Minerals expert Dmitry Przhedetsky says lignite processing is a “miracle waiting to happen” and has shared 20 processing options to be considered.
Several times I have been asked by people outside the mining industry, where do I see the most important technological breakthrough to be expected? This breakthrough would affect not only mining and processing, but would make a far greater impact on the industrialisation of the planet. However, the chances are slim.
Whether it is going to be an accidental discovery or a result of many years of scientific work, it will be almost a miracle – a universal, self-sufficient, economically viable for remote regions and environmentally friendly lignite processing technology.
Today, there are many lignite processing technologies and alternative uses of lignite around. While none of them are “a silver bullet” some could be further developed.
In 2013-14, Australia produced 57.8 million tonnes of brown coal. This is about 10 per cent of the global output. The Australian output of brown coal has been consistent and, most likely, will remain consistent, as it is produced entirely for the local market. The Australian resources of brown coal are estimated at over 400 billion tonnes.
The current economic downturn and low prices on thermal and metallurgical coal, as well as low oil prices, may delay the development of some lignite projects and, most importantly, the development of lignite processing technologies.
Yet, one day the industry will face the same question – how to process, or in broader terms, how to utilise vast deposits of brown coal, especially those located in remote and unindustrialized parts of the world.
While in Australia the main use of lignite is power generation, there are other uses of brown coal, which can be supplementary to the power generation or, theoretically, may be justified on their own for an abstract lignite deposit. It is important to remember that apart from technological challenges involved in each particular alternative application, there is also a substantial amount of commercial risk. For example, if an alternative brown coal processing technology has not yet been commercially viable in Europe, in the USA or in Australia, where the deposits were located close to the industrial areas, it is less likely that it would be feasible to develop such a technology in remote areas.
At the same time, prior to discarding any project based on the fact that the coal has a low calorific value and is located in a remote area, it is worth considering several alternatives, as millions of tonnes of the resources of brown coal are present. Hence, it may be worthwhile considering in first instance increasing the calorific value of coal or, in simple terms – upgrading it. There are several coal properties affecting upgrading and they should be closely looked upon when choosing a particular method
Dmitry Przhedetsky’s experience and knowledge base covers a broad range of minerals – from iron ore to salt mining, underground coal to uranium mining, and quarrying to tunnelling. He has been involved in many major international mining projects and worked all across Australia, as well as New Zealand, New Caledonia, USA, Canada, India, Germany, France, Russia, Ukraine, Kazakhstan, Uzbekistan and Azerbaijan.
Dmitry has been providing consulting services to investors, government officials, equipment manufacturers, and the broader community. He is the founding director of Rock Cognition Pty Ltd.
ACTIVATED CARBON, CARBON FIBRE & CARBON SEMICONDUCTORS
While there are technologies allowing the production of activated carbon, carbon fibre, carbon semiconductors and other materials for consumer use, it would be fair to say that it’s unlikely the quality of coal and the location of many projects would be particularly attractive for these technologies.
There are several processes which can be involved in low-rank coal beneficiation. The most common are gravity cleaning (jigs, hydroclones, etc), fine cleaning (dense media cyclones, dry magnetic separation, etc), and chemical cleaning (acid leaching, molten caustic leaching, etc). Each of these methods has limitations based on the properties of a particular type of coal. Feasibility of either method needs to be thoroughly evaluated, as its implementation and efficient use in relation to any coal deposit will be a costly process.
The inventors claim the new Cat-HTR process uses a proprietary catalyst to convert lignite into syncrude (a synthetic fuel) and high-grade coal. The process uses selective oxygen removal and depolymerisation. It has been claimed that the pilot plant, located near Sydney, has been in operation for more than two years.
A decline in production of smokeless briquettes has been witnessed in Europe in the 90s. Smokeless coal briquettes have been used for domestic heating. Production of coal briquettes requires substantial coal cleaning and low rank coal (e.g., high ash, high sulphur, high arsenic) may present a technological and commercial problem for briquetting.
COAL COMBUSTION PRODUCTS (CCP)
Numerous products can be manufactured from coal combustion products (by-products). These can be: mineral wool, brick and other ceramic products, fillers for metal and plastic product, paint, road base, and controlled strength fill. In all of these applications, it is vitally important to match the properties of the coal by-product with the requirements of the application.
Product specifications for coal by-product application should address environmental, engineering and economic performance criteria together. Additional validation may be required before unrestricted use of these products can be approved. It may be feasible to consider the use of CCP in coarse and fine aggregate road base, which again should only be considered as a by-product of power generation.
COAL SEAM GAS / UNDERGROUND COAL GASIFICATION
This method is traditionally not considered for lignite due to high moisture content. Dewatering of lignite in situ and oxidizing may cause uncontrollable underground fires. There is a reference to technology called enhanced biogenic methane, however it may be a long way away until it’s been practically proven.
DRY LIGNITE POWDER
Dried lignite powder is used in combustion boilers. Germany is also producing pulverised and granular coke for a variety of uses. Dry powder can be pneumatically transported in nitrogen to special storage tanks and trucks to be supplied to customers.
There are several processes and types of equipment available for reducing the high moisture content of lignite. The processes involve evaporative drying techniques, where the coal temperatures remain below 100 degrees Celsius. The drawbacks of these methods are high cost, moisture re-absorption, friability and spontaneous heating of coal.
FLY ASH FOR CONCRETE MAKING
Fly ash is a by-product of the power generation and its utilisation may have some commercial use, such as concrete making. The fly ash alone is insufficient for concrete making as can only substitute up to some 30 per cent of cement, i.e. can only be an additive, even though it has pozzolanic features by itself.
Fly ash can be used for some other purposes, such as a base for backfill in underground mines, but this would not be sufficient to justify an open cut coal project by itself or to offset losses, if power generation is unprofitable.
Formed coke can be produced in different sizes, shapes and qualities to meet the needs of selected markets. Char made from low-ash low-sulphur coal can be a viable alternative to the metallurgical processes requiring high quality carbon, such as derived from wood. Some coal deposits may have the initial coal qualities prohibitive for this option.
Wet coal is heated under pressure to 250-300 degrees Celsius and the coal structure breaks down and shrinks, releasing the water as a liquid.
INTERMEDIATE TEMPERATURE DRYING
These processes (such as Syncoal, hydrothermal, hot oil, etc) involve heating the coal above 240 degrees Celsius, when the coal permanently changes its physical and chemical properties. These processes substantially increase the cost of the final product by approximately 30-50 per cent comparing to the evaporative drying.
LOW RANK COAL WATER FUEL
This technology involves hydrothermal high-temperature and high-pressure treating of coal with the end product being a stable pumpable slurry (a mixture of coal, water and a chemical stabilizing agent). Economic models to date have not been cost-competitive, when compared to the thermal coal in international trade.
At the same time, Coal Water Fuel (CWF) has been produced and used in several countries, mainly for industrial boilers. One of the benefits of the CWF is a possibility to transport the coal via pipeline, which may be economical for some projects. The methods may not be suitable for regions with low winter temperatures and also will require removal of ash, sulphur and other impurities.
Methanol can be considered in co-production with electric power generation. The increasing use of methanol as an additive to the petrol fuel may justify feasibility of some projects. As with other potential by-products considered here, it is important to identify the size of the market and profitability for each particular product. It may well be the case, that there is no demand for a particular product in the area or the efficiency of similar projects in the area is greater.
PORTLAND CEMENT PRODUCTION
As portland cement production is a highly energy consuming process, a cement plant may benefit from being located close to a large source of energy, but most importantly, it should be built around a limestone deposit, which has to be located very close to the coal deposit and limestone has to be suitable for cement production. Even then, the economic benefit of the project can only be viable if there is a market for cement in the area and the final cost of the produced cement is competitive. Even if there is a proven limestone deposit nearby, the evaluation of this option may take a substantial amount of time and investment.
There are several existing proprietary technologies, such as LFC (Liquid From Coal), Formed Coke (FMC), and mild gasification. The techniques are complex and expensive. Past assessments have indicated that upgrading low-rank coals may not be commercially feasible due to the cost of the final product being higher than its market value. There could be some use of the process for producing high added value products, such as activated carbons for use in filtration, however it may have low benefit to many lignite projects due to poor quality of the initial product, as well as the presence of arsenic in some deposits, making any use of the by-products of the coal potentially risky.
SOIL CONDITIONERS & LONG LASTING FERTILISERS
While there are multiple references to the potential production of coal-based soil conditioners and long lasting fertilisers, some references were quite negative, hence not recommending any use of coal-based products in the agricultural industry. In any case, such a use would be only feasible as a by-product and would be only applicable to the regions with high level of agricultural activity.
SYNTHETIC LIQUID FUELS
The technology for producing synthetic liquid fuels has been around for more than 80 years. Due to the high cost of the final product the industrial production of Synthetic Lignite Fuels (SLF) for many years only remained current in South Africa. The processes involved are called direct and indirect liquefaction and there are several types of these processes available.
Due to new liquefaction technologies and volatility of the crude oil price, even despite the currently low oil prices, the feasibility of some projects may be re-considered. It is expected that even if the initial quality of coal is suitable for production of SLF, the investment into this particular technology may be questionable due to the above-mentioned volatility of the market.
SYNTHETIC NATURAL GAS
A synthetic natural gas project in the US has been commercially unsuccessful, even with, presumably, higher quality of initial coal. It will be worthwhile to consider first what are availability and the market for natural gas in the region.
UNDERGROUND COAL TO LIQUIDS (UCTL)
UCTL is a new process invention that occurs within the underground coal seam (in situ), whereby low rank coals are liquefied into crude oil substitute product at 300 degrees Celsius. A substantive heat by-product is returned to the surface potentially creating further commercial applications such as electricity generation. While the inventors are targeting brown coal deposits, the technology has neither been technologically, nor commercially proven.
As the energy consumption increases continuously, there will be more research and development projects related to the utilisation of brown coal deposits. Most likely, we will see that the efficiency of some of the existing methods will improve, and there will be new technologies offered and implemented.
Currently there is a number of research and development projects on the drawing board. Also, it possible that the new technologies being currently developed for black coal will be potentially adapted for lignite (for example, Nano Drying Technologies (NDT), Rapid Nano Drying (RND), CHRONOS, etc). They relate to the more efficient combustion processes, brown coal liquefaction and gasification process and studies of additional application of coal combustion products. There are some other technologies for brown coal being currently developed, but not yet commercially proven, which leaves us with a hope that one day this will become a commercially and ecologically viable reality.