The Beijing problem: reducing its NOx (nitrogen oxides) emissions from boilers for public health. Strict limits on NOx emissions from boilers have been introduced to combat smog in Beijing. Dr. Gregory Zdaniuk, Joël Moreau and Lu Liu explore the use of wet combustion, topic evoked for a long time on Econologie.com in particular via the works of Rémi Guillet who publishes his ideas and works regularly.
Beijing suffers from pollution and seeks solutions
China's very rapid industrial growth has led to significant levels of air pollution, which obviously has an impact on the health of the Chinese, in the big cities more particularly and for many years! The causes are road traffic, the coal industries and heating of buildings. The municipality of Beijing wants to improve air quality and is at the forefront of the fight against air pollution. It is making great efforts to remedy this, in particular by prohibiting new coal-fired installations, limiting traffic and applying new technologies to improve combustion and reduce NOx in particular. The wet combustion is one of these techniques for the future!
"War on Smog": the municipality of Beijing has introduced a series of research measures to combat air pollution:
A ban on coal for new installations
Progressive and mandatory renovation of existing coal facilities
Restrictions on new car registration and daily traffic
Promotion of electric mobility
Promotion of taxis powered by natural gas (methane) and transport by LPG (propane-butane)
Development of carsharing and cycling
Strict limits for NOx in new and existing gas boilers
Since the 1er April 2017, facilities must meet NOx limits for new and existing gas boilers, which are even superior (!!) to European Union standards. The municipality has also put in place incentives to reduce NOx emissions from gas boilers; therefore, 1 500 boilers have been changed to 2016.
The reduction of NOx in boilers is possible in injecting water or steam into the flame zone ; this is what Beijing is using and wants to develop, using a system developed in Europe over the past 15 years, particularly on the work of Rémi Guillet. The post-treatment methods for example, the selective catalytic reduction SCR or selective non-catalytic reduction - treat NOx emissions after training. Combustion control techniques prevent the formation of NOx.
Post-treatment methods tend to be more expensive and are generally not used on boilers with a power of less than 10 MW.
Beijing's strict NOx limits for boilers
In accordance with the Air Pollutant Releases Standard for Boilers (DB11 / 139-2015), new facilities and coal-to-gas NOx limit of 30mg / Nm3 , while existing installations have a limit of 80mg / Nm3. In comparison here in Europe, the equivalent NOx limit set by the European Directive is 100 mg NOx / Nm3… It's 3 times more than in China!
In addition to the strict legal limits, Beijing has set up an economic incentive program to reduce NOx for existing gas boilers. Renovation projects are rewarded based on the amount of NOx they save. 1 gas boilers were modified in 500. In 2016, Beijing modified the equivalent of 2017 GW of cumulative gas boiler thermal power, or approximately the thermal power of 2 nuclear reactors!
The formation of NOx varies almost exponentially with the temperature of the flame. The main method of controlling NOx is to reduce the temperature of the flame. This can be done in several ways:
- Improved flame distribution to reduce hot spots
- Change the air / fuel or fuel ratio and reduce excess air
- Adding the flue gas recirculation (EGR): the blue flame oil boiler
- Layered Combustion (HCCi)
- Inject water or steam during combustion
The challenge for engineers is therefore to lower the flame temperature, while maintaining flame stability and the efficiency of the boiler. Security is also critical, especially when it comes to EGR, due to the risk of carbon monoxide (CO) explosion potential present in the exhaust gas!
The wet combustion system by water vapor pump (PAVE)
The injection of water or steam causes the stoichiometry (the quantitative relationship between oxidant and oxidized) - and therefore the temperature of the adiabatic flame - to change in the air-fuel mixture. The addition of water also “disperses” the calories generated by combustion. Both phenomena cause a decrease in the combustion temperature - the color of the gas flame, logically blue, becomes substantially yellow orange. If the flame temperature is sufficiently reduced, NOx will hardly be formed any more and the thermal performance of the boiler will be preserved.
Figure 1: Same burner operating in wet combustion mode (top) and dry combustion mode (bottom)
The Water Vapor Pump, WVP, or Water Vapor Pump, PAVE) is a method of wet burning of Ph.D Rémi Guillet developed and patented in 1979, of the CIEC company based in Paris and which has been part of the ENGIE group since 2004. It consists of a preheating and humidity saturation of the combustion air with recovery of the sensible and latent heat of the combustion gases. To do this, two sprayers are placed in the air flow: one in the fresh air inlet and the other between the condenser and the chimney, as shown in Figure 2. All the components are made of stainless steel and the burner is made to manage combustion air saturated with humidity. The geometry of the water injection burner has nothing to do with that of a typical low NOx burner (a single double wall)
As the dew point of the combustion gases entering the condenser is, of course, increased (from ~ 58 ° C in the case of regular combustion to ~ 68 ° C in the case of wet combustion), much more latent heat is recovered in the condenser. This compared to an ordinary condensing boiler operating at the same flow and return temperatures. In addition, the additional heat recovery that occurs in the exhaust spray tower cools the flue gases to much lower temperatures compared to an ordinary boiler. As a result, the system PAVE is much more efficient than an ordinary condensing boiler.
Figure 3 compares the efficiency of the PAVE combustion system and a regular condensing boiler as a function of the condensing return temperature. It shows that the onset of condensation is shifted to a higher return temperature, making the PAVE system an ideal candidate for renovation applications where it is not easy to reduce the return temperature of the building (conventional radiators with high temperature)
The PAVE system is characterized by very low flame temperatures, so it is capable of achieving very low NOx production. The limit of 30 mg / Nm3 is easily reached as long as the combustion air is preheated to 60 ° C and adjusted to an optimal temperature. On the other hand, "dry" burners with low NOx and very low NOx emissions can only achieve comparable levels of NOx emissions by using a high proportion of EGRs and, potentially, oversized combustion chambers.
In a conventional combustion system (with atmospheric air), reducing the flame temperature below a certain temperature can lead to the formation of CO, but this is not the case for a PAVE boiler that burns natural gas therefore a fuel which a priori easily accesses its complete combustion.
Furthermore, the performance of the PAVE cycle is neither inclined to cause the combustion temperature to drop as low by means of excessive water recycling, or even to reduce the O2 rate in the oxidant by this same means: and the risk of CO formation is a priori eliminated by the PAVE cycle.
Reducing the production of NOx and reducing the risk of water plume leaving the chimney (via lower humidity in the smoke) have the fortunate consequences: less risk of smog (which is in the case of natural gas combustion) the result of the combination of water plume + NOx) at the same time as the thermal performance of the cycle which are maximum ...
First Water Vapor Pump project in China by CIEC
During the last 15 years, the company ICCS deployed the PAVE system in several European countries, mainly in France, but also in Germany and Italy. As the NOx limits are less stringent in Europe, the system is installed as energy saving measure.
In 2016, Beijing United Gas Engineering and Technology obtained a contract from a Beijing university to renew its boiler room. It involved changing the coal-fired boiler room and installing a new gas system. It was decided to set up the PAVE system in China for the first time.
The system includes two gas condensing boilers of 5,6 MW each to heat the campus on approximately 160 m000 of heating surface. The system has been dimensioned for a capacity of 2 m200000 in anticipation of future expansion works. The heat distribution network is designed for a flow and return temperature of 2 ° C / 70 ° C. All terminal units are controlled by three-way valves, which makes the return temperature variable. Only one of the 50 boilers is currently equipped with PAVE, the second boiler is equipped with a standard burner with low NOx emission. This will allow for comparative tests over time.
Commissioning was carried out in March 2017, NOx emissions being tested at 23 mg / Nm3 (corrected to 3,5% O2), well below the limit of 30 mg / Nm3. The overall efficiency of the boiler was 107% - at a return temperature of 45 ° C and the CO emissions were measured at 0 mg / Nm3!
A bright future for steam pump boilers ...
PAVE is a combustion technology capable of achieving ultra-low NOx emissions and at considerably high yields (109% on PCI) and lower maintenance costs than conventional condensing boilers. The PAVE can be installed on an existing boiler without significant loss of capacity, while the typical renovations of the burner with low NOx emission can significantly reduce it. Faced with a serious smog problem, Beijing is at the forefront of the fight against air pollution and these actions should be observed by political decision-makers around the world…
We participated in the development of this article:
Dr. Gregory Zdaniuk, Senior Director of Engineering, Engie China
Joël Moreau, Deputy Director General of the ICCS
Lu Liu, deputy chief engineer at Buget
Translation by Christophe Martz, engineer and editorial manager of Econologie.com