Atritor’s Andrew Rigg shows how existing technologies can be used to simultaneously dry, deagglomerate and classify difficult ‘waste’ ashes...
The global cement industry faces several challenges at present, not least that of sourcing sufficient supplementary cementitious materials (SCMs). Not only are both slags and fly ash supplies in decline, but the cost to transport them has risen significantly in the past 12 - 24 months, due to a heady mix of post-Covid construction demand, high shipping costs and astronomical fuel prices. At the same time, there is an increasing emphasis on reducing the clinker factor of cement blends, prompted by the sector’s aim to reach net-zero CO2 emissions. SCM supplies are falling just as more are needed.
Against this backdrop some SCM users are considering alternative sources that were previously uneconomical. One such source is waste ash, found in ponds, dumps and silos at (former) coal-fired power stations all around the world. In my native UK, there are an estimated 100Mt of fly ash. In the US, the volumes could be as much as 10 times greater. And these are just two of dozens of markets crying out for SCMs. There are many more markets with large dumped fly ash reserves.
Upgrading ‘waste’ ashes
So how can such ashes be upgraded? Pointers come from the US, where harvesters have sprung up to remove and screen ashes from ponds and other dumps. After drying and classification, they - or third-party marketers - can sell it to cement and concrete producers.
There are several well-established methods to upgrade ‘waste’ ashes. One of these is the Atritor Dryer-Pulveriser, a well-established technology that has been used for ash applications for around 15 years. The Dryer-Pulveriser suspends the wet ash while it is dried, deagglomerated and then classified over a 0.5s timeframe. A high temperature differential across the mill ensures efficient removal of moisture and a dry product. To do this, the air in the Dryer-Pulveriser has traditionally been heated to up to 550°C using natural gas. However, an increasing number of users make use of alternative fuels like bio-gas and hot air heated with renewable electricity. The use of waste heat from the cement plant is also possible. In the future, green hydrogen could also be used. The dry fly ash, which rarely exceeds 60°C, is disentrained from the air in a bag house, conveyed and stored as required.
The moisture in the exhaust gas, typically at 100°C, will not condense until it reaches the dew point, typically 50-60°C, so it will exit through the stack. Another option is to capture the moisture for later use, the latent heat recovered in a heat exchanger. There are also possiblities to reuse the dry exhaust air in the process.
Applications in the cement sector
There are several ways that the Atritor Dryer-Pulveriser could be used in the cement sector. Given its relatively small 100m2 footprint, the components could be built at the site of waste ash reserves by harvesters or marketers, which could remove the moisture for economical transport. Alternatively, the unit could be built at the cement plant itself, enabling it to produce its own SCM as required.
Indeed, on-site production is particularly appealing, as it enables users to be in control of their own fly ash supplies. Rather than being at the mercy of international markets, where prices - and CO2 emissions - are high, they can reduce costs and emissions and develop guaranteed local supplies.
The Dryer-Pulveriser can handle capacities up to 20t/hr, with products that conform to EN450. If larger capacities are needed, the user can install multiple lines. In addition, the process can be tuned to some degree. The moisture levels and particle size distribution can be tuned to match the requirements of the cement mix design rather than what happens to be available. The Dryer-Pulveriser can even process some amounts of bottom ash, which is not usually suitable for use as an SCM, as part of the feed.
Conclusion
The challenge of securing sufficient SCMs is becoming more intense every day and it isn’t going away. Harvested ash that has been dried, deagglomerated and classified presents a low-cost way for cement and concrete producers to take control of making their own SCMs from locally-available resources. Based on its analysis of the market, Atritor believes that the US has reached an economic tipping point for fly ash harvesting. The rest of the world is not far behind. Let the ash harvest commence!
