WO2018115499A1

WO2018115499A1 - Process and plant for denitrification of combustion gases

Info

Publication number: WO2018115499A1
Application number: PCT/EP2017/084508
Authority: WO
Inventors: Frédéric Duong
Original assignee: Suez Groupe
Priority date: 2016-12-22
Filing date: 2017-12-22
Publication date: 2018-06-28
Also published as: FR3061031A1

Abstract

Process for denitrification of a gaseous effluent originating from a combustion system, said gaseous effluent being liable to contain nitrogen oxides. The process comprises a first step of treating the gaseous effluent with a first system (11) for reducing the nitrogen oxide content which is located upstream of a second system (6) for reducing the nitrogen oxide content. The second system (6) is a selective catalytic reduction reactor, or SCR reactor, and the process is characterized in that only a first portion of the effluent leaving the first system (11) for reducing the nitrogen oxide content is brought into contact with the SCR reactor (6), a second portion of the effluent leaving the reduction system (11) being diverted in a controlled manner to an exhaust fan (7).

Description

PROCESS AND INSTALLATION FOR COMBUSTION FUELS DENITRIFICATION

The technical field of the invention relates to the treatment of industrial fumes and more particularly the denitrification of combustion fumes.

The combustion of any hydrocarbon material leads to the production of gaseous effluents rich in carbon dioxide (C0 ₂ ) and nitrogen oxides (NOx). These gases obtained by combustion are subject to special monitoring because of their health and environmental impacts, one being a greenhouse gas and the others being classified as regulated air pollutants. Thus, industries that use incinerators must limit their C0 ₂ emissions and NOx for which the rejection must be less than the regulations imposed by the Directive 2000/76 / EC on the incineration of waste of 4 December 2000 (Waste Incineration Directive) (200 mg / Nm ³ ), or much lower according to environmental permits (80 mg / Nm ³ ). Several types of known methods allow the purification of acids from combustion fumes. The main ones are:

The wet process is based on iso-enthalpic cooling of fumes to bring them in the presence of water at their saturation conditions. Under these conditions, the cooled and saturated fumes will undergo a series of treatments that will have the effect of transferring the pollutants from the gas phase into the liquid phase. The purified gas phase may be rejected in accordance with the regulations; the liquid phase must be treated either specifically or by a return to the top of the station, as long as this return is admissible.

In contrast to wet processing, the dry process processes the fumes as they are at the output of the energy recovery unit by ensuring that the temperature is compatible with the technologies applied. This assumes a controlled cooling system that can be either indirect because of the presence of an exchanger, either direct by addition of ambient air or by water spray.

Finally, the semi-wet system is a hybrid system between wet and dry systems trying to retain the benefits and limit the disadvantages of each system. The wet system has two major drawbacks: the production of water which contains dissolved salts is particularly penalizing when the dust load is very high; the need for treatment of the purges from the different washing steps. The dry system has two major drawbacks: the need to cool the fumes before entering the treatment chain (this conditioning is particularly strict when it comes to bag filters) and the dry system-specific reaction overstoichiometry which represents a significant operating cost.

The semi-wet system therefore consists of preceeding the wet treatment, that is to say the scrubbers, by a reaction tower in which the purges are sprayed and evaporated. This system is only possible if the sensible heat contained in the fumes makes it possible to compensate for the evaporation and overheating of the water contained in these purges. The crystallization salts are recovered at the bottom of this tower or in the bag filter. This principle is only suitable for fumes with little dust, so it applies well to pyrolysis processes in which case it is very economical both in investment and in the costs of reagents but difficult to incineration processes such as bed fluidized.

Furthermore, the treatment of combustion fumes resulting in the reduction of pollutant emissions, in particular NOx, can be achieved

or by a selective non-catalytic reduction process (Selective Non Catalytic Reduction or SNCR) implemented by a selective non-catalytic reduction reactor, referred to as the SNCR reactor, or by a selective catalytic reduction process (Selective

Catalytic Reduction or SCR), implemented by a selective catalytic reduction reactor, called SNCR reactor,

or by coupling these two processes. SNCR is one of the most cost-effective NOx reduction techniques. It results from reduction reactions that take place at high temperature (850 to 1050 ° C) between NOx and ammonia, urea or cyanuric acid injected into the fumes. The oxidation-reduction reactions are as follows:

Main reaction: 4 NH ₃ + 4 NO + 0 ₂ → 4 N ₂ + 6 H ₂ 0

Secondary reaction: 4 NH ₃ + 5 0 ₂ → 4 NO + 6 H ₂ 0 The SNCR must be carried out at precise temperatures in order to favor the main reaction of reduction, the latter being in competition with the secondary reaction of oxidation which is to be avoided. Indeed, at lower temperatures too few NH radicals, active are formed while at higher temperatures, other reactions appear which promote the production of NO. Other parameters may be involved in the optimization of the NOx reduction in response to load variations at the furnace, such as, for example, the initial NO content, the residence time, the presence or absence of additives.

Under entirely optimal conditions, it is possible to reach a denitrification rate of the order of 90%, equivalent to that of the SCR. But in practice, the rate is usually between 40 and 65%.

This non-catalytic process has the advantage of being simple to implement. However, if the reducer is incorrectly dosed, the residence time is too short or the temperature is too low (<300 ° C) then the fumes are more concentrated in ammonia, the emission rate of which is also controlled.

SCR is a catalytic process that reduces NOx emissions. Urea or ammonia precursor ΝΗ ,, is injected into the flue gas upstream of the SCR reactor where the catalyst is located. The reducing agent is sprayed before being injected into the flue gas. The NOx is then reduced by the injected reagent to form molecular nitrogen (N ₂ ) and water (H ₂ 0) on the catalyst surface according to the following chemical reactions.

Using ammonia as a reducing agent:

4 NH ₃ + 4 NO + 0 ₂ → 4 N ₂ + 6 H ₂ 0

8 NH ₃ + 6 N0 ₂ → 7 N ₂ + 12 H ₂ 0 Using urea as reducing agent:

2 (NH ₂ ) ₂ CO + 4 NO + 0 ₂ + 2 H ₂ 0 → 4 N ₂ + 6 H ₂ O + C0 ₂

4 (NH ₂ ) ₂ CO + 6 N0 ₂ + 2 H ₂ 0 → 7 N ₂ + 12 H ₂ 0 + 4 C0 ₂ SCR is characterized by a high reduction rate, often greater than

90%, for an ammonia dosage close to stoichiometry, limiting the risk of leaks. The operating temperature, very widely between 170 ° C and more than 540 ° C, is generally optimal between 300 ° C and 400 ° C. Ammonia losses are mostly negligible once the installation is optimized. The life of the catalyst depends largely on the fuel composition and varies with the amount of "poisons" (heavy or other metals) it contains. It is however easy to replace a catalyst reached at the end of life but the cost for such a replacement is very high. In addition, regular maintenance is also necessary to avoid clogging catalysts that eventually turn off and it becomes necessary to change them. This process has the advantage of operating at low temperatures but the operating expenses are very important.

In other words, the SCR makes it possible to cut down large quantities of NOx, but at the cost of major economic and environmental disadvantages, whereas the SNCR, which is more economical, does not make it possible to achieve a nitrogen oxide removal efficiency. as high as the SCR.

Energy From Waste (EfW) units are often equipped with catalytic flue gas denitrification (SCR) reactors to achieve a release threshold below 80 mg / Nm ³ of NOx. These reactors and their peripherals represent, most of the time, a bottleneck within the installation that either needs to limit the load of the furnace below its nominal capacity, or not be able to increase it when the deposit of the waste allows this, which represents a significant loss of load.

On the other hand, the pressure drop of the catalyst and its associated equipment, especially the cross exchanger in the case of a fume treatment wet, is high representing a second bottleneck for the draft fan.

Moreover the replacement of these equipment SCR + exchanger is expensive and it is difficult to increase their size due to a large footprint.

Thus, because of the advantages of reducing NOx and the high costs of SCR operation, many flue gas denitrification processes provide for the coupling of SCR and SNCR processes.

Thus, US Pat. No. 5,853,683 and patent application CN 102716665 describe processes for reducing the level of NOx in flue gases in which an SNCR is placed upstream of an SCR. This prior passage in a SNCR reduces the fouling of the SCR due to a high content of pollutants such as NOx and limit operating expenses. In addition, the coupling of a pre-SCR SNCR optimizes the reducing capacity of the SCR NOx, which is particularly useful for the treatment of fumes with a high NOx content.

However, these systems do not meet the strict discharge regulations and require a NOx release threshold of less than 80 mg / Nm ³ , or meet the maximum Nh discharge threshold in fumes subject to excess of SNCR reagent.

Consequently, there is a need to provide a method of optimizing the denitrification of combustion fumes without modifying the characteristics of an existing catalytic reactor (SCR) and making it possible to release the bottleneck of the fumes.

One of the aims of the present invention is therefore to provide a process for the denitrification of a gaseous effluent from a combustion system, said gaseous effluent being capable of containing nitrogen oxides, said process comprising a first stage of treatment of said effluent gas with a first system for reducing the nitrogen oxide content upstream of a second system for reducing the nitrogen oxide content which is a reactor to selective catalytic reduction, said SCR reactor, and said method being characterized in that only a first portion of the effluent at the outlet of the first system for reducing the nitrogen oxide content is brought into contact with the SCR reactor, a second part of said effluent at the outlet of the reduction system being diverted in a controlled manner to a draft fan.

In the context of the present invention, it is considered that the terms "gaseous effluent" and "combustion fumes" are synonymous. The step of bringing into contact with two systems for reducing the nitrogen oxide content according to the invention has the advantage of limiting the use of the second SCR reactor thus limiting operating and maintenance expenses.

In addition, the controlled diversion of a portion of the effluent at the outlet of the reduction system makes it quite surprising to optimize the combustion furnace charge by significantly reducing the static pressure loss of the flow rate. gaseous effluent at the suction of the draft fan.

The first system for reducing the nitrogen oxide content may be located above the combustion system firebox.

According to one particular embodiment of the invention, the method comprises a second step of defining the flow rate of the second part of the effluent by means of measuring the nitrogen oxide content of the effluent at the outlet of the effluent. first system for reducing the nitrogen oxide content.

According to the invention, this second step of defining the flow rate of the second part of the effluent is carried out by the implementation of measuring means which make it possible to control the degree of opening of the means for diverting the second part of the effluents. so that the reduction rate of nitrogen oxides is at least 30%, and preferably at least 50%, relative to the initial content measured at the inlet of the SNCR reactor.

To do this, the measuring means are connected to a calculation unit for the definition of the flow rate of the second part of the effluent, which interacts directly or indirectly on the means to divert the second part of the effluents to allow to obtain a rate of abatement in nitrogen oxides in accordance with the regulations in force.

According to the invention, the term "measuring means" means any means known to those skilled in the art capable of measuring the content of nitrogen oxides, or even other acidic gases, such as a sensor and / or or analyzer.

According to the invention, the term "means for diverting" any means, known to those skilled in the art, allowing the computing unit to modulate the amount of effluent to divert, such as a motorized register or a valve.

Adaptation of the diversioned gaseous effluent flow as a function of the measurement of the NOx content limits the "bottleneck" effect usually observed shortly before the chimney outlet, in particular at the draft fan. Thus, the emission of NOx at the outlet of the stack is stabilized and the load of the combustion furnace can be optimized.

The method according to the invention may furthermore comprise a third step of analysis of the effluents that have passed through the SCR reactor, and which have passed through the reduction zone above the furnace, and that measure the oxides content. nitrogen of said effluents.

In an advantageous embodiment of the invention, the amount of the second part of effluent diverted upstream of the SNCR is between 10 to 50%, preferably 10 to 30%, of the quantity of effluent coming out of the first system for reducing the nitrogen oxide content.

In a particular embodiment, the quantity of the second effluent portion may be controlled in particular by a control means using as a control parameter the measurement of the nitrogen oxide content of said effluents.

The process according to the invention, and whatever the combination taken into consideration, makes it possible to obtain a reduction rate of nitrogen oxides at the outlet of the SNCR reactor of at least 30%, and preferably at least 50%, by relative to the initial content measured at the inlet of the SNCR reactor to nitrogen oxides.

As a result, said method is able to cope with the variability of nitrogen oxide rejection regulations while guaranteeing a relatively low operating expense (OPEX).

Another aspect of the invention relates to an installation for carrying out the process for the denitrification of gaseous effluent from a combustion system according to the invention, comprising in a flow direction of the effluent:

at. a first reduction system that can be above the focus of the nitrogen oxide content,

b. a second system for reducing the nitrogen oxide content which is a downstream SCR reactor,

vs. means for feeding the effluent from the first system to the second system,

d. means for diverting a second portion of the effluents at the outlet of the first system for reducing the nitrogen oxide content to a draft fan,

e. means for measuring the nitrogen oxide content of the effluent at the outlet of the first system for reducing the nitrogen oxide content.

In particular, said means for diverting the second part of the effluents are controlled by said measuring means as a function of the nitrogen oxide contents measured at the outlet of said first reduction system.

Indeed, the means for measuring the nitrogen oxide contents at the outlet of the first system for reducing the nitrogen oxide content make it possible to control the degree of opening of the means for diverting the second part of the effluents so that the the nitrogen oxide reduction ratio is at least 30%, and preferably at least 50%, relative to the initial content measured at the inlet of the SNCR reactor. To do this, the measuring means are connected to a computing unit, which interacts directly or indirectly with the means for diverting the second part of the effluents to enable a rate to be obtained. nitrogen oxide abatement in accordance with the regulations in force.

In an advantageous embodiment of the invention the means for diverting the second portion of effluent are adapted to divert an amount of the second part of effluent between 10 to 50%, preferably between 10 to 30%, of the quantity of effluent at the outlet of the first system for reducing the nitrogen oxide content.

In an advantageous embodiment of the invention, the first system for reducing the nitrogen oxide content is a thermal system for reducing the NOx content, and may in particular be a staged combustion or "overfire", a means of vapogasification with water supply, or a selective non-catalytic reduction reactor, said SNCR reactor.

In the context of the present invention, the term "staged combustion" or "overfire" is understood to mean any staged combustion as practiced on the grate furnaces, that is to say any combustion comprising at least two distinct reaction stages where the first The combustion step is reducing at a relatively low temperature (about 1000 ° C) followed by a second stepped oxidation step of CO at higher temperature (> 1150 ° C).

In the context of the present invention, the term "vapogasification with steam supply" or "vapogasification", any system based on the principle of a supply of water vapor and combustion air.

When said first system is an "overfire" or a vapogasification, it has the advantage of reducing the temperature of the combustion chamber which is favorable to the limitation of the formation of NOx. Other features and advantages of the invention will emerge from the detailed description of an embodiment and the figures without being limited thereto.

Figure 1 is a diagram illustrating an example of a denitrification plant and a combustion system known from the state of the art.

Figure 2 is a diagram illustrating an example of a denitrification plant and a combustion system according to one embodiment of the invention.

The numbering of the various elements of the installations are identical for the two figures.

The installation of Figure 1 includes a waste incinerator (1) and a boiler (8)

The gaseous effluents, at the outlet of the boiler (8) whose temperature approaches 200 ° C, pass through an electrostatic precipitator (2) where an electrostatic current draws the dust contained in the fumes to limit the health impacts, and in particular the nuisance to the respiratory system due to this dust.

Once the fumes have been removed from the flue dust, the gaseous effluents circulate through a desaturator (3). The desaturator (3) is a first crossed exchanger. After the first passage of the effluents through the desaturator, the fumes are fed to a scrubber (4) before circulating again through the desaturator (3) during a second pass.

The scrubber (4) cools the gaseous effluents at about 60 ° C to neutralize all the active gases, such as S0 ₂ , S ₂ and Cl. The gaseous effluents thus washed are then reheated to about 126 ° C. of their second passage in the desaturator (3). At the outlet of the desaturator, the effluents are conveyed to a second cross exchanger (5) from where they are directed to a catalytic reduction reactor (6) and then to the second exchanger and then to the draft fan (7). The scrubber has the effect of neutralizing the acid gases. The installation of Figure 2 comprises a non-catalytic reduction reactor (11) as the first reduction system. This SNCR reactor is located between an incineration furnace (1) and a boiler (8).

The SNCR is composed of at least one level of ammonium injection, preferably three levels of ammonium injection, in order to limit by at least 60% the formation of NOx in the gaseous effluents.

Then, the gaseous effluents, whose temperature approaches 220 ° C, pass through the electrostatic precipitator (2) where an electrostatic current attracts the dust contained in the fumes to limit the health impacts, and in particular the nuisances to the respiratory system due to these dusts.

Once the fumes have been cleared of dust, the gaseous effluents circulate through a desaturator (3) from which the fumes are fed to a scrubber (4) before circulating again through the desaturator (3) so that first part of the effluent is finally conveyed to the cross exchanger (5) and a second part is bypassed towards the draft fan (7) by means of a motorized valve or register which separates the effluent stream into a first part and a second part. This circulation of the gaseous effluents is allowed thanks to the cross organization of the currents within the desaturator (3). The gaseous effluents entering the desaturator (3) have a temperature close to 210 ° C and are cooled to about 140 ° C during their first pass.

The scrubber (4) cools the gaseous effluents at about 60 ° C to neutralize all the active gases, such as S0 ₂ , S ₂ and Cl. The gaseous effluents thus washed are then reheated to about 126 ° C. of their second passage in the desaturator (3) before a first part is conveyed to the cross exchanger (5) and a second part is diverted to the draft fan (7).

As well as during the passage in the desaturator (3), the gaseous effluents leaving the desaturator (3) pass first through the cross exchanger (5) where they are heated to 220 ° C before being fed to the second reduction system (6), which is a catalytic reduction reactor (SCR). The SCR reactor (6) limits by approximately 61% the formation of NOx in the flue gases. The gaseous effluents leaving the reactor (6) were heated to 240 ° C to be cooled to 140 ° C during their second pass in cross exchanger (5) before being brought to the draft fan (7) by which the gaseous effluents will be released into the atmosphere.

The installation of FIG. 2 comprises an SNCR reactor (11) providing a preliminary treatment of the fumes to reach a value close to 180 mg / Nm ³ over the entire flue gas stream at the boiler outlet and the SCR reactor (6) ensures finishing on 75% of the flow with an adjusted rejection to reach the maximum overall rejection value.

Consequently, the addition of an SNCR reactor (11) relieves the load of the cross exchanger (5) and the SCR reactor (6) by approximately 25%, thus limiting the energy impact of the various bottlenecks on the treatment of combustion fumes and to obtain a significant operating operating margin. The objective is to stabilize the boiler load at nominal walking (MN) for 8000 h / year.

Comparative results obtained with installations comprising either an SCR reactor alone or an SCR reactor combined with an SNCR reactor which can be combined with an exchanger are shown in Table 1 below.

Table 1 - Comparative table of the various data relating to installations comprising an SCR reactor alone or in combination with an SNCR reactor which can be combined with an exchanger.

In addition, the installation as described in Figure 2 has the following advantages:

- reduction of the pressure drop of the cross exchanger (5) by 40%,

- reduction of the electrical consumption of the 100 kWe draft fan,

- restoration of an acceptable excess of air with 7% of 0 ₂ ,

- Possibility of increasing the overall flue gas flow rate when needed (lower heating value, or PCI, low due to the high humidity of the waste),

- reduction of the formation of ammonium sulphate in the SCR reactor (6), and

- Improved efficiency.

The results obtained are due to a controlled regulation of the denitrification at the SNCR reactor in order to obtain an optimal reduction of the discharges at the boiler outlet and also to limit the excesses of reagent in form N H3.

The regulation of the SCR reactor will be such that the mixing of the flow from the SCR reactor and the diverted flow makes it possible to reach the guaranteed regulatory threshold.

The SCR bypass rate setting will be established based on the effectiveness of the SCR; the higher the efficiency, the more it will be possible to work around it and vice versa. This will allow on the one hand to increase the load of the furnace and also reduce the power consumption of the draft fan, since the pressure drop of the catalysts will be significantly reduced.

Claims

A process for denitrifying a gaseous effluent from a combustion system, said gaseous effluent being capable of containing oxides of nitrogen, said process comprising a first step of treating said gaseous effluent with a first reduction system (11) of the nitrogen oxide content upstream of a second nitrogen oxide content reduction system (6), said second system (6) being a selective catalytic reduction reactor (SCR) and said process being characterized in that only a first portion of the effluent at the outlet of the first nitrogen oxide content reduction system (11) is brought into contact with the SCR reactor (6), a second portion of said effluent output the reduction system (11) being diverted in a controlled manner to a draft fan (7).

Process according to Claim 1, characterized in that it comprises a second step of defining the flow rate of the second part of the effluent by means of the measurement of the nitrogen oxide content of the effluent at the outlet of the first system reducing (11) the nitrogen oxide content.

Process according to any one of the preceding claims, characterized in that it further comprises a third step of analysis of the effluents having passed through the SCR reactor (6) and measuring the content of nitrogen oxides of said effluents.

Process according to any one of the preceding claims, characterized in that the amount of the diverted second effluent portion is between 10 to 50%, preferably 10 to 30%, of the quantity of effluent leaving the first reduction system. (11) the nitrogen oxide content.

Denitrification plant for carrying out the process according to claims 1 to 4, comprising in a flow direction of the effluent: at. a first system (11) for reducing the nitrogen oxide content, b. a second system (6) for reducing the nitrogen oxide content which is a downstream SCR reactor,

vs. means for supplying the gaseous effluent from the first system (11) to the second system (6),

d. means for diverting a second portion of the effluents at the outlet of the first system (11) for reducing the nitrogen oxide content to a draft fan (7),

e. means for measuring the nitrogen oxide content of the effluent at the outlet of the first system (11) for reducing the nitrogen oxide content.

6. Installation according to claim 5 characterized in that said means for diverting the second part of the effluents are controlled by said measuring means as a function of the nitrogen oxide contents measured at the output of said first reduction system (11).

7. Installation according to claim 5 or 6 characterized in that the first reduction system (11) of the nitrogen oxide content is either a staged combustion, a vapogasification means with water supply, or a reactor of selective non-catalytic reduction, says SNCR.