METHOD FOR REMOVING NITROGEN OXIDES FROM COMBUSTION FUMES WITH ON-SITE GENERATION OF AMMONIA

Abstract

A method for the control of nitrogen oxides content in the combustion fumes of a thermal power plant is disclosed; the method comprises the on-site production of ammonia by the steps of: electrolysis of water as a source of hydrogen; separation of air as a source of nitrogen, formation of a make-up gas and synthesis of ammonia in a suitable synthesis loop; said on-site produced ammonia, or a solution thereof, is used for a process of reduction of nitrogen oxides in the combustion fumes.

Claims

1. A method for the control of nitrogen oxides content in the combustion fumes (G) of a power plant for the production of electric energy where said combustion takes place, the method comprising the steps of: producing ammonia in the site of said installation, with a process including: producing a hydrogen current by means of electrolysis of water; producing a nitrogen current by means of separation of nitrogen from air; forming an ammonia make up gas containing hydrogen and nitrogen from said hydrogen current and nitrogen current respectively, and reacting said make up gas at a suitable ammonia synthesis pressure; and reducing nitrogen oxides contained in said combustion fumes using said produced ammonia; wherein the production rate of ammonia is regulated according to the cost and/or availability of electric energy, and/or according to the load of said power plant and/or according to the demand of electric energy, such that the production of ammonia is increased during electricity off-peak hours when the demand for electricity is low, and is reduced or stopped, thus maximizing the net output of said plant, during electricity peak hours when the demand for electric energy is higher.

2. The method according to claim 1, said process of reducing nitrogen oxides being a process of selective catalytic reduction or selective non-catalytic reduction.

3. The method according to claim 1, said installation being any of: a thermal power plant for production of electricity, a waste incineration plant, a reformer, a fired heater.

4. The method according to claim 1, wherein said nitrogen current is obtained by one of the following means: molecular sieves; pressure swing adsorption (PSA); vacuum pressure swing adsorption (VPSA); temperature swing adsorption (TSA); cryogenic separation.

5. The method according to claim 1, wherein the synthesis pressure of ammonia is in the range 80 to 300 bar.

6. The process according to claim 1, where excess ammonia is produced during said electricity off-peak hours, or when production of ammonia is greater than actual need of the process for NOx reduction, and said excess ammonia is stored in a storage vessel (ST), either in anhydrous form or in the form of an aqueous solution.

7. A method for modification of an installation comprising a unit for reduction of nitrogen oxides from a combustion flue gas (G), wherein: an on-site ammonia plant, for the generation of ammonia and for use in said reduction unit, is added to said installation, said ammonia plant comprising at least: a water electrolysis section (WE) for production of a hydrogen current from water and electric energy; an air separation unit for production of a nitrogen current from air; means to form an ammonia make up gas by using said hydrogen current and nitrogen current, and an ammonia synthesis section (SL); wherein said installation is a power plant for the production of electric energy and the production rate of ammonia is regulated according to the load of said power plant and/or according to the demand of electric energy, such that the production of ammonia is increased during electricity off-peak hours when the demand of electricity is low, and is reduced or stopped, thus maximizing the net output of said plant, during electricity peak hours when the demand of electric energy is higher.

8. The method according to claim 7, said installation being a thermal power plant for production of electricity.

9. The method according to claim 7, said unit for reduction of nitrogen oxides being a selective catalytic reduction or selective non-catalytic reduction unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a block diagram showing a preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] Referring to FIG. 1, the dotted boundary line 11 denotes an ammonia plant which is added on-site to a thermal power plant. The thermal power plant is generally denoted with block C and converts a fuel F into electric energy E. The stream G denotes combustion flue gas of said thermal power plant.

[0026] In this example the ammonia plant is added to a thermal power plant but in other applications of the invention, the ammonia plant 11 is added to other kinds of a fixed installation, where a combustion takes place and a flue gas is produced. Electric energy E may be produced or not in said installation.

[0027] Turning back to FIG. 1, the gas stream G could be, for example, the combustion fumes of a steam generator in a steam-turbine plant, or hot exhaust gas of a gas turbine or engine. Said stream G contains an amount of nitrogen oxides (NOx) formed during the combustion of the fuel F and which need to be removed, until an acceptable low concentration of NOx is reached.

[0028] The reduction of NOx takes place in a selective removal unit 13 and with the help of ammonia or an ammonia solution 12 produced in the on-site ammonia plant 11. In a preferred embodiment, in particular for a thermal power plant, said unit 13 is a SCR unit but a SNCR unit could also be used. For example, the hot combustion gases are first cooled in a heat exchanger, for example an economizer; they enter the SCR unit 13 around 350° C.; then the purified fumes are usually directed to a further heat recovery (e.g. preheating of combustion air) and/or they are treated (e.g. filtered) before release into atmosphere.

[0029] Said ammonia plant 11 comprises: a water electrolysis unit WE, an air separation unit ASU, a gas compressor MUC, a synthesis loop RL and a recycle compressor RC. The water electrolysis unit WE is fed with demineralised water 2 and electric power 1, and delivers a current 3 composed mainly of hydrogen. The ASU is fed with air 4 and produces a current 5 composed mainly of nitrogen.

[0030] The process of electrolysis of water, which takes place in the unit 1, is known in the art and need not be described. The nitrogen current 5 is preferably obtained with molecular sieves or with a process selected between pressure swing adsorption (PSA), vacuum pressure swing adsorption (VPSA) or temperature swing adsorption (TSA), or with cryogenic separation.

[0031] The hydrogen current 3 and nitrogen current 5 are mixed together and form a make-up gas 6. Said gas 6 is compressed to a synthesis pressure, preferably in the range 80 to 300 bar. The block MUC denotes the make-up gas compression and purification (removal of impurities) in order to obtain a compressed make-up gas 7 almost solely composed of hydrogen and nitrogen. The compressed gas 7 is sent to the synthesis loop SL; said loop SL is operating at said synthesis pressure, and comprises at least an ammonia reactor.

[0032] The product gas of said reactor contains ammonia and a certain amount of reagents (hydrogen and nitrogen). Ammonia is separated from said product gas and the remaining reagents are recycled to the reactor via a recycle compressor RC and currents 8, 9. In some embodiments, the recycle compressor RC is replaced by an additional stage of the gas compressor, namely the current 8 is sent to said additional stage of the compressor and returns into the loop SL via the stream 6.

[0033] The stream 10 is the ammonia product of the synthesis loop SL. Said ammonia product 10 is stored in a suitable storage vessel ST and is injected via the flow line 12 into the SCR unit 13. In some embodiments, the ammonia is stored in the form of an aqueous solution (ammonia-water).

[0034] The on-site ammonia plant 11 consumes a part of the energy E produced by the power plant, in particular for the production of the hydrogen current 3 in the water electrolysis unit WE. Hence, it may be stated that electric energy is basically the source of ammonia, provided that water 2 is available.

[0035] Preferably, the production of ammonia 10 follows the peaks of the demand of electricity. During off-peak hours ammonia can be produced in excess over the the amount necessary for the SCR unit 13, so that some ammonia is stored in the vessel ST; during peak hours, on the other hand, the production of ammonia is preferably reduced below the actual need of unit 13, or even stopped, in order to increase the net production of electricity; the ammonia stored in the vessel ST is then used to form at least part of the ammonia stream 12. In a more general way, the actual production of ammonia can be regulated according to the availability and/or cost of electric energy.