Beijing's problem: reducing its NOx (nitrogen oxides) emissions from boilers for public health. Strict limits on NOx emissions from boilers have been introduced to fight smog in Beijing. Dr Gregory Zdaniuk, Joël Moreau and Lu Liu discuss 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, coal industries and heating of buildings. Beijing Municipality 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 banning new coal-fired installations, limiting traffic and applying new technologies to improve combustion and reduce NOx in particular. The wet combustion is one of those techniques of 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 higher (!!) 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 area ; this is what Beijing uses 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 - addresses post-formation NOx emissions. Combustion control techniques prevent the formation of NOx.
Post-treatment methods tend to be more expensive and are generally not used on boilers with an output below 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… That's 3 times more than in China!
In addition to the strict legal limits, Beijing has implemented 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 boiler efficiency. Safety is also critical, especially when it comes to EGR, due to the risk of carbon monoxide (CO) explosion potential present in the exhaust gases!
The wet combustion system by water vapor pump (PAVE)
The injection of water or steam causes the modification of the stoichiometry (the quantitative relationship between oxidant and oxidant) - and therefore of the temperature of the adiabatic flame - of the air-fuel mixture. Adding 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 noticeably yellow-orange. If the flame temperature is sufficiently reduced, NOx will hardly be formed and the thermal performance of the boiler will be maintained.
Figure 1: Same burner operating in wet combustion mode (top) and dry combustion mode (bottom)
The water vapor pump system (WVP, or Water Steam 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 components are made of stainless steel and the burner is designed to handle the 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 is compared to an ordinary condensing boiler operating at the same flow and return water temperatures. Additionally, the additional heat recovery that occurs in the exhaust spray tower cools the flue gases to much lower temperatures compared to a regular 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 retrofit applications where it is not easy to reduce the building return temperature (conventional radiator at high temperature)
The PAVE system is characterized by very low flame temperatures, so it is able to achieve very low NOx productions. The limit of 30mg / Nm3 is easily reached as long as the combustion air is preheated to 60 ° C and adjusted to an optimum temperature. On the other hand, “dry” low NOx and very low NOx burners can only achieve comparable levels of NOx emissions by using a high proportion of EGR and, potentially, oversized combustion chambers.
In a conventional combustion system (with atmospheric air), reducing the temperature of the flame below a certain temperature can lead to the formation of CO but this is not the case for a PAVE boiler which burns natural gas therefore a fuel which a priori easily accesses its complete combustion.
In addition, the performances of the PAVE cycle are neither inclined to make the combustion temperature drop so low through too much water recycling nor even to reduce the O2 rate in the oxidizer by this same means: and the risk of CO formation is a priori eliminated by the PAVE cycle.
Reduction in the production of NOx and reduction in the risk of a plume of water leaving the chimney (via less humidity in the fumes) have the happy consequences: less risk of smog (which is in the case of the combustion of natural gas the result of the combination of water plume + NOx) at the same time as the thermal performances 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 was awarded 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 PAVE system in China for the first time.
The system includes two condensing gas boilers of 5,6 MW each to heat the campus over approximately 160 m000 of heating surface. The system has been sized for a capacity of 2 m200000 in anticipation of future expansion work. 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 low NOx burner. This will allow comparative tests to be carried out over time.
Commissioning was made in March 2017, with NOx emissions tested at 23 mg / Nm3 (corrected to 3,5% O2), well below the limit of 30 mg / Nm3. The overall boiler efficiency 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 considerably high efficiencies (109% on PCI) and lower maintenance costs than conventional condensing boilers. PAVE can be installed on an existing boiler without significant loss of capacity, while typical low NOx burner retrofits 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 policy 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