MIM has committed to the Australian Governmentís Greenhouse Challenge. This Federal and State Government sponsored initiative requires companies to reduce emissions of carbon dioxide and other greenhouse gases by accepting specific targets. Greenhouse Challenge staff have been using MIMís outstanding record in energy management and carbon dioxide reduction at Mount Isa as a case study in promoting the Challenge.
Until early 1997, MIM was the main generator of electrical power for north-west Queensland. Transmission of the power external to the MIM leases was carried out by the Northwest Queensland Electricity Board (NORQEB).
Local discoveries of many new orebodies meant the inevitable increase in power demand for new mines. In addition, its own expansion at Mount Isa meant that MIM had either to invest in new generating capacity or utilise existing generating plant more effectively. More effective use meant that MIM was required to undertake rigorous energy management within its own operations. Energy management relied on a personal computer based system and more energy efficient operation of plant.
MIMís Mount Isa operations are situated in northwest Queensland and form part of the Carpentaria Minerals Province. The Province is remote from the electrical grid systems of both the Northern Territory and Queensland. Most of the electrical power for the province is generated from Mica Creek Power Station (MCPS) and the Isa Mine Power Station (MPS). Until recently, the total generating capacity of MCPS was 198MW of which 165MW was coal fired and a further 33MW was from a distillate fired gas turbine set. In addition at MPS, a maximum of 29MW can be generated from a cogeneration plant using waste heat from the smelters and if necessary distillate.
The generation of electricity at MCPS and MPS is responsible on average for about 72 percent of total emissions from Isa operations. The remainder of the emissions are from explosives (less than 1 percent), transport (2 percent), copper smelter (14 percent) and lead smelter (11 percent). The emissions from the smelters are from fuels such as coal, coke, distillate, gas and the 1 percent carbon content of the ore, which in the smelters combines with oxygen to form CO2.
Cleaner Production Initiative
If energy management was to be a viable option to the continued growth in energy use, large gains would quickly have to be made and seen to be made. Key areas of Isa operations where such gains could be made was in underground operations and to a much lesser extent, on the surface.
On the surface, energy efficiency was improving:
- MIMís proprietary smelting process (ISASMELT), which generates much of its own heat internally thus reducing its need for external energy, was being used in both the copper and lead streams. The ISASMELT process breakthrough came from MIMís own research and development in association with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), resulting in reduced operating costs and considerably cleaner production. Millions of dollars were being spent on ISASMELT and other smaller scale projects to make the surface plant more efficient.
- Waste heat from the smelters was being used for cogeneration at the Mines Power Station.
Most of the dramatic rise in overall load was caused by underground plant. It was therefore decided to concentrate on the underground operations. Major reductions in energy consumption, peak demands and greenhouse emissions were achieved by implementing initiatives in five specific areas.
- A 1,000kW impulse turbine (a turbine which uses high pressure water driven bucket wheel principles) and generating set was installed 1,000m underground. Chilled water at 1oC is discharged at around 100l/s down a vertical pipe from the surface to underground. Prior to the installation of the set, the water gained around 2.5oC between the surface inlet and the underground outlet, resulting in a temperature of 3.5oC. The installation of the set not only generated emission free electrical energy but, recooled the water by 2oC down to 1.5oC, reducing the chilled water requirement by around 11 percent. Required generating capacity could also be lowered by running the set during times of peak demand.
- After passing through the impulse turbine the chilled water is collected in a cold water dam on 20 level. This water was initially pumped through fan cooling units in all of the underground cribrooms. Pumping costs were very high. Many of the cribrooms were subsequently fitted with dedicated refrigerated air conditioners, reducing pumping costs and the need for chilled water.
- The twelve 1MW or 2MW axial ventilation fans on the surface are mounted over vertical shafts, which are typically 1,000m deep. Operation of the fans is to either extract or supply air to the underground workings. The pitches of the fan blades are automatically changed at regular intervals during the day by a process controller installed on the surface. Fan blades are driven to minimum pitch during times such as change of shifts, when ventilation of the whole mine is not required.
- Dispersed throughout the mine are around 1,000 smaller ventilation fans, each fan having an average connected load of 11kW. These fans increase general air movement underground and direct ventilation to priority areas. Many of the fans have been fitted with ripple frequency controllers. Two NORQEB owned and operated ripple frequency transmitters inject into the NORQEB electrical reticulation control pulses at 750Hz. Initially these transmitters were only used to control domestic hot water heaters in the city and surrounding properties, which used off peak electricity supplied at a lower tariff. The transmitters are now used to control both the hot water heaters and fans underground. Fans are turned off at the end of each shift.
- Desynchronising skip hoisting. The hoisting control systems of the R62 (lead mine) and U62 (copper mine) ore skips operated independently. Coincidently, during their hoisting cycles, full copper and full lead skips would be accelerated from rest at the same time. The mass of each skip and contained ore is 40 tonnes, the acceleration time for both is about 20 seconds. This meant that there were random occurrences of high current, short duration demands on the generators. To allow for these occasions, the maximum sustainable load of MCPS has been set at 5MW below the station's maximum steady state generating capacity. The two winders are now controlled by interconnected Programmable Logic Controllers (PLCs) such that only one skip can be accelerated from rest at a time.
Underground and surface plant
It was realised that even greater energy efficiency and reduced emissions would result if operators were made more accountable for energy use. In early 1997, MIM sold an 80 percent interest in MCPS to a specialist power generator, which assumed management of the plant requiring MIM to order its electricity 24 hours in advance. These two factors gave impetus to the development of a lease wide personal computer based energy and emission management system (PC: EMS).
PC: EMS was designed to allow easy data input together with meaningful displays. Plant operators are the key to PC: EMS. All plant operators were supplied with a PC. Each PC was connected to an area network which covered a surface area of about 28kms by 2kms and it also reaches deep underground.
Plant operators are set the task of entering daily operational forecasts at half hour increments via their terminals. These forecasts are entered into Forecast Advice Sheets such that there are always displayed, eight days of forecasts for each plant area. Forecast Advice Sheets for the current day cannot be changed by the plant operators as they form the basis of the order for electricity. Plant operators are not expected to directly forecast electrical loads and greenhouse gas emissions. Instead, operators forecast the fraction of their plant that is expected to be operational. In the central database, the forecast demand and energy requirements are calculated by multiplying the proportion of plant estimated to be operating by the full load rating (MW) of the plant. During the current day, a copy of each plant forecast is modified by the MPS Load Controller to reflect actual operation. This modified forecast is also saved. Therefore, two sets of daily forecasts are both available and saved each day.
Greenhouse gas emissions are calculated by applying a conversion factor to the plant forecasts, changing MW to tonnes of CO2 . Emissions of greenhouse gas can be fairly accurately calculated because both the type of fuel and station efficiencies are known. All operators can call up a display to see how much CO2 their plant will emit during the day. The forecast displays are only used as a management tool for the operators.
Each plant operator, therefore, is able at any time to see the energy and environmental costs of running his or any other plant in dollars, energy consumed in MWh, peak demand in MW, and tonnes of carbon dioxide emitted to generate the necessary power. Previous to these displays, most operators rarely had any idea of the environmental and monetary costs of their operating practises. They had even less appreciation that they could significantly affect those costs.
Power meters with electronic outputs are being installed and new connections are continuously being made with plant control systems so that comparisons of forecasts with actual or measured loads, and energy costs can be displayed. At present the target is for forecasts to be within a band between 110 percent and 90 percent of actual. Because local operators may not be immediately aware of influences outside their control, their forecasts are subject to adjustment by the PC: EMS Administrator. These adjusted or final forecasts are then issued to MCPS. Final forecasts are usually within 105 percent and 95 percent of the actual and are typically 104 percent of the actual or measured value.
Advantages of the Process
Energy management has resulted in reduced energy consumption and the postponement of further capital outlay for generating plant. As a direct result, carbon dioxide emissions have been substantially reduced.
- The company was been able to open the deepest mine in Australia, with all the additional power requirements that entailed, and still reduce total energy use.
- Carbon dioxide emissions related to metal produced have fallen by 11 percent since 1995/96.
- When both power stations are converted to gas, there will be a further considerable reduction. In fact, by the year 2000, the carbon dioxide emissions from Mount Isa will be at least 40 percent below those of 1990/91.
PC: EMS was conceived and designed solely to manage energy costs and greenhouse gas emissions more effectively. However, it soon became apparent that energy forecasts by plant operators also showed planned plant status as a by-product. Forecasts reflect times when plant is non operational. This information is easily available for maintenance, managing and ordering of supplies and other equipment. The result has been that there are now more operators with read only access to PC: EMS than forecasters.
Financial results of actions described above were the subject of a 1996 Bureau of Industry Economics report (Energy Efficiency and Greenhouse Gas Abatement: the role of cooperative agreements in Australia)
The report concluded the following:
- ISASMELT. Sufficiently attractive economically, such that its capacity is being expanded to smelt all copper ores at Mount Isa.
- Cogeneration at MPS. Without using the waste heat to generate environmentally clean power, further expenditure would have been necessary for cooling steam from the smelters, extra electricity would have needed to be purchased and capacity charges would have been higher. MPS supplies about 10 percent of the electrical energy used by plant at Isa operations.
- Cogeneration underground. Purchase and installation of the impulse turbine/generator set cost around $1 million. The payback time was under a year. Without the generator higher production costs would have eventuated from purchasing more electrical energy, higher capacity charges and the need to chill and circulate more water.
- Local air conditioners, where installed, had payback times measured in weeks. As an example, one underground cribroom previously cost $95,000 per annum to cool using reticulated chilled water. A local refrigerated air conditioning unit reduced the cost to around $11,000 per annum.
- Installing a Mine Processor to control fan pitch and main pumping cost just over $500,000, the audit showed that the pay back was less than a year. The processor was also used for control of fifteen 400kW dewatering pumps, reducing the number of operators, energy use, peak demand and maintenance costs.
- Skip hoisting. At the time that desynchronising skip hoisting took place, natural gas was not available for generation purposes. The removal of the 5MW derating of MCPS generating sets was equivalent to installing new coal fired generating capacity. Coal fired generating sets were estimated to cost $2.7 million perMW installed. Desynchronising the sets resulted in a potential cost saving of over $13 million in deferred expenditure from a cost of around $20,000 in labour and material.