This article covers experiences about NOx reduction in Europe, USA and China as well as the challenges and opportunities to comply with the revised emission norms in a cost efficient way
The Indian Ministry of Environment had on August 25, 2014 published new nitrogen oxide (NOx) emission limit values for the cement industry. It means rotary kiln plants commissioned before the date of notification have to comply with a new NOx emission level value of maximum 800 mg/Nm3 dry flue gas at 10 per cent O2, measured as NO2. For plants commissioned on or after the date of notification, the ELV is 600 mg/Nm3. There have been discussions between the cement industry and the ministry. The final outcome regarding the NOx levels and date for compliance was not yet official at the time of writing this article.
This article covers experiences from NOx reduction in other parts of the world such as Europe, USA and China as well as the challenges and opportunities to comply in a cost efficient way. Some of the points being discussed are:
NOx emission limit values in India
The new emission limit values in India were published on August 25, 2014. Table-1 is an extract from the notification from the ministry.
All monitored values for NO2 shall be corrected to 10 per cent oxygen and on dry basis. After some initial uncertainties, it has been clarified that the NOx values are to be measured as 24-hour average values. The industry has also been given an extension of the time to comply. According to the author´s understanding, 18 months after September 2015 if the plant can give proof of having started activities with the aim to reduce the NOx.
NOx emission limit values in Europe
Refer to Table-2, which is showing the present NOx emission regulation in the European Union. The values are given as Associated Emission Levels (AEL), associated with the Best Available Technique (BAT). The BAT and the AEL are set by the European Union and it also takes into account what is reasonable economically. The most common applicable AEL for traditional pre-calciner clinker plants is today 500 mg/Nm3. But more and more alternative fuels are being used. The ELV for waste to energy plants is 200 mg/Nm3 @ 11 per cent O2. This means that cement plants having a certain amount of waste as fuel, will have to comply with 200 mg/Nm3. Yara Environmental Technologies has been and is involved in several projects for exchanging or converting old SNCR systems to new high performance systems that are able to reduce the NOx down to 200 mg/Nm3. To reach these high reductions of sometimes 80-90 per cent with SNCR, ammonia solution has to be used. Using urea, reductions over 50 per cent in most cases cannot be reached.
SCR in the cement industry
SNCR has been the totally dominant deNOx method in the cement industry. A small number of SCR (Selective Catalytic Reduction) systems has however been installed. The main reasons for this seem to have been:
The main operational cost for SCR is still the ammonia or the urea, even if the chemical efficiency is higher for SCR than for SNCR. To this comes the catalyst cost, which is a big part of the operational costs. It includes regular regeneration of the catalyst and replacement with new catalyst after some years of operation. The electrical power is also much higher for SCR, 2-5 kWh per ton clinker. This comes mainly from increased fan consumption due to pressure drop over the catalyst and from dust cleaning when running high dust SCR.
The SNCR process
The SNCR process is a process of chemical reactions taking place very soon after the combustion. The main reaction is NH3 + NOx ? N2 + H2O (Simplified formula). To be efficient, it requires a certain temperature window. For ammonia it is between 970 - 850°C; for urea it is 1100 - 950°C. This is one of the reasons why ammonia is much more efficient than urea in pre-calciners and pre-heaters. So for an efficient reduction and low reagent consumption, it is important to install a certain number of injectors at the right locations GÇö typically, somewhere between the kiln inlet and the last cyclone but after secondary combustion. Refer to Figure-3 If injection is done at a very high temperature, more NOx will be formed. Injection at low temperatures will create ammonia slip. Both result in unnecessary consumption of reagent.
The SNCR system and its components
Figure-4 shows the main parts and flow of an SNCR system. An SNCR system normally consists of the following main parts:
The tank can be a single wall or double wall type and it should be manufactured in stainless steel. The single wall tank will require a retention bund in concrete around the tank to capture any possible leaks. The tank is designed for pump filling from a tank lorry and with a return line for displaced gas back to the tank lorry. The tank has to have certain instruments like overfill protection, pressure/vacuum valve with flame arrestor, leakage indicator, level transmitter for indication of the tank volume and a tank pressure transmitter.
The emergency shower should be a combined drench shower and eye/face wash for installation outdoors close to the storage tank for reduction agent and filling point.
Pump module for filling of the storage tank
The pump module is for filling of the reduction agent, from a tank lorry without own pump, into the storage tank. The pump module will be located outdoors near the storage tank.
Pump module for reduction agent
The pump module is for transporting and pressurising reduction agent from storage tank to the process units. The pump module is normally located outdoors close to the storage tank.
Pump module for softened water
There should also be a pump module for transporting and pressurising softened water. For some applications with low reagent consumption, it is needed for dilution of the reduction agent solution to the concentration needed in the furnace. The water is pumped to the process unit where it is blended with the reagent. For cement applications, the consumption of reagent is normally quite high and thus no dilution water is needed. But it will still be needed for the flushing of the system when the system is stopped for maintenance, etc.
The process unit should be located as close as practically possible to the injection area. The unit includes one blending module and injection modules, one for each injector. The blending module produces the required quantities of reduction agent for the current operating condition demands and each injector module manages one injector. The injector module is equipped to open and close reduction agent flow, manage atomising and cooling air and purging with water when stopped. The unit includes all necessary valves, flow transmitters and pressure transmitters for automatic operation.
The unit is also equipped with a gas alarm, giving a light signal and sending an alarm to the control unit at moderate gas concentration of ammonia. If level of ammonia gas is high the complete SNCR system will be stopped automatically. The process unit communicates with the control and management module, normally via Profibus DP to distributed I/Os.
The special injectors developed by Yara are manufactured in stainless steel and managed from the process unit. The spraying length and width as well as the size of the droplets are adjustable. The possibilities of adjustments give a broad flexibility and means that the spraying dose can reach a large area of the ducting. Compressed air is used to atomise the droplets of reduction agent. For injectors in stand-by the compressed air is used for cooling of the injector. When the injection is stopped the injectors will be purged with softened water to secure that no reduction agent is left in the system.
Control and management module
The control and management module is equipped with PLC intelligence of well-known brands and distributed I/Os that normally communicate via Profibus DP. CMM controls, manages, co-ordinates and monitors all process functions to an automatic controlled system. It communicates with the superior control system (DCS) of the plant, receives signals from the plant, takes care of the recipe arithmetic, calculates set points for the reduction process and communicates with operator interface. The local operator interface (HMI) is an industrial PC and a touch screen. All commands to the system are given through the operator terminal. Process pictures are used for operation and management of the system. The pictures make the operation easy since commands often are given by pushing buttons on the touch screen. The control panel also collects and stores the process information. Necessary process data shown in the operator terminal can be transferred to the superior control system (DCS) of the plant for normal operation/supervision. The normal daily operation and supervision of the system is made from the DCS of the plant. The system can be equipped with a VPN connection for remote control, trimming and supervision by Yara.
Installation and commissioning.
The installation works on site require piping for reagent, water and compressed air between the modules. The pipes for the reagent shall be made in stainless steel. The electrical works are limited to power connection of the modules and communication cables between them and to the DCS.
Ammonia or urea?
So far there are two different types of reagents that have been used in SNCR systems.
Which one to use?
There are several aspects to take into account when choosing the reagent.
CAPEX and OPEX
Below, see Table-6, are two examples showing the investment and the annual operational costs for a typical SNCR installation at a cement clinker plant at two different NOx reduction rates. The estimations are based as far as possible on conditions in India.
The first example is with a NOx reduction from 1200 to 800 mg/Nm3 dry at 10% O2 and the other example from 1000 to 800 mg/Nm3. Typical comparison between 25 per cent ammonia solution and 40 per cent urea solution is done.
From the examples it is clear that the dominant cost is by far the consumption of reagent. And ammonia solution gives a substantially lower cost than urea due to its normally much higher efficiency.
It can also be noted that the annual OPEX is on the same level as the total investment. So in order to minimise the OPEX, the design and installation of the SNCR is worth paying careful attention to. An SNCR installation which has an efficiency of, 20 per cent more than another one, will quickly offset a possible higher investment.
Some of the main factors that make an SNCR installation efficient are:
High performance SNCR
This article has been authored by CH Persson, Yara Environmental Technologies AB, Sweden
Table-2 NOx emission limit values in Europe
Table-6 OPEX and CAPEX examples OPEX and CAPEX