USE OF HYDROXIDE IONS AS A HEAT SOURCE

Abstract

The invention provides the use of hydroxide ions as a heat source in a CO.sub.2 absorption process.

Claims

1. Use of hydroxide ions as a heat source in a CO.sub.2 absorption process.

2. The use as claimed in claim 1, wherein the hydroxide ions are in aqueous solution together with one or more cations.

3. The use as claimed in claim 2, wherein the one or more cations are selected from the group consisting of potassium (K.sup.+), sodium (Na.sup.+) and lithium (Li.sup.+).

4. The use as claimed in any of claims 1 to 3, wherein the concentration of hydroxide ions is up to 30 mol %, preferably 2 to 30 mol %, more preferably 5 to 20 mol %.

5. The use as claimed in any of claims 1 to 4, wherein said process produces up to 50% more heat, relative to an identical process wherein hydroxide ions are absent.

6. The use as claimed in any of claims 1 to 5, wherein said hydroxide ions have been generated via a process using energy which has not produced CO.sub.2 emissions, preferably renewable energy.

7. The use as claimed in any of claims 1 to 6, wherein said process employs an aqueous hydroxide solution as a sorbent.

8. The use as claimed in any of claims 1 to 7, wherein said CO.sub.2 absorption process occurs at a temperature below 100? C., preferably 40? C. to 80? C.

9. The use as claimed in any of claims 1 to 8, wherein the CO.sub.2 absorption process is part of a decoupled CO.sub.2 absorption and desorption system.

10. The use as claimed in claim 9, wherein said desorption uses low CO.sub.2 producing energy sources, preferably renewable energy sources.

11. The use as claimed in claim 9 or 10, wherein the desorption generates hydroxide ions, which are recycled back to the absorption process.

Description

DESCRIPTION OF FIGURES

[0039] FIG. 1: Example of a decoupled CO.sub.2 absorption and desorption system

[0040] FIG. 2: Example of a decoupled CO.sub.2 absorption and desorption system for a waste incinerator using KOH as solvent

[0041] FIG. 3: Simplified process flow scheme with main modelling results

EXAMPLES

[0042] The following simulation data has been obtained to demonstrate the invention.

Method and System

[0043] The system is shown in FIG. 3, which is a simplified process flow scheme with main modelling results. A modelling tool was used with good enough thermodynamic packages for salts that are relevant for NaOH based CO.sub.2 capture (mainly carbonate and bicarbonate). The exhaust is chosen to be the exhaust from a typical steam reformer.

[0044] The Table below gives the constant input parameters

TABLE-US-00001 Flue gas flow (STDm.sup.3/hr) 194000 Flue gas flow (tonne/hr) 221 Flue gas temperature (? C.) 200 CO.sub.2 mole fraction in flue gas 8.57 Lean solvent inlet temperature (? C.) 30 Rich solvent outlet temperature (? C.) 40 CO.sub.2 outlet mole fraction 0.5 Storage volume for period between discharge 7 of (bi)carbonate and OH.sup.? refill (days) Cold water inlet temperature (? C.) 30 CO.sub.2 captured (tonns/yr) 257820 Capture rate (%) 95 Number of stages in absorber/evaporator 5

[0045] 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 CO.sub.2-rich exhaust. So, it is smaller and has lower CAPEX. Moreover, the cleaned exhaust also contains the exothermic heat of reaction of CO.sub.2 and OH.sup.? 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.

[0046] 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 SO.sub.x removal from flue gasses (FGDflue gas desulphurization) and various drying technologies.

[0047] The following 2 cases were simulated in the modelling tool with different NaOH concentrations in the lean solvent entering the absorber evaporator: [0048] Realistic conservative with 9 mole % NaOH in lean solvent (see FIG. 3) [0049] No capture, only evaporation of water with 0% NaOH in lean solvent, rest same as Realistic conservative. By comparing this case with the Realistic conservative it is possible to estimate the contribution due to capture.

[0050] 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.

Results and Conclusions

[0051] The results are given below

TABLE-US-00002 Realistic No capture NaOH mole in lean solvent % 9 0 Lean solvent flow (tonne/hr) 211 13.7 Lean solvent flow (STDm3/hr) 190 13.7 Lean solvent storage (m3) 31920 NA Temperature out of absorber (? C.) 75.1 69.5 Condenser duty (MW) 28.6 24.0 Cond hot water temperature (? C.) 65.2 59.5 Cleaned exhaust temperature (? C.) 49.5 49.1 Slightly acid water production (tonne/hr) 40.5 33.9 Rich solvent cooler duty (MW) 7.21 0 Cooler hot water temperature (? C.) 61.9 NA Rich solvent flow (tonne/hr) 221.7 0 Rich solvent flow (STDm3/hr) 208.2 NA Rich HCO.sub.3.sup.? concentration (mole %) 3.73 NA Rich CO.sub.3.sup.2? concentration (mole %) 2.67 NA Rich solvent storage (m3) 34977 NA Sum of heat production (MW) 35.81 24 Total storage volume (m3) 66897 NA Storage per tonne captured CO.sub.2 12.36 NA (independent of discharge/refill interval)

[0052] The following observations, conclusions and recommendations can be made: [0053] The Realisticno precipitation case extracts 35 MW low grade heat, while the no capture case only 24 MW. So, the addition of CO.sub.2 capture to the evaporation increases the heat production with 49%, which comes ultimately from renewable sources. This is evidence of a significant upside of this technology. [0054] For scaling up and down one can use the storage per tonne captured CO.sub.2, which is independent of discharge/refill interval. In the realistic case it is around 12 m.sup.3/tonne CO.sub.2. It is expected to decrease with higher NaOH concentration with more precipitation. This value will always be above 2, also if pure or extremely concentrated NaOH (above 90%) is used. The reason is that two tanks are needed and the molar mass of CO.sub.2 is 44 and of NaOH 39 and 1 mole NaOH reacts with one mole CO.sub.2 and needs water. Extremely concentrated NaOH solutions are not likely to be used since they are probably very viscous hindering mass transfer and there will be not enough water for the evaporation and reactions. [0055] The temperature of the produced warm water is modest. This is not high enough for all heating applications and district heating networks (e.g. Trondheim has up to 120? C.), but it can work for some. Alternatively, the heating in the CO.sub.2 capture/evaporator can be used as a pre-heating step. The water can be heated more in a heat recovery system in the exhaust prior to the CO.sub.2 capture/evaporator unit. [0056] The water balance is not closed, but this is highly dependent on inlet and outlet temperatures. These temperatures can be regulated to a certain degree. So, this technology can both produce water and be a water consumer. [0057] The heat from the rich caustic cooler can be used to pre-heat the lean solvent. In most cases this will be a useful heat integration and save CAPEX, but does only move heat production from one heat exchanger to another. The overall conclusions will be the same.