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HAZELWOOD POWER Leading the way to a cleaner environment By Sammy Isreb When you burn coal, you get ash. However, these days no-one tolerates a thick plume of ash-laden smoke emerging from a chimney – or in the case of a power station, multiple, highvolume chimneys. But with exhaust temperatures of 200°C or more, it’s not quite as simple as putting a filter in the flow . . . About two hours east of Melbourne, nestled in the heart of Victoria’s Gippsland region lies the Latrobe Valley. While the valley thrives on farming there is another noticeable industry: power generation. It’s been a major part of the valley since the first station was synchronised to the Melbourne grid on the afternoon of Tuesday, 24th June, 1924. In fact, power stations are so much a feature of Gippsland that anybody who has driven through the region will surely remember the myriad of chimney stacks rising into the sky. So why did the former SECV (State Electricity Commission of Victoria) 4  Silicon Chip decide to build more than 80% of the state’s generating capacity in the area? Under the region’s pastures lies billions of tonnes of brown coal, one of the richest deposits in the world. The brown coal is burnt to turn water into superheated steam, which drives the turbines. As a byproduct of burning the coal, ash is produced, which brings us to the topic of this article: dust removal by electrostatic precipitation. Hazelwood Power Station Hazelwood Power Station was commissioned during 1964-1971 (in four stages). When completed the plant had a total capacity of 1600MW, made up of eight 200MW units. Each unit has an independent boiler, turbine, generator, condenser, precip-itator and draft systems, along with independent controls. Two of these units make up what is known as a “stage”, sharing a bare minimum of equipment. Common equipment is basically limited to forms of data logging and the “Pondage” – Hazelwood’s 506 hectare, 30,850 megalitre cooling pond. The Pondage eliminates the need for cooling towers, providing a relatively cool supply of water for the eight condensers. Brown coal is supplied to the plant from an open cut mine, a massive hole in the ground with a perimeter of approximately 12 kilometres and a depth of around 100 metres in places. Dredgers weighing up to 1800 tonnes remove coal from the mine face, sending it down conveyor belts to bunkers from where it is sent to the plant. The station uses roughly 15 million tonnes per year or about 1700 tonnes per hour. With this sheer amount of coal, one can see how rigorous environmental procedures must be put into place in order to avoid waste products polluting the area. Privatisation In August 1996, Hazelwood Power Corporation was sold to a private consortium for $2.35 billion. Since the sale, one could only describe the renovations to the station as staggering. When it was purchased, the operational capacity of the plant was 1200MW, with Unit 7 damaged (due to an overheating incident in the boiler) and Unit 8 mothballed. Shortly after the purchase, Unit 8 was recommissioned. In January 1998, the newly rebuilt Unit 7 was brought into service, ending a yearplus long project worth tens of millions of dollars. During the following summer months of 1998, the operational capability of the plant was restored to 1600MW, with a peak of 1679MW recorded. Why clean the emissions? One of the biggest problems in coal fired power generation is pollution from the ash in emissions from the chimneys. Fortunately, the brown coal in the area has very low sulphur content, eliminating the acid rain which plagues some other countries. The major constituent of pollution from brown coal is ash, formed in the combustion process. If no action is taken, it is ejected from the chimney. It is the role of the electrostatic dust precipitator (EDP) to remove this dust from the exhaust gas, allowing it to be collected for disposal. When the SECV commissioned Hazelwood the best EDP technology available at the time was installed., The old EDPs still met the Environmental Protection Agency (EPA) licensing requirements, but only just. In 1977, Hazelwood Power management decided to replace the current precipitators. Several factors led to the program being conducted on a unit-by-unit installation basis, with completion not being scheduled until 2007. Unit 3 was chosen to be the first recipient of the new precipitator due to it being the worst performer on an emission basis. What’s wrong with a screen? Commonsense would suggest the application of some sort of particle screening or filtration solution. And why not? All around us common devices use a multitude of screening techniques, from vacuum cleaners to dust masks, to keep unwanted airborne particles under control. So why deviate from this seemingly Hazelwood Power Station, looking across one of the ash-settling dams with Hazelwood Pondage (supplying cooling water) at the rear right. The eight units which make up the station are capable of generating 1200MW. AUGUST 1999  5 Out with the old, in with the new: three of the old EDP flows at Hazelwood (you can just see the new flows behind). The old units were only just capable of meeting environmental specs but the new ones are significantly better. simple solution into electrostatics, fluid flow and vibration mechanics? The answer lies in economics and practicality of scale. On full load, each unit’s boiler at Hazelwood Power Station emits roughly 10 tonnes of ash per hour at a temperature in excess of 200°C. The high temperature is only one problem; the smoke also contains a high moisture content, due to that of the brown coal. Standard filtration through a filter medium is impractical. Even if the medium did not immediately try to combust due to the high temperature or become clogged due to the high moisture content of the exhaust gas, the need to constantly replace or clean the filters would prove to be the downfall of the system. Also, the twin 1768kW induced draught (ID) fans, responsible for extracting the exhaust gases from the boiler, would be unable to to pull the gas through the extremely fine filters necessary to remove ash. In fact, EDP is one of the most efficient and convenient solutions to many gas cleaning situations. It is impervious to high temperatures and high moisture content, allowing removal of filtrate while in operation and providing little resistance to the gas flow. How does this technology work? As its name suggests, electrostatic rather than mechanical forces are the key. In a nutshell, the ash particles are charged by a high voltage while in the gas flow and are then attracted to an opposite-charge collection plate. The particles cake on the plate, the cake drops and the ash is transported away. (For a more detailed explanation, see the separate panel). Modern EDP design The first few generations of EDPs used thyristor-controlled transformer/rectifier HT sets, with 6  Silicon Chip little control of the output. While this arrangement achieved dust extraction, several flaws existed in the design. Firstly, there was no accurate method to determine optimum voltage/current settings to maximise ash collection. Second, arcing would often occur between the discharge and collection electrodes. When arcing was detected, the thyristor would simply be turned off for several cycles, allowing quenching of the spark. However, this minimised power flow through the EDP. Manufacturers of new generation EDPs have recognised the advantage of automation in improving efficiency. The biggest breakthrough has been the use of “pulsed” controllers for the removal of “tough” particles from gas. Rather than simply increase the voltage for tough dust, which would The specially-made trolleys can be seen underneath the EDP unit with one of the enormous prime movers carefully moving it into position. Each 400-tonne EDP was manufactured on site, then moved into place during the generating unit’s scheduled maintenance shutdown. be ineffective due to arcing and back-corona, the new microprocessor controlled EDPs send high “pulses” of power into the EDP. Overall, the average power entering the EDP will be the same but this method results in increased efficiency. While it might be imagined that between pulses of power, dust-laden gas would be escaping the EDP, there is no loss of collection capacity. In fact, the dust/ash layer represents a resistive/capacitive circuit with a time constant significantly greater than a second or so. Therefore the pulses of high power can break down the resistive dust layer and before back-corona or arcing occurs, power is reduced greatly, with no net effect due to the slow step response of the dust/ash system. As well as implementing this “pulsed” system, modern controllers determine optimum power levels for performance right up to the point at which back-corona and arcing occur. Hazelwood’s EDP system Adapted directly for Hazelwood Power by ABB, the new EDP consists of three “flows”, basically separate units connected in parallel. The rationale behind this modular setup is not only for ease of construction and installation but also for maintenance. If one or more flows are to be taken out of service for repair, the remaining flows are able to operate under heavier load in the meantime. Each of the three flows consists of main components integrated to form the EDP: • Main support structure. • Six collection hoppers at the base to collect waste ash. • Three electrically isolated bus sections containing emitting and collecting electrodes (along with the associated rapping equipment) and T/R (Transformer/Rectifier) sets. • Inlet and outlet distribution evases/transitions which contain gas distribution screens designed to maintain optimal gas flow distribution within the EDP. • Roof structure, comprised of HT chambers and T/R sets. • Insulation around the unit to minimise heat losses during operation. • Ash disposal system, consisting of conveyor from ash hoppers into a mixing system which forms a slurry Each of the 8 units at Hazelwood consumes nearly 200 tonnes of coal and emits 10 tonnes of ash each hour. That ash would be a major source of pollution if it wasn’t removed from the exhaust. to be discharged into sluice ways. Controlling each of the three T/R sets per flow is the EPIC II (the initials standing of Electrostatic Precipitator Integrated Controller), a microprocessor based system, mounted in the switchboards on ground level. Therefore, there are nine EPIC II systems per unit, as there are three T/R sets per flow, and three flows per unit. Each of these nine EPIC II microprocessors feed into a remote terminal unit (RTU) in the control room. Information on each EDP such as sparks, general alarms and trends can be displayed. Mode settings can be altered for each of the EPIC II units, with anything from the standard mode, to “sootblowing” mode, in which current is kept artificially high, even during sparking. Rapping sequences are also available to be viewed and altered from the RTU. As part of Hazelwood Power’s reporting obligations to the EPA, dust monitoring equipment is installed throughout the station. On each chimney is an Erwin Sick opacity dust monitor, which log the dust levels to remote control rooms and dataloggers throughout the station. Wired into the engineering office via the internal network, a dedicated dust monitoring PC logs half-hour averages of dust levels throughout the units against their megawatt outputs. Monthly databases are then stored for record keeping and for reporting to the EPA. Installation The new 26m high EDP flows, ready for installation, are dwarfed by Hazelwood’s eight chimney stacks. Construction of the new Unit 3 EDP began about six months before the unit was taken offline, in March 1998. The EDP was constructed in three separate flows, with the plan being to remove the old EDP casings and place their newly constructed successors on the same foundations. AUGUST 1999  7 How An Electrostatic Dust Precipitator Works... 1: Corona generation Inside an EDP are alternating rows of collecting electrodes (rigid steel plate curtains) and emitting electrodes. A high voltage negative DC supply, typically -50kV or more, is connected to the emitting system. In the region known as the corona (near the emitting electrodes where the electric field strength is greatest) the gas is ionised. The ionisation of gas produces positive and negative ions. The positive ions are attracted to the negatively charged emitting electrode and the negative ions are attracted to the grounded collecting electrodes. Fig 1: a cut-away view of an 2: Particle Charging Along the way, the negative ions collide electrostatic dust precipitator. with suspended dust particles, charging them proportionally as a squared function of their size. Once charged, the dust particles are attracted towards the collection plates. Hence, the particles “migrate” towards the plates with a velocity dependent on their size (larger particles travelling faster). When they reach the collection plate, they stick and begin to form a layer. It is at this stage in the process that problems sometimes occur. As the dust begins to build up on the collection plate, it will exhibit a resistance to the flow of current. If the resistance of the particles is too low, a high current flow will occur, causing the particles to quickly lose their charge and possibly re-enter the gas stream. Conversely, if the accumulating layer is of high resistivity, an abnormally high electric field will be present in the dust layer. A “back-corona” can occur, breaking down interstitial gas and producing ions and spontaneous electrical discharges from the dust layer. The resulting reduction in performance is twofold: the electrical discharge from the dust layer allows collected dust to re-enter the gas stream and the positive ions counteract the approaching charged particles. The resistivity of the particles will depend on the type of fuel and how well it has burnt. Luckily for Hazelwood, the ash passing through the EDPs is of moderate resistivity and causes no problems of this nature. Each of the three flows of the Hazelwood EDPs are divided into three equal-sized fields (or zones) operating in series. Because the larger particles are much easier to collect, the first field removes approximately 80% of the ash and dust entering the EDP, with the second field removing around 15% and the third field 5%. 3: Rapping The layer of ash and dust particles on the collector plate is removed by a process called “rapping”. This simply uses heavy metal hammers to strike an anvil on a shockbar, to which four collection plates are attached by huckbolts. The hammers produce a force of up to 300Gs. This effectively shears the dust from the plate surface, dislodging “cakes” of dust which fall into hoppers below. From there it is carried away by screw conveyors before being mixed with water and removed via sluiceways to settling dams. 8  Silicon Chip Built on temporary foundations, the three flows each measured 26m high x 13.5m x 19.5m and weighed around 400 tonnes. Once the new Unit 3 EDP flows had been successfully fabricated about 50 metres from their final resting places and with the unit offline for its major outage, it was time to commence the gargantuan task of moving and installing them. To begin with, each old flow was disconnected from its foundations and placed on a hydraulic trailer, containing 144 wheels on nine separate axles. With the aid of three prime mover trucks the flows were moved to their storage place. The new EDP flows, also jacked up and pre-positioned onto a similar trailer, were then guided into place and anchored. The entire operation took just 13 days. Once the new EDP had been positioned and with the relevant ducting and electrical connection work completed, it was time for the commissioning. The ultimate test While computer models predicted what modifications to the inlet and outlet screens and deflectors were needed to ensure uniform airflow throughout each flow, these models were only a guide, being no substitute for real testing. Flow testing began, in late October 1998. The 12 painstaking tests, conducted around the clock and requiring modifications after each test, took eight days. The tests were conducted with the unit still offline, with the test team running the ID fans and taking air flow readings in a multitude of places in the EDP. Finally, the unit was brought back into service on the 7th January 1999,. Dust emissions for Unit 3 on full load dropped from around 300-400mg per cubic metre to a new level of less than 100mg/m3. Acknowledgement Fig 2: a somewhat stylised representation of the inside of the precipitator above. Exhaust gases flow in the direction of the arrow. I would like to thank the following engineering staff from Hazelwood Power for their extensive help in the compilation of this article: Tony Innocenzi, Chris Morley and Daryl Anderson, along with Sara Stigsson, Wayne Bassee and Jason Price from SC ABB.