Make the difference
The concept of optimisation is considered one of the keyways of permanently resolving issues impacting OPEX and helping a business be sustainable. This study shows how measure the gap between the present and optimised operation levels of a dust collector system and decide on how convenient it is to proceed given existing conditions by focussing on particular areas of expertise – performances, reliability of technology, quality and savings.
In this case, the existing dust collectors on kiln / raw mill of the cement plant are not performing according to the design conditions, the initial guaranteed value of pressure drop is always exceeded inevitably leading to the main following conditions:
- premature bags failure, mainly due to overcleaning to manage the very high pressure drop across the filter
- extremely high compressed air consumptions
- high power consumptions of the fan
Some examples of the criticalities highlighted by a preliminary analyse were related to the internal filter design, the exit available area reduced, the bottom, vertical and lateral gas velocity. Rendering the existing filter with a 3D model suitable for running a CFD simulation combined with a tailored process data collection campaign, allow both to perform a detailed gas behave analysis and rectify the mechanical criticalities affecting the equipment performances. This new approach “OTP OPTIMISE PERFOMANCESR” passes through the deep process analysis and equipment validation and the study of a comprehensive solution to optimize the existing bag filter with low impact modifications and saving capex.
The main results and sustainable goals achievable have been:
- optimized gas distribution restoring the optimal and safe operating conditions
- compressed air consumption reduced from 25% to 40%
- number of existing cleaning valves reduced of 50%
- extension of bags and cages lifetime of 100% (from 30 to 60 months)
- reduction and stabilization of filter pressure drop (≤ 150 daPa)
- reduced maintenance costs: no need expensive filter media
- Total Cost Ownership (TCO) < 17,70%
- Environment: 1.668.858 Am3/h Gas volume cleaned / 84t/h of dust collected
- H&S: rarely maintenance interventions (lower frequency)
The case we are going to present is related to an existing process dust collectors installed in a cement plant and dedusting the kiln/raw mill exhaust gas from two different cement production lines (respectively approx. 6000tpd and 4000tpd each).
End-user, after trying filter bags replacement and other actions without a successful result, requested OTP analysis and recommended solution due to the following main recurring issues on both dust collectors :
- High dust emissions at stack;
- High compressed air consumption compared to the equipment datasheet;
- Continuos failure and replacement of filter bags;
- High differential pressure across the filter impacting on FAN performances and related energy consumption as well as on upstream process handling.
Below we report some key data related to the bigger dust collector which we are going to present in next pages :
After a preliminary analysis of the main key indicators as air to cloth ratio and can velocity (ascend speed) it is pretty clear that there are criticalities to be deeply analyzed.
On a different perspective and especially considering this is a dust collector resulting from a previous electrostatic precipitator conversion where existing casing and dust transport system have been reused, approach cannot be limited to a theoretical calculation of the a.m. key indicators but requires to deepen our study and analyse in detail the gas behave and the real criticalities.
Thanks to the cooperation with the end user by providing existing 2D documentation and some measurement from site taken together with our supervisor, OTP mechanical department could render a faithful 3D model (pic. 1) of the existing dust collector and its internal parts (i.e. inlet duct, chambers dirt side, chambers clean side, outlet duct, dampers etc.).
This activity allowed to set a step-by-step approach
1) Deeply analyse the process conditions and the possible ameliorations to gas distribution through CFD study;
2) Verify the perfomance and quality of installed filter components (cleaning system, filter bags, filter cages etc.) and identify any possible improvement;
Process analysis and CFD study
The above 3D model, in combination with the process data provided by end-user and confirmed by OTP site measurements, allowed to OTP internal CFD department to simulate the gas behave (pic. 2) and draw an exact “as is” condition of the dust collector in the different operation:
Above 3D model creation and CFD study, together with the CTP Team / OTP experience and continuous R&D in dust collection technologies, allowed to highlight some criticality to be corrected as, but not limited to, the direct gas flow approaching the first rows of filter bags and a bad overall gas distribution.
What above provided also the confirmation that results coming from the theoretical key indicators calculations (i.e. air to cloth ratio and can velocity) could not reflect the real situation.
After internal OTP review of all the results a first solution was elaborated consisting in following main activities :
- Taking advantage of the existing casing dimensions, bags and cages have been designed 2m longer taking the overall length from 8m to 10m and recovering the needed cloth area to comply with the high volume of flow ;
- A series of internal baffles have been designed in order to better distribute the gas into the filter and avoid a direct and harmful approach to the first raw of filter bags (pic. 3)
A new CFD study on the new filter configuration was executed highlighting several benefits on overall gas distribution and avoiding a direct impact on the first rows and previously most damaged filter bags (pic. 4 and pic. 5)
Other actions and improvements
After the optimisation carried out on process and gas distribution, a second step has performed to evaluate the room of improvement on other dust collector components.
A first important action was performed on the installed cleaning system based on four rows of compressed air reservoir with in total 334 diaphgram valves and where each valve was cleaning n. 12 height meters filter bags.
Thanks to the CTP Team SWAP cleaning system (below briefly described) and its high performances, two entire rows of cleaning tanks were removed allowing to halve the total number of installed cleaning valves (167 Vs. 334).
In the new configuration each valve cleans 24 ten meters length filter bags (pic. 6)
Cleaning System SWAP (Sonic Wave Acceleration Pulse)
The system get improved efficiency results and doesn’t require any injectors, Venturi or other accessories on the top of the bags, to increase the secondary air in bag regeneration.
To get sufficient energy to separate the dust from the bags surface is necessary a cleaning system capable of strong accelerations to the “dust-bags” system, leading the separation with high forces in very short time.
This feature can be achieved through two basic principles:
- Special valves that allow a speed of mechanical opening of the membrane of about 20-40ms. (See section Pulse valves open and integrated membrane valves)
- Maximum use of the law aerodynamics (impulse force).
This effect is related to physical parameters connected with:
- Cleaning pressure (from 2.0 to 3.0 bar)
- Optimized ratio between diameter holes in the blow pipe and diameter of bag.
Optimisation of the parameters mentioned above, allows the cleaning of the bags with sonic waves which pass through the bags with a speed of ~Mach 1, in progressive way, avoiding to make a clean not only swelling the bags like other traditional systems normally done.
The SWAP cleaning system can clean in ON-LINE mode bag with a length up to 12m.
Low pressure cleaning system
(SWAP (Sonic Wave Acceleration Pulse))
In addition, the forces located at the sonic wave, have much higher efficiency in comparison with traditional cleaning system (5-6 bar) with bags maximum length of about 6m with Venturi. For this reason, the traditional systems with bags longer than 6m (eg. 8m) are forced to perform clean-OFF-LINE
Bags cleaning and filter control is done through a control board which control the bags cleaning procedure sending information to the main control room concerning the pneumatic valve position and status.
Bags cleaning cycle can be controlled both by filter Δp and by timer.
From an apparently “simple” filter bags premature failure issue and the OTP approach allows to identify the following issues:
- Too high flow rate compared to the installed filter cloth area;
- Not optimal gas distribution into the dust collector;
- Some dust collector’s design criteria and geometry to be reviewed;
- Some misalignment between blow pipes and tube sheet holes;
- Obsolete and not effective adopted cleaning system and cleaning logic.
These main corrective actions have been implemented :
- Filter bags and cages elongation to cope with the gas volume handled;
- New baffles and internal modifications to correct and optimize the gas distribution;
- New cleaning system for an effective filter bags cleaning including new adaptive cleaning logic;
- Engineering, supervision and commissioning by CTP Team and OTP.
The benefits reported:
- Controlled dust emission at stack complying with local authority regulations;
- Differential pressure reduction across the filter :
- Stabilization of ID fan operation lowering the FAN energy consumption;
- Stabilization of cleaning demand to filter bags lowering mechanical stress to the filter cloth, increasing filter bags lifetime and lowering compressed air consumption;
- OPEX reduction thanks to the reduced maintenance demand from the system
OTP is the business unit that assists all customers, providing a reliable and prompt actions during all the life of the Air pollution control systems
Ensuring technical support upon the request in terms of engineering, quality and technical specifications, the division acts in three main areas of focus:
- Existing process conditions and installed equipment Overall Evaluation, identifying main Critical Points
- Analysis and Optimization of Gas Distribution by in-house filter model rendering and CFD modelling
- Analysis and Optimization of Existing Equipment Key Components Specifications and Design