The US NESHAP regulations provided a rich talking point at Cemtech Americas 2015 and discussions continued during the associated Technical Workshop. At the forthcoming Cemtech Middle East & Africa Technical Workshop on 8-10 February 2016, attention turns to the progressive levels of optimisation and understanding their interaction.
The turn of the year is a very busy time for the Technical Forum, CemNet and ICR. Along with everything else, the festive season brings additional demands beyond technical considerations of the manufacture of cement, but at a time when there is a lot happening. Cemtech Americas 2015, with the associated Technical Workshop, marks the start of this busy period in early December. No sooner has this conference concluded, than thoughts turn to Cemtech Middle East and Africa in early February – with less than two months between the two very important events and the festive season in between.
NESHAP regulations and compliance in focus
Speakers at Cemtech Americas, which took place in Orlando, USA, on 7-10 December 2015, presented a range of very good and interesting papers, thus maintaining the high standards to which delegates have become accustomed.
The impact of increased regulatory responsibilities and adapting plant operations to new environmental rules formed a key part of proceedings. The New National Emissions Standards for Hazardous Air Pollutants (NESHAP) regulations for conventional cement kilns came into force in September 2015, with a further 12 months being required to demonstrate compliance. If a kiln is firing alternative fuels it is considered as a Commercial Industrial Solid Waste Incinerator (CISWI) kiln and the compliance deadline is February 2018. This had been expected in the US for some time but now applies for most kilns with the clock ticking to demonstrate compliance. This, and in particular, the possibilities to comply with mercury emissions were also major topics of conversation during the Cemtech Americas Technical Workshop.
As part of the main conference, Al Linero of Koogler & Associates (USA) delivered an excellent paper on the options to comply with mercury emission limits in Florida. For conventional kilns those limits are 55lb of mercury/Mt of clinker produced, based on 30-day average emissions. For CISWI kilns they will be 11mg of mercury/dry standard cubic metre of exhaust at seven per cent O2 (ie virtually the same emission rate, according to the EPA). In Florida the mercury content of limestone is not an issue. The main mercury input source is fly ash added to raw mix to raise the alumina content of the kiln feed and clinker. Mr Linero emphasised the importance of mercury speciation (elemental or combined mercury) in continuous mercury emissions monitoring and how this changes dependent on whether the kiln is operating in combination with the raw mill or the raw mill is standing alone. The raw mill is a very good scrubber of mercury: mercury captured in the raw feed can be removed, thereby breaking the mercury cycle and enabling the plant to meet its emissions limits. In the US this is referred to as ‘dust shuttling’.
Technical Workshop talks
Back in the Technical Workshop these options were discussed in detail along with some interesting solutions to mercury compliance that are being promoted in the US.
One such solution is from Mercury Capture Systems, whose proposition is for a thermal desorbtion gas reactor. This involves heating the cement kiln dust (CKD) to volatilise the mercury and then directing the volatilised fumes to “a modified particulate scrubber with a constantly-recirculating stream of reagent catalyst. In the normal configuration, vaporised mercury is carried into the scrubber and then removed from the gas stream through contact with an aqueous reagent catalyst solution”.
No doubt that should work, but one problem with such a solution is that this is a standalone system, which might require extra manpower and will require additional thermal energy. However, the suggestion to use this to clean fly ash of mercury before adding it to the raw mix is sound. More concerning are the company’s assertions regarding the true cost of dust shuttling. It claims that CKD is essentially the same as clinker with an equivalent value to clinker, neither of which is true. It also states that it is usual for raw mill capacity to be a limitation of the cement manufacturing process, which it is not. Furthermore, the company claims that there are losses of clinker or cement production with dust shuttling, which again there are not.
A more impressive proposition is the Scheuch Ex-Mercury solution with the separate preheater to which the dust is delivered independently of the kiln feed (see ICR January 2016). This is a robust process which operates continuously with the kiln with virtually no additional manpower or thermal energy requirements. However, it does involve major plant modification with associated capex implications. Scheuch acknowledges the effectiveness of dust shuttling but has developed a different solution due to the quantity of dust that can be involved.
One interesting point of discussion is why extract (shuttle) dust when the raw mill is better at absorbing mercury? This is because with a three-fan kiln and raw mill system some of the exhaust is always going to the filter. The filter efficiently captures mercury on the bags and the mercury that directly reaches the filter is added to the mercury that has been absorbed in the mill. So this dust has a greater concentration of mercury than the mill product because it has mercury from more of the exhaust gas stream. Therefore, this is the material stream to shuttle to break the mercury cycle. To bypass some of the downcomer exhaust gas is a further improvement to the mercury abatement process as that is where the mercury is most concentrated in the process.
From a technical perspective the moderator thinks the new NESHAP regulations are a wonderful development. Irrespective of the environmental improvements they will deliver, they also make us think about and develop an understanding of the complexities of the cement manufacturing process. Only when we have developed that understanding can we identify the best solutions to meet the new NESHAP requirements.
Exploring the progressive levels of optimisation
That brings us to considerations of the forthcoming Cemtech Technical Workshop in Dubai. This three-day, classroom-based course will follow the theme of the progressive levels of optimisation (as discussed in Technical Forum, ICR January 2016). The workshop will begin by looking at raw materials and their preparation, moving on to the fuels for firing a cement kiln and the ventilation of the exhaust gases from the process. An understanding of all these processes and how they interact is necessary to control and optimise the cement kiln process. The intention of the Dubai workshop will be to provide and enhance that understanding of the individual processes and their interaction.
We have already received positive feedback regarding our thoughts on level 1 optimisation relating to raw materials, as presented in the previous Technical Forum. The above considerations of NESHAP and emissions control and mitigation illustrate the importance of level 3 optimisation, relating to exhausting the gases from the cement manufacturing process. No aspect of the cement manufacturing process can be considered in isolation. All the different aspects interact with one-another. We will be seeking to explore this in the workshop in Dubai in February, as well as at the Cemtech Asia, Europe and Americas conferences later this year.
Article first published in International Cement Review, January 2016.
