International Sugar Journal

An innovative solution for air pollution control in spent wash-fired incineration boilers: A case study from Daurala Sugar Works

Abstract

Following the inception of  “Spent Wash fired Incineration Boilers” to reduce the water pollution, as part of the “Zero Liquid Discharge” policy enforced by the Central Pollution Control Board of India for molasses-based distilleries, the Industry has been facing severe issues with “Air Pollution Control” (APC) on such installation. Due to the complex nature of dust generated from the combustion of concentrated spent wash, the conventional APC devices have not been able to satisfy the end-users on most fronts, i.e. Results, Operability & Maintenance. The authors, being at the forefront of developing & bringing new technologies in this field, recognised the limitations of conventional technologies on this particular application and started looking for an economically viable solution. After extensive research, an integrated model was concluded in the form of MEPA (maximum efficiency particulate abatement)  and the first installation on a spent wash-fired incineration boiler was successfully installed and commissioned at Daurala Sugar Works in Oct 2019. MEPA is an integration of DESP with a highly efficient polishing filter known as EGB Precipitator.

Abbreviations: MEPA- Maximum Efficiency Particulate Abatement, DESP-Dry Electrostatic Precipitator, EGB-Electrified Gravel Bed, BF-Bag Filter, WS-Wet Scrubber, WESP-Wet Electrostatic Precipitator, MCR-Maximum continuous rating, TPH-Tons per hour, EFB-Empty Fruit  Bunch, WFGD-Wet Flue gas Desulphurization, DSW-Daurala Sugar Works, EEPL-Enviropol Engineers Pvt.  Ltd.

Introduction

MEPA is designed for two-stage separation (Figure-1). The primary cleaning is achieved using conventional dry ESP.

Figure 1: MEPA

The final stage separation is done using  EGB to achieve relatively uniform results with no possibility of rapper re-entrainment.

The EGB can be designed to achieve outlet emissions of even below 10 mg/Nm3. 

Why MEPA

The fly ash from spent wash incineration boilers contains a high percentage (+ 40 %) of potassium and sodium salts, which are sticky and hygroscopic. Besides,  the particle size distribution of this ash reflects a very high percentage of ultra fines ( about 40 % below 10 microns).

The presence of such complex dust in high quantity (about 30 gm/Nm3) is always a challenge for any standalone equipment like Dry ESP or BF. Wet Technologies like WS or WESP are not viable for this application as the same will re-produce liquid effluent.

The prevailing technologies like Dry ESP and BF deployed on this application do still have the following concerns :

Dry ESP

  1. Inconsistent results due to rapping entrainment and escaping ultra fines
  2. Sticky nature of fine dust causing higher build-up & less effective rapping
  3. Varying dust resistivity due to combination firing

Bag Filters

  1. Restricted operating range due to varying flue gas temperature ( 180-225 ºC)
  2. Limited life of costly bags- high recurring cost
  3. Fire hazard

These limitations on prevailing technologies led the authors to develop a two-stage solution in the form of MEPA under a technology tie-up with a US company.

MEPA- how it works

MEPA is an integrated system where the primary cleaning is done using conventional DESP of 2/3 fields. The fields are designed with multi-stage rapping allowing the ultra-fines to escape the polishing filter called EGB.

Salient features of EGB

The EGB is another form of the high-performing dry precipitator. The EGB is used as the final stage separation to remove sub-micron particulates with other left-out impurities from process and flue gases. This technology is best suited for all such complex situations as spent wash, municipal solid waste, black liquor, EFB, palm shell and fibre, besides other biomass.

This technology has also been used successfully to upgrade existing ESP installations to meet the most stringent emission norms in power & cement on coal and lignite-fired applications.

How EGB Operates

 The ionizer electrode; The particulate in the gas is negatively charged in the ionizing section. It usually runs at 20-40 kV. The highly effective process electrically charges the particulates to near saturation point. The polarity is negative. The ioniser can be avoided if the EGB is placed downstream of DESP as part of MEPA.

The bed electrode; The gas is forced through the gravel bed (Figure-2). The gravels are polarized using embedded electrodes. It normally runs at 10-20 kV.

The negatively charged dust adheres to the positive side of the polarized gravel surface.

Figure 2: Polarized Gravel Bed

System description

The gases (1) entering the filter body (2) pass through an ionizing chamber (3) down into the filter body (4). In the ionizer, the particulates are negatively charged. If necessary, the ionizer is cleaned periodically by a suitable automatic brush system weekly (figure 3). The periodicity of cleaning varies once in 2 to 7 days, depending on the nature of the dust. However, for MEPA installations, an ionizer is not required as the particles are already carrying a negative charge while escaping from integrated ESP.

The gases are transported by the fan (5). The gases distribute equally and pass through the polarized filtering bed (see figure 3).

Figure 3: EGB system

The gravel bed is a cylindrical ring contained internally by a series of taper-shaped steel rings (louvers), installed so that the gravel cannot spill through them and that the fumes can enter freely.  Inside the gravel bed, there is installed an electrodes cage.  The positive electrical potential is 10÷20 kV. As a result, the particulate in the gases, previously negatively charged, is attracted by the gravel and sticks to it.

The gravel moves slowly downwards. At the bottom, the dirty gravel flows through the manifold to a bucket elevator (6). Nest, the gravel flows through a wind-sieve (8) before the cleaned gravel enters the surge bin (11).

Also, the gravel can be dumped separately from any single unit in case of maintenance without stopping the filter (see figure 4).

   Figure 4: Pea-sized gravels

 

The particulate released from the surface of the gravel is separated and transported by the fan (10), settled in the cyclone (9).  The transport gravel re-enters the flue-gases upstream of the ionizer.

Collection efficiency 

Collection efficiency of the filter, in the function of the particulate dimensions and of the bed electrodes voltage is shown in figure-5.

It can be noticed that, even without a high voltage field, a good collection of particulate with dimensions above 2.0 µm is achieved.

Figure 5: Efficiency curve 

 

Major advantages of EGB as compared to other technologies

 

  • No slipstream and even gas distribution 

The gases are evenly distributed over the entire precipitation surface.  Therefore, all the gases have to pass through the bed and the precipitation zone.

  • Energy saving 

The EGB filter is equipped with few electrical devices; all have low power except the ID fan. The power need for both High Voltage fields is extremely low. The power consumption to clean a gas flow of 100.000 m3/h to lower than < 10 mg/Nm3 can be as low as 1 kW. 

  • Excellent for high resistivity dust 

In the middle of 2018, DSW started exploring the installation of a spent wash incineration boiler for their distillery plant. Accordingly, they started building the project specifications in consultation with original equipment manufacturers. However, considering the boiler specifications (34 TPH), the team at DSW was not comfortable with either of the technologies (ESP & BF) deployed till then for Air Pollution Control on this application. 

Case study – Daurala Sugar Works (DSW)

 In the middle of year 2018, DSW started exploring the installation of spent wash incineration boiler for their distillery plant. Acccordingly, they started building up the project specifications in consultation with original equipment manufacturers. Having frozen for the boiler specifications        (34 TPH), team DSW was not comfortable with either of the technologies ( ESP & BF) deployed till then for Air Pollution Control on this application.

Phase-I: Study and decision making

Team DSW visited various installations of incineration boilers and found issues with the prevailing technologies at most places. With ESP, the outlet emissions were reported to be fluctuating as the day progressed after periodic boiler cleaning and during rapping cycles. While on bag filter installations, even though the emissions were not an issue, end-users were not satisfied with its restricted operating range (180-210 ºC), need for highly vigilant operation, the threat of fire and limited life of costly bags.

Viewing this, team DSW contacted the authors, who were already working on an alternative technology to resolve these concerns. This is how the MEPA came into the discussions on this application.

The team visited Sweden to see a working installation of EGB (Figure 6) on a wood-fired application. The data collected from this site revealed that the EGB installation was running for the last 15 years on 30 TPH wood-fired boiler and consistently producing results below 10 mg/Nm3. On the maintenance front, only the electrode cage was replaced after 14 years.

After a detailed analysis, the team at DSW concluded in favour of MEPA.

Figure 6:  EGB in Sweden

Phase-II: Manufacturing and installation

 The Boiler GA drawings were re-drawn to accommodate the new APC system – MEPA, as shown in figure 7. This economised the overall length of the boiler by about 10 m compared to conventional technologies.

                Figure 7: Elevation-incineration boiler with MEPA

The order for a complete APC and ash handling system was awarded to Enviropol for the design, manufacturing, supply, installation and commissioning of the entire package as a sub-package to the ongoing boiler project.

Figure 8: Actual MEPA Installation at DSW

The complete package was manufactured within 5 months and commissioned with the boiler in October 2019 (Figure -8). 

Phase-III: Commissioning and adjustments 

The MEPA system was implemented and showed exemplary emission control results. The initial figures recorded were 20-27 mg/Nm3 at 60% MCR loading as against the designed figure of 50 mg/Nm3.

However, after 2 weeks of operation, current fluctuations were noticed in the second field of DESP beyond 6 TPH of spent wash firing.  An increased pressure drop across EGB followed this.

Data analysis over the next 2 months of boiler operation, under varying load conditions, provided a clear direction to improve the working and running of the system at Boiler MCR conditions while maintaining the desired pressure drop.

Accordingly, an improvement plan was drawn and executed in phases during planned shutdowns over the next 6 months.

The rapping mechanism was strengthened on the ESP front while improving the gas distribution in both fields. The EGB was also upgraded by increasing the gravel speed to deal with extra dust load from ESP.

This proactive approach led MEPA to perform consistently even at Boiler MCR conditions – (i.e. spent wash incineration @ 11 TPH ) with bagasse/rice husk as supporting fuel for the last 24 months. The outlet emissions are within 50 mg/Nm3 @ 135-150 mmwc (millimetres per water column) pressure drop. 

Phase-IV: Operation and testing

The boiler has been operating for almost a year. This being the first installation in the world on this application, a series of tests were conducted to capture real-time data to validate the design calculations and create a feedback mechanism for the next installation.

The complete MEPA & Mechanized Ash Disposal System is controlled and monitored through a dedicated control desk, as shown in figure 9.

Figure 9: Control panel – MEPA Screen

The fly ash from all boiler streams is collected in a typical ash silo and directly loaded in trucks. This ash, being rich in potash, is usually sold @ INR300-350/ ton generating a revenue of about INR9 million-10 million annually. Further opportunity also exists to generate more revenue by producing saleable gypsum using WFGD downstream of EGB.

EGB has recently been tested for SOx reduction using hydrated lime as sorbent, and the initial findings are encouraging. Team Enviropol will conduct a few more tests to validate its use as “Multi-Pollutant Removal System”  across varying applications. 

Maintenance

Almost a year of operating experience on this particular application and past data available from over 90 installations of EGB by the technology provider reveal that no special maintenance needed for this equipment. It is quite a sturdy and stable device requiring only the replenishment of gravels @ 8-10 % per month. The overall maintenance cost is limited to <  50 % of the cost incurred for maintaining bag filters /ESP.

The integrated DESP follows the standard maintenance schedules already established for these applications. 

Other facts 

EGB, as a stand-alone APC device, can also be used directly with low ash fuels as a Multi-Pollutant Removal device. Opportunities also exist for those who are due to upgrade their old ESP for lower emissions but unable to do so due to space constraints or are susceptible to achieving the desired emissions even after adding extra fields due to the remaining dust being very fine  (all below 5 microns).

The existing ESP can be upgraded by adding EGB downstream, as shown in figure 10.

Applications 

·         CPP– Coal /Lignite fired Boilers

·         Paper– Black Liquor Recovery Boilers

·         Cement – Kilns & Clinker Coolers

·         Sugar- Bagasse fired Boilers

·         Distilleries– Spent wash Incinerators

·         Chemical – Other Incinerators

·         WTE –  MSW Incinerators

·         Palm Mills– EFB, Palm shell & Fibre

·         Cogen– Mustard, Trash, Rice husk etc.

Figure 10:  ESP upgrade

Conclusion

MEPA/EGB Technology is best suited to achieve very low emissions on demanding applications such as spent wash, EFB, Palm, black liquor & MSW etc., where the dust resistivity is high or unknown with the sticky nature of dust. This has compact footprints and low power consumption compared to other conventional technologies. MEPA can be further upgraded to remove other pollutants.

This technology can also be retrofitted old ESPs to meet the most stringent emission norms.   EGB, being very compact, can be fitted easily within a limited space.

 

Acknowledgements

The authors are grateful to the Management of DSW for providing this opportunity and taking another commendable initiative (after putting up the first of its kind Hybrid Dryer in 2018), to opt for the installation of the first MEPA on their new spent wash fired incineration boiler.

Further, the authors would also like to thank their Swedish & US Technology partners for their active cooperation during the course of the development of this concept and successful execution.

Bibliography

PhD thesis -1978 @ MIT by Dr Alexander, Data analysis & Literature published -1995 by J.I & Env.AB