BY RACHAEL GARDNER-STEPHENS

BY ROBIN SAMUELS, JOURNALIST, PI MAGAZINE ASIA & VICHARN TANETSAKUNVATANA, PRODUCTION DEPARTMENT MANAGER.

MAE MOH POWER PLANT PRODUCTION DIVISION, EGAT

A ROOM WITH A VIEW

MAE MOH POWER PLANT:

60 JULY/AUGUST 2012 POWER INSIDER

This special issue continues our collaboration with EGAT, and also sees a new plant review

feature as we take a tour in one of the largest lignite fired power generating facilities in South East Asia. Located in the Lampang province, in the northern reaches of Thailand, the Mae Moh plant is one of the more notable power generators in Asia, with a great story of tribulation, continuous expansion and consolidated effort by the state owned giant to effectively reduce emissions in consideration of neighbouring settlements. The plant was constructed in four phases from 1977 to 1996, as part of Thailand’s national energy development strategy to reduce the countries high dependence on imported fuel, by developing indigenous energy resources.

Almost 30 years on and the impressive facility stands tall with a total installed capacity of 2,625 MW over 13 units. Each of the first three units has installed generating capacity of 75 MW with stack height of 80 meters. Units 4 to 7 and 8 to 13 each

have installed generating capacity of 150 MW and 300 MW, respectively, with stack height of 150 meters. The last two units 12-13 were brought into operation in 1995.

Lignite used by the power plant to generate the electricity is excavated from the Mae Moh mine located adjacent to the power plant. Fifty thousand tons of lignite per day is used by the power plant. The average sulfur content in lignite from Mae Moh mine is 3% by weight on a dry basis. Sprawling across a total area of 135 square kilometres, mitigating emissions was always going to be a challenge with technology of the past.

THE INCIDENT AND IMPACT ON THE MAE MOH VALLEYDespite the huge role that the plant obviously plays in electrifying Thailand and ensuring a stable and steady supply, it has faced continuous opposition from neighbouring settlements and environmental corners

in relation to inevitable emissions from firing lignite. During 1992, in early October, the weather was

changing from rainy season to winter season in Thailand; the first sulfur dioxide incident caused by the Mae Moh power plant was officially reported. The seasonal transition ensured that high pressure atmosphere moved from China toward Thailand causing the air around the plant to engross in the phenomenon known as the Inversion Layer. The maximum hourly ground level ambient SO2 concentration was observed at 3,418 mg/m3 (hourly average standard of SO2 is 780 mg/m3). Back then, there were only 11 units in operation with a total installed generating capacity of 2,025 MW. The episode occurred soon after Unit 10 and Unit 11 (600mw combined) were brought into operation, resulting in additional SO2 emission of approximately 160,000 ton/year.

During the incident, large numbers of people living in several villages located downwind from the power plant sought medical attention for symptoms which included stinging nose and throat, cough, chest tightness, asthmatic attack, nausea, vomiting, dizziness, malaise and occasionally wheezing and shortness of breath. However, no death was reported. In addition to the reported health impact, damages to plantation and animals were also reported. The leaves of many trees, vegetable, and plants were reported to wither and fall to the ground overnight. There were also reports of livestock sickening and dying. The unfortunate events led to swift actions by EGAT to pioneer Thailand’s first multi-unit FGD configuration.

LONG TERM MEASURESIn order to solve the problem completely, the cabinet approved the additional installation of FGD system for units 4-7 of Mae Moh. Environmental responsibility had led EGAT to investigate retrofitting FGD systems at units 8-11, a complex feat for any global engineering contractor, this process was already under development at the time of incident. Unit 12-13 fortunately had an FGD system installed during the construction of the power plants. Mae Moh’s beginnings as a power plant at Fig 1. The Mae Moh power plant

MAE MOH POWER PLANT

‘EGAT’S MAE MOH POWER PLANT, HAS UNDERTAKEN A CONTINUOUS STUDY OF THE ENVIRONMENTAL IMPACT SINCE INCEPTION IN 1977, TO NEUTRALIZE THE NEGATIVE IMPACTS OF BURNING LIGNITE AND TO KEEP THEM WITHIN THE STANDARD VALUE OF THE OFFICIAL OF NATURAL ENVIRONMENT’BOARD.’

A ROOM WITH A VIEW

POWER INSIDER JULY/AUGUST 2012 61

Fig.2 Lignite Consumption at Mae Moh during 2011

Fig.3 Ambient Sulfur Dioxide levels in the Mae Moh valley over a 20 year period

unit 1 – 3, was coming to the end of its lifecycle and retrofitting to accommodate and operate FGD would simply not be cost effective. So in compromise and consideration of the environment the power plant unit 1-3, now only operates under clear and favourable weather conditions.

SHORT TERM MEASURES TO KEEP THE DOORS OPENThe long term measures were clear in strategy and expected results, but since the installations of the FGD units 8-11 were under construction and would not be complete until 1998, EGAT had given a strict set of rules for the plants to operate during winter periods of 1993-1997 to alleviate the problems that will affect the health of locals.

The plant’s capacity should be reduced under the unfavourable weather condition. (Between 1AM-12AM in 1993-1994 and 6AM-1PM in 1994-1997, the capacity was reduced to 700-1000 MW range)

The plant’s reparation (minor inspection/major overhaul) should be scheduled on winter period.

Mae Moh mine should stock up low-sulphur lignite (less than 2% sulphur) for using under the unfavourable weather condition during the winter of 1993-1997, and stock up on lignite that contains sulphur percentage of less than 1% from the external sources such as Lanna Lignite Co., Ltd., Banpu Public Co., Ltd., Chiang Muan Mine Co., Ltd. and Chaitharin Ltd, all for using during the winter of 1994-1997.

Operator should switch from general diesel to low percentage-sulphur diesel (0.5-0.6 percent sulphur) in an emergency situation whereas at least one of the pollution monitoring station shown that the concentration of sulphur dioxide (SO2) is rising.

Air quality monitoring system should be improved into Real-time Air Quality Monitoring and link its result to power plant’s control room (8 stations in total before November, 1994, and up to 12 stations after November, 1994).

Equipments for monitoring air quality and meteorological instruments should be purchased and installed properly. Currently, there is 12 continuous emission monitoring stations in total.

COOPERATION FROM OVERSEAS & HEAVY INVESTMENTEGAT’s Mae Moh power plant, has undertaken a continuous study of the environmental impact since inception in 1977, to neutralize the negative impacts of burning lignite and to keep them within the standard value of The Official of Natural Environment Board. In 1990, EGAT had already planned to install wet type FGD systems for units 12-13, which have the efficiency of eliminating approximately 92% of harmful sulphur dioxide. The installation of FGD units 12-13, went successfully along with the construction of the plants, which were commissioned later in 1995.

The technology installed to effectively monitor emissions has been world class with respect to air pollution control and combustion optimization, all thirteen units have electrostatic precipitators for particulate removal with control efficiency of 99.9%, and low NOx burners are used to control emission of nitrogen oxides. As for responsibility surrounding sulphur dioxide, EGAT instantaneously decrease power generation when sulphur dioxide concentration

is higher than standard value in the Mae Moh valley.In addition, EGAT also hired KBN Engineering

and Applied Sciences, Inc. from America to monitor and analyze the data of the environment surrounding Mae Moh power plant. At the same time, Pollution Control Department also co-worked with EGAT, while having budget supported from United States Agency for International Development (USAID), and Sargent & Lundy Engineers Corporation were hired to undertake the “Development of System Wide Emission Control Strategies Application to Mae Moh Power Plant” project. This study concluded that the optimized standard value of sulphur dioxide concentration for Mae Moh power plant should be 1,300 microgram per cubic meter per hour, and installation of wet type FGD system for units 4-7 should be undertaken.

During November 1999, units 1-3 were shut down, and configured to operate on a cold stand by basis; this was mainly because of the oncoming expiry date and the unit’s small size, which wasn’t worth the installation and operational investment of FGD. The units are now demolished.

62 JULY/AUGUST 2012 POWER INSIDER

UnitBegan Construction Finished Construction Started Up Invest-

ment Cost

(Million Month Year Month Year Month Year

4

October 1997Novem-

ber1999

May 2000

1,160.506

5

6December 1999

1,160.506

7

8

November 1994 April 1998

November 1997 656.00

9 September 1997 656.00

10 April 1998 656.00

11 February 1998 656.00

12October 1993

May 1995 1,081.00

13 September 1995 1,081.00

MAE MOH WET FGD SYSTEMWet flue gas desulfurization is installed on Mae Moh for units 4 to 13, this FGD process can be divided into 3 major subsections as described below;1. Reagent Preparation System2. Absorber System3. Gypsum Dewatering System

limestone silo through limestone unloading conveyer and bucket elevator in the respective order. The limestone enters the ball mill through the weigh feeder, which can vary its speed according to the amount of limestone need to be crushed.

The ball mill contains a large number of iron spheres in different sizes, which consists of 63.5 mm, 50.8 mm, 38 mm. and 25.4 mm. spheres. The ball mill rotates through a 6.6 kV electrical motor source and limestone is crushed whilst being mixed with the grinding water. The crushed limestone has a smaller size and flows out from the mill along with grinding water through trommel screen, a cylinder used to separate material by size, into mill recycle tank. This limestone solution is referred as Reagent which is diluted by adding dilution water in order to maintain the concentration of reagent in the mill recycle tank between 50-55% solid by weight.

ABSORPTION SYSTEMAbsorption system is the most crucial part of the FGD system for removing sulphur dioxide. This system consists of an absorber tower for absorbing SO2, a tower with 15 meter diameter and 27.7 meter height. Reagent is filled into the absorber sump along with process water and gypsum, becoming seed slurry while carefully controlling its pH between 5.0-5.5. Slurry dwell within absorber sump is circulated by the absorber recirculation pump; there are 5 pumps in total in which the slurry is sprayed through spray headers on the 5 respective levels of the tower. Each header composes of nozzle spray headers so that the sprayed slurry covers the entire layer. With the flow rate of nozzles and the gravitation force, sprayed slurry falls to the sump while absorbing sulphur dioxide gas flows in the opposite direction. Slurry is then pumped and sprayed over and over again, known as recirculation.

When high temperature flue gas, (approximately 169oC), flows from the boiler through heating elements of the gas-gas heater the temperature is reduced to approximately 130oC. Then, upon entering absorber tower, flue gas will exchange its heat again with slurry sprays from spray nozzles until its temperature reduces to saturated point about 61oC. From this point, sulphur dioxide mixes within flue gas and is dissolved by slurry mixture becoming sulphurous acid, reacting with limestone result in gypsum crystalline.

At the same time, flue gas flows through sprayed slurry from nozzles while SO2 is being absorbed. As it reaches the highest part of the absorber tower, water droplets which flow along with the clean flue gas are trapped by 2 layers of mist eliminators. This way only dry-clean flue gas is be able to reach the outlet side of the GGH and have its temperature heated up (from 61oC) to approximately 90oC. There’re 2 advantages of heating the clean gas; one is to reduce dew point rate and another is to help flue gas flow easier. Before flue gas reaches the stack, it needs to pass a long route of ducts, GGH and the absorber system, which all contribute to dropping pressure. To compensate all the draft loss incurred, booster fans are needed to be installed at the outlet side of FGD, between the reheating side of GGH and the stack, to increase the pressure of flue gas

Fig. 4 Mae Moh Generation Statistics 2011

REAGENT PREPARATION SYSTEMWet FGD unit 4-13 are use limestone (calcium carbonate; CaCO3) from a limestone mine, which is located in the adjacent area behind Mae Moh power plant. Generally the purity of the limestone used varies within the range of 90-99%. Limestone is transported by truck for weighing at the load cell station. It is then dispatched for storage in the

Fig. 6 Iron Spheres used in crushing process Fig. 7 Hydrocyclone & Reagent Tank

Figure. 5 Detailed Data of Flue Gas Desulfurization Installation Time/Cost at Mae Moh

MAE MOH POWER PLANT

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64 JULY/AUGUST 2012 POWER INSIDER

before it is released to the atmosphere.After the sulphur dioxide mixture within flue

gas passes through the absorber tower, limestone concentration of the slurry is reduced to its forward chemical reaction, which is gradually consume limestone reagent.

This can be observed from the pH value of the slurry; once the reaction moves forward, slurry’s pH value is reduced relevant to the amount of limestone consumed. There is a control valve, which receives the feedback from absorber sump, and adjusts itself to an appropriate degree, for precisely controlling the amount of limestone fed to balance the pH within its appropriate range. However, as the reaction progresses, the amount of gypsum crystalline within absorber sump is increased along with the slurry’s density. The gypsum bleed pump is activated to bleed out the gypsum for maintaining the density of the slurry. DEWATERING SYSTEMThe final products from the FGD process are gypsum and carbon dioxide while having limestone, sulphur dioxide, oxygen and water as reagents for the reaction. As mentioned at the end of absorption system, gypsum mixed with the slurry is bled out via a gypsum bleed pump, to a Hydrocyclone, the device which separates gypsum from limestone reagent. Limestone, the lighter substance, overflows from the Hydrocyclone by centrifugal force then return to absorber sump for reusing again in the absorption process, while gypsum, which is heavier, falls down from the Hydrocyclone (underflow) to a local

gypsum slurry tank. This process between gypsum bleed pump and

local gypsum tank can be referred as the primary dewatering system. Upon entering local gypsum slurry tank, gypsum crystalline mixing with the water has density controlled between 50-60% solid by weight, for example. As for the last process, secondary dewatering system, water is drained out from gypsum and drawn downward through the filter. The main components of this process are: a dewatering belt filter, a belt which stretched up by head and tail pulley which driven by motor drive through speed reducer. Along the belt’s length, there is a horizontal groove which houses a large number of 1 cm.-hole in the centre. These holes are connected to vacuum trenches which are lined up under the belt, from the head to the tail of belt filter. The water separated through vacuum belt is stored at filtrate receiver tank.

After gypsum slurry arrives at the dewatering belt, water is drained out of the slurry along with the air through the porous surface of filter cloth into filtrate receiver tank. Then, only dry gypsum remains at the end of the belt. The water which is separated from gypsum slurry in this process is referred as reclaim water, for recycling and then is reused again in the system. Reclaim water is stored within the reclaim tank which is later used in absorber tank or the ball mill process.

The installation of these advanced air pollution control technologies have been a significant investment, but many are unanimous in their benefit. They have allowed Mae Moh to continue operation at full capacity which has subsequently

been a significant factor for stabilizing the grid in Thailand. Official monitoring and reports from the Pollution Control Department indicate that S02 levels surrounding the plant on a yearly average are significantly lower than those of Bangkok, which is an indication of how far EGAT’s major lignite fired facility has come. As population grows and the summer demand increases through Thailand’s notoriety as a popular tourist destination, Mae Moh’s importance is ever present.

REPLACING AGING UNITS AND KEEPING UP CAPACITYUnits 4-7 are rapidly approaching expiry which means a significant loss of 600 MW for the Thai grid. The decommissioning period has been noted by EGAT for some time, and subsequently they have had a contingency plan in place, announced officially with the Thailand Power Development Plan 2010. To replace the aging units, a brand new plant will be installed at the site, utilizing the most advanced and high efficiency technology. Feasibility is underway, as the proposed plans are currently in the midst of rigorous environmental health impact assessments. The plant will consist of a Lignite-fired Supercritical Pressure Boiler, Steam Turbine Generator, Electrostatic Precipitator, Flue Gas Desulfurization and Selective Catalytic Reactor for De-NOx. EGAT are aiming for a commercial operation date in January 2017, and the lead contractor is expected to be announced in the following months so please keep tuned to PI Magazine Asia for details of the selected vendor.

Fig. 8 Recirculation Pumps for spray nozzles

Fig. 10 Vacuum belt filter for gypsum separation

Fig. 9 Absorber Towers

Fig. 11 Average yearly S02 concentration - Bangkok vs Mae Moh

MAE MOH POWER PLANT

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