Cleaning of combustion gas including the removal of co2

[0001] The invention relates to exhaust gas ultra-purification systems and methods followed by CO2 capture and recovery.

BACKGROUND [0002] Most of the energy used in the modern world comes from the combustion of fuels containing carbon and hydrogen, such as fossil coal, oil and natural gas. In addition to carbon and hydrogen, these fuels contain oxygen, moisture and impurities such as ash, sulfur, nitrogen compounds, chlorine, mercury and other trace elements. Awareness of the destructive effects caused by pollution released during combustion entails even more stringent emission limits from power plants, refineries and other industrial processes. The operators of such enterprises are under increasing pressure focused on achieving near zero emissions and reducing CO2 emissions.

[0003] The prior art includes various processes and technologies aimed at reducing exhaust emissions. Bag dust stations, electrostatic precipitators and water scrubbers are usually used to collect dust pollution; various chemical processes are used to reduce emissions of sulfur oxides, HCl and HF; combustion reduction and NOx reduction processes are used to reduce NOx emissions; processes are being developed to capture mercury and other trace elements from exhaust gases.

[0004] In the last 20-30 years, significant progress has been made and today's industrial plants are much cleaner and safer for the environment than in the past. However, there is an increasing number of indications that even a low concentration of particulate pollutants, especially very fine particles less than 2.5 micrometres (PM2.5), sulfur oxides, acid mists and mercury, is harmful to human health and must be controlled.

[0005] Controlling residual emissions is still a challenge, and with existing technologies, the cost of capturing the last few percent of harmful pollutants is very high.

[0006] Furthermore, in the last few years, there has been a growing concern about the accumulation of CO2, a greenhouse gas in the atmosphere. The accelerated increase in CO2 concentration in the atmosphere is attributed to the growing use of fuels such as fossil coal, oil and natural gas, resulting in the emission of billions of tonnes of CO2 into the atmosphere every year.

[0007] Reduction of CO2 emissions can be achieved by increasing energy efficiency, switching to fuels containing a lower concentration of carbon, and using CO2-neutral alternative energy sources. However, due to the lack of a major breakthrough in energy, CO2-emitting fuels will remain the main source of energy in the foreseeable future. As a result, there is a demand for a cheap and low-energy process of CO2 capture and sequestration to reverse the global warming trend.

[0008] CO2 capture technologies known in the art are not suitable for use with low CO2 concentration, low pressure and pollution, and oxygen containing exhaust gas. Commercial CO2 capture technologies are energy consuming and expensive. Their use would be a heavy burden due to the cost of energy.

[0009] A suitable currently available CO2 capture process after combustion is an amine process using Mono-Ethanol-Amine (MEA) or similar amines to react with CO2. The MEA process enables high capture efficiency and generation of concentrated CO2 stream for sequestration. However, this process has several disadvantages, including:

• MEA reagent is expensive and decomposes in an oxygen and CO2 environment.

• MEA is corrosive and can only be used in a relatively diluted form.

• The reaction of MEA with CO2 is highly exothermic.

• Recovery is energy consuming.

• The process consumes large amounts of heat and auxiliary energy.

[0010] The cost of the amine process and system is very high and the net energy efficiency of a power plant equipped with an amine system for CO2 capture is severely reduced.

[0011] From GB 899,611 a gas separation process is known. This document describes the delivery of an ammoniacal solution to the absorption column at 20 ° C or higher, and the exhaust gas is supplied to the absorption column at an even higher temperature.

[0012] From US 2,043,109 a process for recovering carbon dioxide from waste gases is known. In this process, the flue gas enters the column at a temperature of about 20-25 ° C and is heated in the column to 40-60 ° C by contact with a potassium carbonate solution.

[0013] To achieve clean combustion of fuels with near zero emissions, including CO2 emissions, a low-cost and low-energy process is needed which:

• captures residual impurities, • captures CO2 and releases it in concentrated and highly compressed form for sequestration.

[0014] Accordingly, the development of new systems and methods to overcome current problems and disadvantages would be considered as progress in the prior art. SUMMARY OF THE INVENTION [0015] The invention is an integrated method and system for efficiently and economically reducing residual emissions such as SO2, SO3, HCl, HF and particulate pollutants including PM2.5 from flue gas, located behind conventional air pollution control systems, to a level near zero. Furthermore, the system of the invention reduces CO2 emissions by capturing it and delivering it in sequestered form under high pressure. The object of the invention is that this process should be relatively uncomplicated,

- 3 used a cheap reagent, did not generate additional waste streams, and most importantly, consumed a small amount of funds and energy.

[0016] The invention is a wet method and system by which saturated exhaust gases after conventional air pollution control devices and systems are cooled to a temperature much lower than their ambient saturation temperature. Cooling is carried out by direct contact with cold water in appropriate vessels. Direct contact between gas and liquid, combined with severe moisture condensation from saturated gas, is a very effective water scrubber. Alternatively, alkaline substances such as sodium or ammonium carbonate may be added to the direct contact cooler to improve the capture of acidic substances from the gas. Direct cooling to low temperature can be achieved during one or more cooling stages. Continuous discharge from the direct contact cooler prevents the accumulation of trapped dirt in the direct contact coolers.

[0017] Cold water will be produced in cooling towers with additional cooling to lower the temperature to a range of 0-20 and even 0-10 degrees Celsius, by efficient mechanical vapor compression, in which the water itself serves as the refrigerant.

[0018] According to the invention, gas cooling significantly reduces the moisture content. Cooled gas with low humidity has a relatively small volume and a relatively high concentration of CO2, thus providing efficient CO2 capture in an easier and cheaper way.

[0019] The invention further relates to the mass transfer and reaction of CO2 gas from flue gas with a CO2-poor ammonia solution, resulting in a CO2-rich ammonia solution. According to the invention, the absorption reaction occurs in a CO2 absorber operating at approximately atmospheric pressure and in a temperature range of 0-20 or even 0-10 degrees Celsius. Low temperature increases the penetration of CO2 mass into the solution, while significantly reducing the vapor pressure of ammonia and preventing its evaporation into the gas stream. One or more CO2 absorption steps may be used depending on the capture capacity required.

[0021] Then, according to the invention, the pressure of the CO2-rich solution from the CO2 absorber is increased by means of a high-pressure pump to a range of 2-138 bar (30-2000 psi) and the solution is heated to a temperature in the range of 100-150 degrees Celsius. Under the above conditions, CO2 is separated from the solution and separated as a relatively pure and high pressure gas stream. The high pressure CO2 gas stream contains a low concentration of ammonia and steam that can be recovered by scrubbing the CO2 gas stream cold.

[0022] The recovery reaction is endothermic. However, the heat of reaction is low and the overall heat consumption of the process is relatively low. In addition, high pressure recovery minimizes the evaporation of ammonia and water, thereby reducing energy consumption in the process. Furthermore, low-quality heat can be used to recover CO2 to further reduce the impact of CO2 capture on the overall efficiency of the plant. The CO2-poor solution used in the absorber for CO2 capture contains the ratio

- 4 molar NH3 / CO2 in the range 1.5-4.0 and preferably in the range 1.5-3.0. The CO2-rich solution sent for recovery contains a NH3 / CO2 molar ratio in the range 1.0-2.0 and preferably in the range 1.0-1.5.

[0023] The invention is characterized by the advantage of efficient and cheap capture of residual pollutants from the exhaust gas, followed by efficient and cheap capture and recovery of CO2. Low-temperature absorption and high-pressure recovery are crucial for the proper operation of the process and system. A simple, cheap and efficient system has a noticeable advantage over other CO2 purification and capture processes and represents a real breakthrough in achieving the near zero emission target.

BRIEF DESCRIPTION OF THE DRAWINGS [0024] The above and other advantages of the invention will become more apparent from the following description combined with the accompanying drawings, among which: FIG. 1 schematically illustrates an integrated system for capturing residual impurities and CO2 from flue gas located behind conventional air pollution control systems. This system includes gas purification, CO2 absorption and CO2 recovery.

[0026] FIG. 2 is a diagram of the gas cooling subsystems and deep cleaning of residual impurities.

[0027] FIG. 3 is a diagram of the subsystems for CO2 capture and recovery. It includes a CO2 absorber operating at low temperature and a CO2 regenerator operating at moderate temperature and high pressure.

DETAILED DESCRIPTION OF THE INVENTION [0028] The invention provides a method and system for removing most contaminants, including CO₂, from gas streams. These gases usually come from the combustion or gasification of fossil coal, liquid fuels, gaseous fuels and organic waste. Contaminants include residues, for example SO₂, SO₃, HCl, HF, CO₂, dust pollution, including PM2.5, mercury and other volatile substances. High removal efficiency is achieved by saturating and effectively cooling the gas to a temperature below its adiabatic saturation temperature, as low as 0-20 or even 0-10 degrees Celsius. Fine particles and acid mist are nucleation sites for water condensation. Therefore, virtually all fine particles and acid mist are removed from the gaseous stream. Low temperature creates an environment of low SO vapor pressure₂, SO₃, HCl, HF, mercury and other volatile substances that also condense into cold water.

[0029] The cooling of the flue gas allows an efficient CO2 capture of a CO2-poor ammonia solution or suspension. CO2 absorption is carried out at low temperatures, even 0-20 degrees Celsius or even 0-10 degrees Celsius. The absorbent is regenerated by increasing the temperature of the solution or suspension to a range of 50-200 degrees Celsius (not according to the invention) or to a range of 100-150 degrees Celsius and pressure to a range of 2-138

- 5 bar (30-2000 psi). Low absorption temperature and high recovery pressure give high CO2 capture efficiency, low energy consumption and low ammonia evaporation loss.

[0030] CO2 absorption takes place in the NH3-CO2-H2O aqueous system, in which the ammonia may be in the form of an ammonium ion, NH4⁺, or dissolved molecular NH3. CO2 can be in the form of carbonate, CO₃^2-, bicarbonate, HCO₃^- or dissolved molecular CO2. The ability of the solution to absorb CO2 and the form in which it occurs depends on the concentration of ammonia, the NH3 / CO2 molar ratio, and temperature and pressure.

[0031] The high NH3 / CO2 molar ratio increases the vapor pressure of ammonia and leads to ammonia losses by evaporation. The low NH3 / CO2 molar ratio increases the vapor pressure of CO2 and reduces its capture capacity. Therefore, the optimal NH3 / CO2 molar ratio for absorption is in the range 1.0-4.0 and preferably in the range 1.5-3.0. High temperatures increase the vapor pressure of both ammonia and CO2. As a result, the absorber should operate at the lowest practical temperature, preferably in the temperature range of 0-20 degrees Celsius or even in the temperature range of 0-10 degrees Celsius.

[0032] At high concentration and at a lower temperature, limits of solubility and solid particles can be precipitated. These solid particles are usually in the form of ammonium carbonate (NH4) 2CO3 at a high NH3 / CO2 ratio and ammonium bicarbonate NH4HCO3 at a low NH3 / CO2 ratio.

[0033] Mass transfer and absorption reactions for concentrated low-temperature suspensions are as follows:

• CO2 (g) CO2 (aq) • CO2 (aq) + H2O --- ^ - H + + HCO3^- • (NH4) 2CO3 (s) --- ^ - 2NH4 + + CO3^2- • H + + CO3^2- HCO3 • HCO3^- + NH4 + --- ^ - NH4HCO3 (s) [0034] Where the CO2 captured from the gas converts ammonium carbonate to ammonium bicarbonate. The above reactions are reversible and CO2 is stripped from the liquid phase at elevated temperature.

[0035] Depending on the operating conditions, undesirable side reactions may occur, such as:

• NH4 + + CO3^2- NH3 (g) + HCO3^- • NH4 + + HCO3^- NH3 (g) + CO2 (g) + H2O [0036] Causing NH3 emissions to the gas phase. Lower temperature and lower ratio

NH3 / CO2 in the absorber inhibits these adverse reactions. However, ammonia gas forms during recovery and at elevated temperatures. To prevent ammonia from escaping from the liquid phase (and for other reasons), the regenerator is designed to operate under

- increased pressure and in conditions in which the solubility of ammonia in the solution is very high and the emission of ammonia gas is very low.

[0037] FIG. 1 is a schematic diagram of an integrated process including gas purification and cooling, CO2 absorption in a CO2-poor ammonia solution, and recovery of CO2 from a CO2-rich solution. Stream 102 is a gas stream from combustion or an industrial process containing residual impurities, CO2 and inert gases. The CO2 concentration in gas is usually 10-15% when burning fossil coal and 3-4% when burning natural gas. Subsystem 130 presents a number of conventional processes for controlling air pollution, which, depending on the gas source, may include dust collectors, NO control. and SO2, acid mist capture device, etc. Impurities collected in the system are removed in stream 112. Stream 104, located behind conventional purifiers, contains residual impurities not collected by conventional systems. It is usually saturated with water and has a temperature in the range of 40-70 degrees Celsius. Subsystem 132 is a series of one or more direct contact coolers (DCCs) in which cold water produced in cooling towers and coolers (not shown) is used to leach gas, capture its residual impurities and reduce moisture content. Stream 114 is a bleed from subsystem 132 designed to remove all trapped residual impurities.

[0038] Stream 106 is a cooled gas suitable for capturing CO2 in the CO2 absorber. Subsystem 134 is a CO2 absorber and may contain a number of absorber levels, depending on the required removal efficiency and plant operating conditions. Pure gas with low CO2 concentration, stream 108, is released into the atmosphere. Stream 124 is a cooled, CO2-poor, ammonia solution from the regenerator subsystem 136, and serves as an absorbent to capture CO2 in the absorber. The resulting stream 120 is a CO2-rich ammonia solution sent for recovery.

[0039] The regenerator, subsystem 136, operates at high pressure and elevated temperature and can be one or a series of recovery reactors. The pressure of the ammonia solution fed to the regenerator is increased by means of a high-pressure pump 138 to obtain stream 122, which is rich in CO2 and at high pressure. Typically, stream pressure 122 is in the range of 4-172 bar (50-2500 psi) and is higher than the regenerator pressure to prevent premature CO2 evaporation. Heat is supplied to the regenerator by means of heating stream 126 in heater 140. High pressure and high temperature in the regenerator cause the release of a high pressure CO2 gas stream, 110. High pressure recovery has significant advantages in terms of economy and energy consumption. Low quality heat energy is used to generate the high pressure CO2 stream, not expensive electricity.

[0040] FIG. 2 is a diagram of cooling and purification subsystems that may optionally include waste heat recovery, a heat exchanger 240, to utilize the residual heat contained in the gas. Residual heat in stream 202

- 7 can be extracted in the heat exchanger 240 by transferring heat to the coolant stream 220 and 222. This heat can then be used to recover CO2 downstream.

[0041] The vessel 242 is a wet direct contact scrubber used for adiabatic cooling and gas saturation. If the gas contains a high concentration of acidic compounds, such as gas from fossil-fuel or oil-fired power plants, the 242 reactor is used to flue gas desulphurization. Acid-absorbing reagent (such as limestone), stream 226, is added to vessel 242, and product such as gypsum, stream 224 is removed. Make-up water, stream 227, is added to vessel 242 from direct contact cooler 244 (DCC). Make-up stream contains all contaminants collected in direct contact coolers. These impurities are removed from the system along with the effluent stream 224. Gas stream 202 for fossil-fueled boilers usually has a temperature in the range of 100-200 degrees Celsius, gas stream 204 usually has a temperature in the range of 80-100 degrees Celsius and gas stream 206 is usually saturated with water and usually has a temperature in the range of 40-70 degrees Celsius.

[0042] The direct contact cooling and purification steps in vessels 244 and 246 are shown in FIG. 2. The actual number of direct contact coolers can be larger and depends on the optimization between capital expenditure, energy efficiency and required cleaning efficiency.

[0043] The gas stream 206 is cooled in DCC 244 to a temperature just above the cooling water temperature of stream 230. The temperature of the cooling water, stream 230, depends on the ambient conditions and the operating and process conditions in cooling tower 250. Cooling tower 250 may be of the wet type temperature slightly lower or slightly higher than the ambient temperature, or dry type with a temperature higher than the ambient temperature. The ambient air, stream 212, provides heat from the system and the heat is waste in stream 214, which absorbs heat from the water stream 228. The resulting cooled water stream 230 usually has a temperature in the range of 25-40 degrees Celsius, and the resulting cooled exhaust stream DCC 244 has a temperature about 1-3 degrees Celsius higher. Basic substances, such as sodium or ammonium carbonate, can be added to DCC 244 to neutralize captured acid compounds. Alkaline substances can be added in make-up water, stream 225.

[0044] Stream 208, cleaner and at a lower temperature, flows to DCC 246, which is similar to DCC 244, except that colder water, stream 234, is used for cooling. Stream 234 is a cooled water stream cooled in refrigerator 248, which preferably it is a device with mechanical vapor compression with water as the refrigerant. The heat from the cooler 248 is discharged as waste via stream 236 to the cooling tower 250 with return flow 238. The cooling water stream 234 can have a temperature of just 0-3 degrees Celsius or higher, which means that the exhaust gas temperature, stream 210, leaving DCC 246 has a temperature of 0-10 degrees Celsius or a few degrees higher. The heat absorbed from the gas stream is removed from DCC 246 by means of a water stream 232. In DCC 246, further condensation and further capture take place.

- 8 impurities. These impurities are tempered from the system to vessel 242. (Tempering stream is not shown).

[0045] Gas stream 210, a cooling and purification subsystem product, shown in FIG. 2, has a low temperature; low moisture content and practically no solid particles, acidic or volatile substances.

[0046] FIG. 3 is a diagram of the CO2 capture and recovery subsystem. Stream 302 is a clean and cooled gas stream similar to stream 210 in FIG. 2. It enters the CO2 absorber 350, where CO2 is absorbed by the chilled ammoniacal solution poor in CO2, stream 324 contains a molar ratio NH3 / CO2 in the range 1.5-4.0 and preferably 1.5-3, 0. Depending on the design of the absorber and the number of absorption steps used, more than 90% of CO2 can be captured from stream 302, resulting in a cold and CO2-depleted gas stream 304. Residues of ammonia in stream 304 can be washed in vessel 356 with cold water or cold and slightly acid solution, stream 338. Stream 338 is cooled in a heat exchanger 368. As a result of cooling, purifying and capturing CO2, the effluent gas stream from the system, stream 306, contains mainly nitrogen, oxygen and low concentrations of CO2 and H2O.

[0047] Stream 324 is a CO2-poor regenerator stream that is cooled in a regenerative heat exchanger 354 and then chilled water in a heat exchanger 362. It captures CO2 in the absorber 350 and is discharged from the absorber as stream 312, a rich CO2 stream with a NH3 / CO2 molar ratio in the range 1.0-2.0 and preferably with a NH3 / CO2 molar ratio in the range 1.0-1.5. In a preferred embodiment, stream 312 contains a high concentration of dissolved and suspended ammonium bicarbonate. Part of stream 312 is optionally recycled to the absorber, while equilibrium stream 314 is compressed in a high pressure pump 360, resulting in a high pressure stream of ammonia solution 316. Stream 316 is heated in a regenerative heat exchanger 354, exchanging heat with a hot and CO2-poor regenerator stream, stream 322, which is part of stream 320 discharged at the bottom of regenerator 352.

[0048] The CO2-rich stream from the regenerative heat exchanger 354, stream 318, can be further heated by waste heat from the boiler or other sources. This stream enters regenerator 352, which has one or more recovery steps. More heat is supplied to the regenerator from the 364 heat exchanger, which heats stream 330. The heat supplied to the system from various sources increases the regenerator temperature to 50-150 degrees Celsius or more, depending on the desired CO2 stream pressure 308 and considerations regarding process economy. The higher the temperature, the higher the pressure of CO2 emitted from the solution, stream 308. The higher the pressure, the lower the content of ammonia and steam in stream 308 will be. To produce a CO2 stream 310 at low temperature and high concentration, stream 308 is washed and cooled in a direct contact vessel 358 with cold water, stream 336 from heat exchanger 366. Excess water and NH3 captured in vessel 358, stream 332, flows back to

Regenerator 352, while equilibrium stream 334 is cooled and recycled to the washing chamber, vessels 358.

[0049] The invention has been described in accordance with several embodiments, the purpose of which is to illustrate the invention in all aspects and not to limit it. Therefore, many changes are possible to the invention regarding the detailed implementation which the skilled person can deduce from this description. It is believed that all such changes and other changes remain within the scope and spirit of the invention as set out in the following claims and their legal equivalents.

Dorota Rzążewska Patent attorney