Causes of Poor Performance of ESP's (Electrostatic Precipitators)

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Causes of poor performance of ESP & discussions

There are many causes of poor performance of ESP (Electrostatic Precipitators). Some of the reasons are:

·       Excess gas volume

·       Poor gas distribution

·       Tracking and air inleakage

·       Electrode breakage

·       Ash resistivity

·       Particle size

·       Electrical conditions

·       Over-full  hoppers

·       Defective collector plates

Excess gas volume

If the gas volume is high, the gas velocity through the precipitator will be high so that the time available for the charged particles to migrate to the collector plates is reduced, possibly to the extent that the particles pass out of the working zone before they can be captured. The normal gas velocity is about 2 m/s, but this may be increased by:

·       Tramp air in leakage at the boiler.

·       High air heater gas outlet temperature causing increased volume flow.

·       Poor condition of rotary air heater seals.

·       Operating the boiler with too much excess air.

Poor gas distribution

It could be that the total gas flow is satisfactory but the spatial distribution is poor, causing good performance where the gas velocity is low and for it to be poor where the velocity is high. To be completely acceptable, all the traverse point velocities should be within 25% of the average for the duct.

Tracking and air in leakage

This is a common cause of trouble because precipitator structures are physically large and have numerous access doors, etc, at which leakage can occur. If the in leakage is sufficiently high, a draft of cool and possibly moist air will pass over the insulators, steady bars, etc. As the voltage inside a precipitator is very high (40-50 k V), tracking paths may result. Therefore, it is important that access -door seals, rapping rod covers, expansion joints and other ingress points are kept in good condition and are airtight.

Apart from above hazards, there are the additional disadvantages that in leakage increases the mass flow of the gas and lowers its temperature.

Poor rapping

In the usual precipitator arrangement the wires are negatively charged and the collector plates are earthed. In the immediate vicinity of the wires the electrostatic field is so strong that corona discharge occurs. The corona ionizes the flue gases and the entrained solid particles acquire negative ions, causing them to migrate towards the relatively –positive collecting plates. A few particles acquire positive ions and theses migrate towards the discharge electrodes.

The dust which adheres to the wires and collecting plates is periodically removed by rapping.

The timing of rapping operation is important as damage can be done to the plant if it is too frequent. Also, every time rapping takes place some of the dislodged dust is re-entrained, so reducing the collection performance. On the other hand, if the interval between raps is too great, the dust build-up on the wires and the plates will be sufficient to interfere with the operation of the plant and this also will reduce its performance. Efforts should be made to determine the optimum timing.

Upon inspection, the collector plate dust –layer should be less than 1 mm for high resistance dust and 3 mm for dust from coal with high sulphur. The discharge electrodes should have a very light deposit.

Electrode  breakage

The discharge electrodes consist of relatively thin wires which may be of flat, circular, star or square cross-section. In addition, they may be barbed or plain, twisted or straight and rigidly or loosely fastened, besides being made from a range of materials. Whichever arrangement is adopted, the essential requirement is that the wires should have excellent reliability. Out of the huge number in a zone, it only needs one to break to cause short-circuiting and electrical instability generally, which may seriously affect the performance of the plant.

Spark erosion is a common cause of wire failure.

The ‘unbreakable’ type of wire is becoming popular because of its inherent reliability. One method of limiting the effect on performance by breakage is to divide a complete zone into four sub-zones, so that a broken wire only affects a quarter of the zone.

Ash resistivity

An important factor in precipitator performance is the electrical conductivity of the ash itself. Poor combustion can result in a higher than normal carbon content in the ash. This causes the particles to have a lower resistivity which may cause them to be difficult to collect. This means that they can be readily re-entrained by the gas, particularly during rapping. On the other hand, if the resistivity is too high, the dust will accumulate on the collector plates and form an insulating layer, so preventing subsequent particles from surrendering their charge. Consequently, the charge on the surface layer will repel incoming dust, causing re-entrainment. In addition, a very high voltage- gradient will be built up across the dust layer which could result in flashover. Dust with high resistivity will suppress the corona discharge, seriously lowering the performance of the plant.

The resistivity of the ash is a function of the surface layers of sulfuric acid, salts and moisture, particularly the acid. If the coal contains about 2% sulfur, the resulting sulfuric acid deposition creates a desirable level of dust resistivity. If the sulfur content of the coal is very low, a highly resistive ash may be produced which is very difficult to precipitate. One remedy for this is to inject small quantities of suitable additives, such as ammonium sulfate, into the flue gas.

The desirable range of resistivity is between 10⁶ and 10¹² ohm-cm. Should the normal fuel supplies be low in sulfur and a change to alternate supplies be uneconomic, another possible remedy besides injection is to install ‘pulsed  energization’. In this, high frequency pulses are added to the rectified wave form. The resulting high ion-density corona acts along the length of the discharge electrode instead of just at discrete locations, so the dust particles are more easily precipitated. The improved performance is called the Enhancement factor given by (pulsed migration velocity)/ (unpulsed migration velocity).

Particle size

Particle sizing has an important bearing upon the efficiency of precipitations. Up to approximately 20 µm, the electrical and the drag forces combine to give a deposition velocity which increases in roughly efficient for sizes over 20 µm, the only problem being that of slight re-entrainment. Dust less than 20 µm is more difficult to capture, so the stack emission will contain a preponderance of fine particles.

Electrical conditions

Figure below shows the relationship between electrical power input and   precipitator efficiency. As one would expect, increased power results in improved performance, provided the power is useful.

For example, flashover and tracking will give high power consumption, but much of it is wasted. To find the demarcation between useful and wasted power, a simple graph is determined from tests on the plant. Starting at a low value the current is increased by suitable increments, noting the corresponding voltage. A typical diagram is shown below:

Notice that the current and voltage both increase until the voltage is about 45 k V. Thereafter increasing current is accompanied by reducing voltage. The knee of the curve determines the demarcation between stable and unstable operation, the highest stable current here being a little over 200 m A. The automatic voltage control always tries to operate the plant with the highest stable power input which can be maintained.

Causes of electrical instability include:

·       Tracking , possibly due to air ingress

·       Arcing and poor connections, possibly at sub-zone isolation selectors

·       Ineffective rapping

·       Overfull dust hoppers, causing bridging of electrodes

·       Misaligned electrodes

·       Broken discharge electrode wires

Defective collector plates

The plates are mechanically strong, but can become defective if the gas temperature falls below the dew point, allowing deposition of dilute acid. This can occur in discrete areas if cold air inleakage takes place. Alternatively, general corrosion can occur if the flue gas itself has a low temperature, possibly due to massive air ingress at defective seals on the ducts between the air heater and the precipitator, or if the ducting is corroded and holed. Frequent cold-starting of the boiler can also lead to low- temperature gas at the precipitator.

We undertake all types of works for Dry ESP  under Overhauling, Performance Enhancement, Technical troubleshooting and Erection Works. Visit our site for details.

ESP Troubleshooting Guide

We undertake all types of works for Dry ESP  under Overhauling, Performance Enhancement, Technical troubleshooting and Erection Works. Visit our site for details.

Electrostatic precipitators (ESP's) are used to reduce particulate emissions from blast furnaces, sintering operations in the steel industry, and for fly ash control from industrial and utility boilers. ESPs have been designed to collect particles with diameters of from 0.1µm to 10µm; collection efficiency is considered high, sometimes exceeding 99%. The ability of ESP's is to handle large exhaust gas volumes at temperatures between 175 and 700C.

An electrostatic precipitator contains the following:

• Discharge electrodes

• Collection electrodes

• Electrical system  

• Rappers

• Hoppers

• Shell                         

The discharge electrode is usually a small-diameter metal wire. This electrode is used to ionize the gas that charges the particles and to create a strong electric field. The collection electrode is either a tube or a flat plate with an opposite charge relative to that of the discharge electrode. The collection electrode collects charged particles. The electrical system consists of high voltage components used to control the strength of the electric field between the discharge and collection electrodes. The rapper imparts a vibration or shock to the electrodes, removing the collected dust. Rappers remove dust that has accumulated on both collection electrodes and discharge electrodes. Occasionally, water sprays are used to remove dust from collection electrodes. These precipitators are called water –walled ESPs. Hoppers are located at the bottom of the precipitator. Hoppers are used to collect and temporarily store the dust removed during the rapping process. The shell structure encloses the electrodes and supports the entire ESP.

 Electrostatic Precipitator Troubleshooting Guide

 

Symptom	Cause	Solutions
Corrosion Damage	Air Leakage	- Replace leaking door gaskets - Replace damaged rapper penetration seals - Replace damaged hammer drive seals - Repair leaking duct work test ports - Repair/replace leaking expansion joints - Repair leaking weld seams - Repair holes in casing - Check for reduced gas temperature
	Poor insulation design	- Re-insulate ESP hot roof to penthouse corners - Check/replace/install new hopper heaters
Deposits on Plates & Electrodes	Improper or poor rapping	- Verify rapper / vibrator systems operating - Inspect for binding or broken rapper shafts - Inspect for bound collecting plates
	Moisture / Condensation	- Check for air leakage, repair as noted above - Check for improper or poor insulation
	Change in gas chemistry	- Check for air leakage, repair as noted above - Check for reduced gas temperature - Check for change in operating conditions, fuels
Stack puffing (re-entrainment)	Improper or poor rapping	- Verify rapper / vibrator systems operating - Inspect for binding or broken rapper shafts - Inspect for bound collecting plates - Adjust control rapping schedule & intensity
Heavy dust buildup	Gas flow distribution	- Check for damage to inlet and outlet nozzles, baffles, louvers and perforated plates - Modify turning vanes or baffles
Control tripping	Grounding	- Clean insulators of all dust & condensation to prevent tracking - Check insulators for cracking - Check plate & electrodes for clearances & straightness - Check for broken wires - Check hopper level

Outlook on Cogeneration or CHP (Combined Heat and Power Plant)

Following survey was sent us to present our views on the subject of Cogeneration or CHP (Combined Heat and Power Plant). Our view/response is in red color.

Cogeneration or CHP (combined heat and power) is the simultaneous production of electricity and heat using a single fuel/ mix such as bagasse, natural gas, coal, waste gas, biomass, liquid fuels and renewable gases. The heat produced from the electricity generating process (for example from the exhaust systems of a gas turbine) is captured and utilised to produce high and low level steam. The steam can be used as a heat source for both industrial and domestic purposes and can be used in steam turbines to generate additional electricity (combined cycle power).

1. Average capacity of boiler used in CHP application? (In terms of MW or Tons per hour) –

Varies as per individual plant needs

2. Which industry generally uses boilers having a heat output greater than 80 MW or 100-120 Tonne/hour? (Sugar, Paper. Petrochemical etc)

Iron & Steel, Aluminum & Sponge iron

3. In my research I came to know that the sugar industry has the maximum potential but the boilers used are in the range of 20-30 MW. So I am unsure where these high capacity boilers can be used.

Sugar industry normally uses CHP plants of 20-30 MW capacities.

4. What are the types of boilers used in CHP applications? Are there any other boiler besides those mentioned that are used in the CHP applications?

Some of the types that I have come across are:
a. Multi-fuel for coal, peat, biomass and waste.
b. Waste to energy type – Wastes (MSW - Municipal Solid Waste, others) as fuel.
c. Gas based ones (HRSG, Heat Recovery Steam Generators).

These are the main classification what you have mentioned. It is o.k

5. What is the most common type of boiler used in CHP application in India? (As per your estimate)

Fluidized bed boilers – AFBC/CFBC& HRSG

6. Please indicate the type of design used in each case (e.g. fluidized bed, moving grate, others-please specify)

a. Multi-fuel for coal, peat, biomass and waste

FBC boilers and stoker fired boilers

b. Waste to energy type – Wastes (MSW - Municipal Solid Waste, others) as fuel

Mostly AFBC

c. Gas based ones (HRSG, Heat Recovery Steam Generators)

HRSG

d. Others

7. What are the typical pros and cons of the different types of boilers –

a. Multi-fuel for coal, peat, biomass and waste

In India, we use indigenous and imported coal, lignite, pet coke, biomass, washery rejects or solid waste as the fuel and the type of fuel to be used is dictated by the heating value, cost and distance from its source.

Stoker fired boilers are becoming outdated now thus leaving the choice limited to AFBC or CFBC. For small capacity boilers (say, below 50 TPH), AFBC type is preferred for techno-economical reasons.

b. Waste to energy type – Wastes (MSW - Municipal Solid Waste, others) as fuel

AFBC type is only used.

c. Gas based ones (HRSG, Heat Recovery Steam Generators)

The locations where availability of natural gas is competitive to other solid fuels, cogeneration plants employing gas turbines and HRSGs are favoured.

Heat recovery from waste gases (in industries of steel, sponge iron, copper, etc. ) is made through conventional water tube boilers to generate steam for process needs or power.

d. Others

8. What would be your specific considerations, if any, for choosing a boiler design, and steam conditions (e.g. higher the steam pressure, the better it is) ? i.e. critical things you would want to know from a CHP system owner/buyer, before selling a boiler.(e.g. fuel choices available to the client) ?

The various factors that we consider are:

• The total power requirement (for steam/heat calculations)

• Power for sale or just internal consumption

• Type of fuels available

• Steam/heat requirement of the main process (pressure and temperature)

• Investment available

• Expansion plans

9. What are the most common types of fuels available in India, based on your ideas about the industry ?

We have witnessed following types of fuels in our works:

• Coal – Domestic, Imported, Washery reject

• Lignite

• Pet coke

• Gas – Natural gas, Bio-gas, Waste gas

• Biomass –

o Agricultural wastes –Rice Husk, Paddy Husk, Chicory (coffee beans), date palms, dry leaves, trees, wood extracts etc.

o Industrial wastes – Paper pulp, Rubber extracts

10. What are the key trends affecting the co-generation industry in India?
(Major growth drivers and restraints)

Growth drivers:

o To reduce power and other energy costs.

o To improve productivity and reduce costs of production through reliable uninterrupted availability of quality power from Cogeneration plant.

o Cogeneration system helps to locate manufacturing facility in remote low cost areas.

o Improves energy efficiency, and reduces CO2emissions therefore it supports sustainable development initiatives.

o The system collects carbon credits which can be traded to earn revenue.

o Due to uninterrupted power supply it improves working conditions of employees raising their motivation. This indirectly benefits in higher and better quality production.

o Cogeneration System saves water consumption & water costs.

o Improves brand image and social standing.

o Cogeneration is the most efficient way of generating electricity, heat and cooling from a given amount of fuel. It saves between 15-40% of energy when compared with the separate production of electricity and heat.

o Cogeneration helps reduce CO2 emissions significantly. It also reduces investments into electricity transmission capacity, avoids transmission losses, and ensures security of high quality power supply.

o A number of different fuels and proven, reliable technologies can be used.

o A concurrent need for heat, electricity and possibly cooling indicates suitable sites for cogeneration.

o The initial investment in cogeneration projects can be relatively high but payback periods between 3-5 years might be expected.

o The payback period and profitability of cogeneration schemes depends crucially on the difference between the fuel price and the sales price for electricity.

o Global environmental concerns, ongoing liberalization of many energy markets, and projected energy demand growth in developing countries are likely to improve m