Clean coal is not a new concept, but, new technologies have emerged over the years that have a claim to make coal more ‘eco friendly’ or ‘cleaner’ as an energy source.
In this guide, we:
– Explain what clean coal is
– Explain how clean coal works
– List a range of clean coal technologies that have either been used to date, or, are in development
(*Note – whether or not clean coal is actually clean, or feasible in the present and future, are separate questions answered in other guides on this site)
Summary – What Clean Coal Is, Different Technologies & Techniques, & More
What Is Clean Coal, & How Does It Work?
Clean coal technology involves any part of the coal-for-energy process that reduces either/or both of a) emissions (usually carbon dioxide) and, b) air pollutants (usually sulphur dioxide, nitrogen dioxide, and particulate matter)
There’s several ways clean coal technology can be implemented, and each works differently
It started with washing coal, and advanced to other methods such as CCS, and coal plant with new designs and efficiency levels (like super critical, ultra critical, and HELE plants)
List Of Clean Coal Technologies & Techniques (How To Make Coal Cleaner)
In reality, there are many different clean coal technologies and techniques that help make the burning, or use of coal for energy, cleaner.
‘Cleaner’ usually means reducing the air pollutant emissions (such as nitrogen dioxide, sulphur dioxide and fine particulate matter) or greenhouse gas emissions (like carbon dioxide) released from the coal-for-energy process (but, it could also be extended out to including more eco friendly ways to dispose of or recycle coal ash for example).
As a few examples of clean coal technology designed to do different things – bag filters are designed to catch and filter particulate matter, mercury absorbers absorb mercury, flue desulfurization reduces sulfur, and CCS (carbon capture storage) separates, captures and stores carbon dioxide
Some technologies and techniques directly reduce environmentally harmful substances and gases, whilst others aim to make coal burning more efficient (and reduce the amount of coal and therefore waste products in total).
Other solutions may be indirect e.g. imposing legal regulations or penalties for emissions which encourage certain practices and discourage others
Some of these technologies and techniques have been used for decades or more, whilst some of them are newer and still under development (and may be more speculative than definitive)
Clean coal technologies and techniques can happen anywhere from the mining stage (such as washing coal), through to the coal usage stage (such as capturing gases or contaminants), and even waste product stage (such as re-using or storing coal by-product waste)
Washing coal at the mine site or immediately after, before it is transported, has been used for a long time
Carbon capture and storage (and also carbon capture and use) is one method that gets a lot of attention in the present day.
Storage refers to sequestration under ground or in the ocean, whereas capture and use might refer to carbon use for oil enhancement for example
And recently, newer ‘high efficiency low emission’ coal plants like super critical and ultra super critical plants have been added to the list of clean coal solutions
Some sources say the future of clean coal is gasification of coal, along with carbon capture and storage or re-use.
To be specific, world-nuclear.org says ‘The clean coal technology field is moving in the direction of coal gasification with a second stage so as to produce a concentrated and pressurised carbon dioxide stream followed by its separation and geological storage. This technology has the potential to provide what may be called “zero emissions” – in reality, extremely low emissions of the conventional coal pollutants, and as low-as-engineered carbon dioxide emissions’
Other reports say that clean coal technology is too ambitious, unreliable and costly, and we should pursue other energy sources instead, such as natural gas, nuclear and renewable or cleaner forms of energy
Some of the major barriers to clean coal technology and techniques are cost (both design and operation), along with how energy intensive some coal cleaning or emissions capture and sequester techniques can be.
Some technology can also be unreliable and inconsistent, and new clean coal plants have either been discontinued at the pre construction, construction or operation stages for these reasons, and other reasons.
Read more about the pros and cons of clean coal in this guide.
A list of some of the clean coal technologies outlined below are:
Washing coal and dewatering coal
Electrostatic precipitators and fabric bag filters
Flue gas desulfurization
Increased efficiency of a coal plant
New plants like HELE plants that use pulverized coal combustion, or have a difference in the steam cycly or stream pressure within the boiler (super critical and ultra critical coal plants)
Advanced tech like IGCC and PFBC
Ultra clean coal
CCS, and CCUS
Retiring old ‘dirtier’ plants
Taxes, fines and other penalties on ‘dirty’ plants
In the future – coal gasification with a second stage may be a new potential technology
What Is Clean Coal Technology?
The definition of clean coal has changed over time:
It started out with washing coal at the mine, which removes some of the sulfur and other contaminants, and makes the coal cleaner and cheaper to transport (wikipedia.org)
It then proceeded to CCS (carbon capture and storage technology), whereby the carbon dioxide generated at coal plants it captured and stored underground (usually in rock formations and aquifers)
And most recently, it’s been used to describe coal plants with new designs that are more efficient and/or reduce pollution/emissions (such as Super critical coal power plants, Ultra super critical coal power plants, and other types of High efficiency low emission coal power plants)
The reality is though – ‘clean coal’ can be used as a term to describe any part of the coal-for-energy process (it could be a new design, device, method/technique, action, regulation, or something else) that reduces or eliminates emissions (usually CO2) or air pollution (usually sulphur dioxide, nitrogen dioxide, and particulate matter).
The term ‘clean coal’ is increasingly being used for supercritical coal-fired plants without CCS, on the basis that CO2 emissions are less than for older plants, but are still much greater than for nuclear or renewables
wikipedia.org explains this about air pollution:
[With clean coal technology, the] primary focus was on SO2 and NOx, the most important gases in causation of acid rain, and particulates which cause visible air pollution and have deleterious effects on human health
List Of Clean Coal Technologies & Techniques
Washing coal – after it is mined to reduce emissions of ash and sulfur dioxide when the coal is burned
Electrostatic precipitators and fabric filters – can remove 99% of the fly ash from the flue gases
Flue gas desulfurization – reduces the output of sulfur dioxide to the atmosphere by up to 97%, the task depending on the level of sulfur in the coal and the extent of the reduction
Low-NOx burners – allow coal-fired plants to reduce nitrogen oxide emissions by up to 40%. Coupled with re-burning techniques NOx can be reduced 70% and selective catalytic reduction can clean up 90% of NOx emissions.
Increased efficiency of plant – up to 46% thermal efficiency now (and 50% expected in future) means that newer plants create less emissions per kWh than older ones
Advanced technologies such as Integrated Gasification Combined Cycle (IGCC) and Pressurised Fluidised Bed Combustion (PFBC) enable higher thermal efficiencies still – up to 50% in the future
Ultra-clean coal (UCC) from new processing technologies – which reduce ash below 0.25% and sulfur to very low levels mean that pulverised coal might be used as fuel for very large marine engines, in place of heavy fuel oil [but, technologies under still under development … and wastes from UCC are likely to be a problem]
[Carbon capture, and storage (CCS) technology, or Carbon capture, use and storage (CCUS) technology. Once CO2 is captured and separated, Sequestration refers to disposal of liquid carbon dioxide, once captured, into deep geological strata. The main potential appears to be deep saline aquifers and depleted oil and gas fields]
[Retiring old less environmentally coal plants, and building new supercritical and ultra-supercritical coal-fired plants, also know as high efficiency low emissions plants. An ultra supercritical plant reduces emissions and fuel costs to about 75% of subcritical plants]
[Carbon taxes can indirectly reduce coal emissions by having a penalty for high carbon emitters]
The clean coal technology field is moving in the direction of coal gasification with a second stage so as to produce a concentrated and pressurised carbon dioxide stream followed by its separation and geological storage. This technology has the potential to provide what may be called “zero emissions” – in reality, extremely low emissions of the conventional coal pollutants, and as low-as-engineered carbon dioxide emissions
Fine particle pollution is another hazard [from coal energy] where technology exists to mitigate it — but it’s not always used … Fine-particle bag filters can reduce [coal power plant] fine particle emissions.
Bag filters can help reduce the amount of PM 10, or particulate matter smaller than 10 microns that power stations release. At this size, particles can enter the lungs and bloodstream.
Particles below PM 2.5 pose the greatest threat to health
Electrostatic precipitators are … considered best practice for capturing emissions from brown coal
Mercury absorbers [can also be used to absorb mercury emissions]
Chemically washing minerals and impurities from the coal [after mining]. This step removes some of the sulfur and other contaminants, including rocks and soil. This makes coal cleaner and cheaper to transport.
Gasification [of coal] (see also IGCC)
Improved technology for treating flue gases to remove pollutants to increasingly stringent levels and at higher efficiency
Carbon capture and storage technologies to capture the carbon dioxide from the flue gas
Dewatering lower rank coals (brown coals) to improve the calorific value, and thus the efficiency of the conversion into electricity
[When coal burns, carbon dioxide and air pollutants come out in the flue gas – which is what is released out of the smoke stack of a coal power plant]
[Clean coal technologies work to make coal burning more efficient or to decrease these harmful emissions]
Clean coal technology can include…
Purifying the coal before it burns [e.g. coal washing]
Controlling the coal burn to minimize emissions [e.g. Wet scrubbers, or flue gas desulfurization systems that help remove sulfur dioxide, Low-NOx (nitrogen oxide) burners that reduce the creation of nitrogen oxides, and Electrostatic precipitators that remove particulates]
The gasification of coal
CO2 processing and processing techniques can include carbon capture storage (in the ground or ocean), flue gas separation, oxy-fuel combustion, or pre-combustion capture
Overall, cleaning coal and sequestering emissions raising the price of coal produced energy.
This can be offset to an extent by commercializing coal energy waste and by-products, but a carbon tax would then again add to costs of operation
The type of coal used can impact how clean the coal burning process is in different ways:
Brown coal can be less efficient than black coal and more energy intensive (but, this may not necessarily be better for carbon emissions because brown coal also has a lower carbon content)
CCS (carbon capture storage) can be include:
Pre combustion capture, post combustion capture and oxy fuel combustion
Other carbon capture and storage technologies include those that dewater low-rank coals
Using a carbonate looping process [for post-combustion CO2 capture from power plants ]
Although commercialising coal waste such as coal ash isn’t traditionally seen as part of the clean coal space, we have written about it in our guide on the pros and cons of coal energy.
It can certainly make coal energy more sustainable by making it more sustainable overall.
Newest Clean Coal Power Plants (Subcritical, Super Critical & Ultra Super Critical – also called High Efficiency, Low Emissions Plants)
We thought we’d do a specific section for these newer ‘high efficiency, low emissions’ coal plants.
These plants use pulverized coal combustion technology like regular or older coal plants, but have a difference in the steam cycle or the steam pressure within the boiler.
This impacts the efficiency of each plant.
Some notes on these types of coal plants (paraphrased from reneweconomy.com.au):
Which type of steam cycle is used has no impact on the emissions per tonne of coal burned
The only difference between different steam cycles in terms of emissions is how much power they can generate from one tonne of coal
A typical new subcritical plant will have a thermal efficiency of 38 per cent, meaning that 38 per cent of the thermal energy contained in the fuel is converted into electrical energy fed into the grid.
A supercritical plant will have an efficiency of maybe 42 per cent and a typical ultra-supercritical plant will achieve around 44 per cent (designs going up to 47 per cent are being developed).
The difference between subcritical and ultra-supercritical technology is that the total amount of flue gas emitted from the ultra-supercritical plant is about 14 per cent smaller, and hence the capacity of the SO2 control device can be about 14 per cent lower, resulting in savings in investment and operating costs.
Resulting SO2 emissions associated with a given emission standard will also be about 14 per cent lower.
The same logic applies to the emissions nitrogen oxide (NOx), particulate matter (PM), mercury and other heavy metals.
The air quality and health impacts are directly proportional to emissions.
Read more about the differences in these plants in the sourcewatch.org and reneweconomy.com.au reports