybrids only partly reduces CO2 emissions. Biofuels require large land areas for growing the necessary crops. The cheapest fast-growing biofuel crops compete with food, and non-food biofuels are expensive and/or slow growing. Upgrading the biomass to petrol/diesel is energy intensive, has low efficiency, and still results in significant life cycle CO2 emissions. Current technologies for CO2 capture from mobile/small sources are relatively heavy and large, and require competent personnel for operating and maintaining. They also have complex emission and discharge issues. Compression/liquefaction/storage of small volumes of CO2 is relatively expensive, and has safety issues due to the high pressure under which CO2 is stored.
CO2 capture by decoupled absorption and desorption is a possible technology for CO2 emissions reduction from smaller and mobile sources. This technology employs equipment at the CO2 emission source which is very simple, compact and requires little competence. Furthermore, there are no complex emission/discharge issues and the chemistry involved is often straightforward. It can reduce the threshold for small low manning and competence industries with little space to consider CO2 capture. Such systems can provide simultaneous:
The present inventors have unexpectedly found that hydroxide ions may be employed as a heat source in such processes as well as more generally in any CO2 absorption process. Surprisingly, the level of heat generated through the use of hydroxide ions is significant and of commercial and economic relevance.
In particular, in a decoupled absorption and desorption process, it is possible to convert renewable energy from the desorption site first into chemical energy in the form of OH− and subsequently release it in the form of heat by the exothermic reaction of OH− with CO2 in the absorption site. In this way, renewable energy may be transferred from the desorption site to the absorption site, where it can have a larger value. This is especially useful for CO2 emission sites that already produce low grade heat, like district heating plants. By using hydroxide ions as a heat source, these sites can either increase heat production or decrease fuel demand. In a truck or ship this heat could be used for heating of cargo, driver cabin or deck offices/rooms.
In a first aspect, the invention provides the use of hydroxide ions as a heat source in a CO2 absorption process.
In a particularly preferred embodiment, said CO2 absorption process is part of a decoupled CO2 absorption and desorption system.
In a further preferred embodiment, said hydroxide ions are ions which have been generated via a process using energy which has not produced CO2 emissions
The present invention describes the use of hydroxide ions as a heat source in a CO2 absorption process.
The hydroxide ions are typically employed in aqueous solution (e.g. water) together with one or more cations. Suitable cations are any wherein the corresponding hydroxide compound is soluble in aqueous solution. By “soluble” in this context we mean a solubility in aqueous solution which is high enough to enable the formation of a homogenous solution. The solubility of the hydroxide compound in water may be at least 1 g/L at ambient temperature (e.g. 18 to 30° C.) and pressure (RTP), preferably at least 2 g/L, more preferably at least 5 g/L. Examples of such cations include potassium (K+), sodium (Na+) and lithium (Li+).
In a preferable embodiment, the hydroxide ions are used at a concentration up to 30 mol %, such as 2 to 30 mol %, more preferably 5 to 20 mol %.
The use of hydroxide ions as a heat source unexpectedly leads to a significant increase in the heat produced from the CO2 absorption process. The stored chemical energy in the hydroxide ions is converted and released as thermal energy during the CO2 absorption process. In one aspect of the invention, the heat produced is increased by up to 50%, relative to an identical heat production process wherein hydroxide ions are absent. This degree of heat generation is unprecedented.
Ideally, the hydroxide ions are ones which have been generated via a process using energy which has not produced CO2 emissions, most preferably renewable energy. In this context, the hydroxide ions can be considered to function as a renewable heat source.
The CO2 absorption process in which the hydroxide ions are employed may be any suitable process, but is typically one in which the hydroxide ions also participate directly in the absorption process. For example, the absorption process may employ an aqueous hydroxide solution, which is preferably easy to handle, non-volatile, non-degradable, with minimal or no harm to the environment. This solution (which may also be termed the “sorbent” or “lean solvent”) absorbs the CO2, resulting in the formation of carbonate salts and water. These absorption processes will be well known to those skilled in the art.
The CO2 absorption process may take place in any suitable apparatus, such as apparatus comprising an absorption column or a membrane contactor. The column may be any suitable column known in the art such as a packed column, a tray column, a falling-film column, a bubble column, a spray tower, a gas-liquid agitated vessel, a plate column, a rotating disc contactor or a Venturi tube. The process may also be carried out in a conventional mixer, for example in a co-current or a counter-current mixer.
Unfortunately some sorbents employed in CO2 absorption processes do not have favourable kinetics, and especially in mobile sources there is not much space available. In view of this problem, the CO2 absorption may take place in an enhanced mass transfer device since this is the lightest and smallest option. The enhancement can be done using e.g. rotation, sound, spray, electricity, catalysis, membrane(s) or enzymes. If space and weight constraints are severe, the absorption preferably does not rely only on gravity (falling liquid over a packing) and/or pressure drop (adsorption on solids) alone for bringing CO2 in contact with the sorbent, because this technique may not capture enough CO2 and will likely need enhanced mass transfer.
The CO2 absorption process preferably occurs at as high a temperature as possible, but ideally below 100° C., to keep the water balance in control and avoid expensive cooling and excessive water production from the flue gas. Between 40° C. and 80° C. is preferred.
The CO2 absorption process is preferably part of a decoupled CO2 absorption and desorption system. A decoupled CO2 absorption and desorption system is employed in CO2 capture processes and involves the use of separate absorption and desorption units. The absorption unit is located at the source of CO2 to be captured and the desorption unit is located elsewhere.
CO2 absorption typically occurs as described above. The sorbent, together with absorbed CO2 (termed herein the “rich solvent”) is then transported to the desorption unit
Desorption is typically performed at a stationary site where cheap energy is available from low CO2 producing sources, preferably renewable energy sources. Such low CO2 sources include electricity from wind/solar/CCS/nuclear. The CO2 desorption is preferably done by electrolysis of the rich solvent, preferably combined with H2O electrolysis producing useful side-products O2 and H2. As an alternative or supplement to electricity, heat can be used for the desorption. The CO2 which has been desorbed may then be transported for storage. The desorption site also regenerates the lean solvent (i.e. the aqueous hydroxide solvent) which can be recycled back for use as a heat source at the absorption site.
The CO2 capture system may also serve as a NOx and SOx reduction measure, removing the need for conventional technologies, especially in diesel engines, reducing costs.
An example of a decoupled CO2 absorption and desorption system is described below, with reference to
CO2 is captured from the exhaust of the source apparatus (e.g. vehicle or apparatus) using a compact, lightweight absorption unit. The absorption may be performed using a lean solvent such as KOH and/or NaOH, which is stored in a first tank on the source apparatus, and the resulting rich solvent is stored in a second tank on the source apparatus. The rich solvent may then be transported from the source apparatus, and the CO2 is desorbed at a stationary facility decoupled from the absorber. Performing only CO2 capture on the source apparatus (and not the CO2 desorption) reduces the complexity and weight of the mobile apparatus, and performing desorption at a stationary site where cheap, low-CO2 energy is available further increases the efficiency of the process. An example of this decoupled CO2 absorption and desorption system is shown in
The absorption only is performed at the small and/or mobile CO2 source, resulting in removal of the CO2 from a hydrocarbon-based energy conversion unit, without requiring the conventional energy-intensive desorption and CO2 compression/liquefaction steps at the small/mobile source. This significantly reduces the complexity, size and weight of the apparatus required at the source. Rotating absorbers, membrane contactors or other enhanced mass transfer technologies can reduce the equipment size even more. The CO2 source needs only an absorber, a tank for fresh solvent and a tank for spent solvent. The solvent can be a solid, slurry or liquid. The solvent can optionally be diluted before use and concentrated after use for reducing the stored and transported volumes (e.g. with reverse osmosis).
The supply/storage system transports lean and rich solvent to and from the absorber site and the desorber site. The system typically includes solvent tanks at both the absorber site and the desorber site. If harmless solvents are chosen, these tanks can be atmospheric, simple and cheap. Transport of the solvent can be done by pipelines, sewers, trucks, trains and ships, depending on what is possible and cost efficient. Optionally, if trucks, trains or ships are chosen, these can also have their own small CO2 absorber on board, reducing the overall chain emissions.
The sorbent used for CO2 capture is ideally an aqueous hydroxide solution which is easy to handle, non-volatile, non-degradable, with minimal or no harm to the environment. Preferably, it will have high pH, i.e. with high OH— forming content and rapid reaction with CO2. Candidates are KOH and/or NaOH that form bicarbonate/carbonates with CO2. Lithium may also be possible instead of sodium and potassium. Their mixtures may optimize the performance. Optionally, promotors (like enzymes and amines) may be added if they do not increase the absorber complexity and emission/degradation risk significantly.
Desorption is performed at a stationary site where cheap energy is available from low CO2 sources. Such low CO2 sources include electricity from wind/solar/CCS/nuclear. The CO2 desorption is preferably done by electrolysis of the rich solvent, preferably combined with H2O electrolysis producing useful products O2 and H2. There can be synergy with conventional H2O electrolysis using similar equipment. As an alternative or supplement to electricity, heat can be used for the desorption. This heat can come from gas turbines, boilers, hydrogen/synthesis gas/NH3 production or (bio-)refineries with CCS. For such heat use, a solvent is needed that desorbs the CO2 and produces hydroxide upon heating. K2CO3, Na2CO3, KHCO3 and NaHCO3 do not desorb the all of the CO2, and do not produce KOH/NaOH easily under cheap low temperature heating (<150° C.). So, transferring the CO2 to another cation may be an option if such cheap heat is to be used. Alternatively, higher heat, and/or more expensive heat may be used. The spent solvent is optionally pressurized prior to desorption for reducing CO2 compression costs.
A more detailed version of this system is shown in
The following simulation data has been obtained to demonstrate the invention.
The system is shown in
The Table below gives the constant input parameters
The inlet temperature is on purpose chosen high and comes straight and unsaturated from the process. No pressure differences are modelled. The exhaust is not pre-cooled as done in conventional post-combustion capture, which saves equipment and CAPEX. The absorber is therefore also an evaporator. The heat is taken out from the cleaned exhaust, which is saturated with water. So, the cooler after the absorber/evaporator becomes a condenser with a much higher heat transfer coefficient than a similar cooler in the unsaturated CO2-rich exhaust. So, it is smaller and has lower CAPEX. Moreover, the cleaned exhaust also contains the exothermic heat of reaction of CO2 and OH− to carbonate/bicarbonate. So, there is also more heat to extract. Most of the low grade heat product is extracted from this condenser. But there is also some low grade extraction from the rich caustic cooler.
The detailed design of this absorber/evaporator and condenser can consist of one or more units. It could be one unit with all functions integrated, or multiple units each one performing one (partial) function. Important for the design is which NaOH concentration is optimal, and how possible precipitation can be handled and controlled. Inspiration can be obtained from SOx removal from flue gasses (FGD—flue gas desulphurization) and various drying technologies.
The following 2 cases were simulated in the modelling tool with different NaOH concentrations in the lean solvent entering the absorber evaporator:
One shortcoming of the modelling tool is that is does not have precipitation of (bi)carbonate included in the model. So, only cases without precipitation could be modelled as an example, while the invention optionally includes precipitation. The first case with 9 mole % NaOH in the lean solvent is realistic, because the concentrations are low enough for avoiding precipitation.
The results are given below
The following observations, conclusions and recommendations can be made:
| Number | Date | Country | Kind |
|---|---|---|---|
| 2108256.5 | Jun 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NO2022/050132 | 6/9/2022 | WO |
