Optimising a dynamic process

The cement production process is dynamic, with substantial opportunities for optimisation, improving plant productivity as well as profitability. Cemtech Technical Workshops are an important way of disseminating this information among plant staff and have proven highly successful. We take a look at what lies ahead in the 2017 programme of Cemtech Workshops. By Dr Michael Clark, UK.

As you read this month’s Technical Forum in the first ICR issue of 2017, preparations for the Cemtech conference in Dubai in February will be well underway. The Cemtech conference at the Grand Hyatt in Dubai has become the first major event of the Technical Forum year over the last 10 years and marks a new Technical Workshop to be delivered alongside the Cemtech conference.

Solving a conundrum

The Technical Workshop in 2016 was entitled “The progressive levels of optimisation” and was delivered in Dubai, Manila and Madrid. The workshop was well received by people from cement companies in Azerbaijan, Bosnia, China, Cyprus, Ethiopia, Fiji, Malaysia, Pakistan, the Philippines, Poland, Saudi Arabia, South Korea, Spain, the UK and Yemen. It would be difficult to reach such a diverse audience through any other forum than the Cemtech Technical Workshops held alongside Cemtech conferences in the Middle East, southeast Asia and Europe.

The success of the 2016 workshop presents a conundrum for the moderator who develops and leads these workshops: How to preserve and maintain that success while keeping the workshops fresh and up-to-date? In 2017 this will be solved by retaining the best aspects of the 2016 workshop with the addition of 20-30 per cent new material in a workshop entitled “Optimising a dynamic process”.

Preparing the kiln feed

In 2016, the first of the progressive levels of optimisation reflected on the steps taken to prepare the kiln feed properly for conversion into clinker in the kiln, both chemically and physically. This includes selecting the appropriate raw materials for cement manufacture and bringing those raw materials together in the right proportions to produce clinker of the required mineralogy by process of raw mix design. The ashes from the fuels burnt on the kiln that will be incorporated in the clinker must also be taken into consideration. The cement producer needs to move beyond planning the optimisation of the kiln feed to the practical steps to be taken in the quarries, crushers, pre-homogenisation, raw grinding and kiln feed blending systems of cement factories.

What will be new in 2017? Increasingly modern cement plants use vertical roller mills (VRMs) for raw grinding. The opportunities for optimisation of VRMs are less obvious than with ball mills. All the processes of grinding, drying, classification and regrinding take place within the body of the mill and there are no opportunities to take samples to assess the performance of the separator or the recirculating load in the mill, or the progress of size reduction along the grinding path.

However, this does not mean there are no opportunities for optimisation. The gas flow and velocities through the different parts of the VRM have a critical impact on the external recirculation of materials, the pressure loss across the mill and the electricity consumed by the mill induced draught fan. The depth of the bed of material on the table and the hydraulic pressure on the rollers impact on the grinding efficiency and production rate of the mill. Any adjustments to affect the gas flow and velocity through the mill, the depth of material on the table or the pressure on the rollers will have multiple impacts on the performance of the mill. The process is dynamic and adjustments lead to multiple changes.

Fuels and their combustion

The second of the progressive levels of optimisation in 2016 was the fuels used to fire the cement kiln and the appropriate combustion of those fuels.

Fuel types

Cement manufacturers continuously look to optimise the fuels used to fire their cement kilns because these are usually the highest variable cost item for manufacturing clinker and cement. That is why ICR and Cemtech conferences are full of articles and presentations about projects to burn alternative fuels in cement kilns or switch from one type of fuel to another. The complexities of burning alternative fuels should not be underestimated, nor should the complexities of switching traditional fuels between fuel oil, natural gas, coal and petcoke.

Optimising the combustion of the fuels in the kiln involves much more than providing sufficient fuel for the thermal energy requirements of the process. The fuel at the main burner must be burnt with sufficient intensity to reach the maximum temperature required in the burning zone of the kiln for clinker combination, but also to have the required temperature profile through the kiln to produce clinker of optimal hydraulic reactivity when the clinker is converted into cement.

Again, what will be new in 2017? The impact of the combustion conditions, the temperature regime through the process, and the incorporation of the ash from the fuels on the ultimate quality of the clinker will receive more coverage. To really monitor and understand the impact of combustion, temperature history and incorporation of trace components into the clinker requires more than simply calculating the Bogue mineralogy of the clinker.

Ideally the real mineralogy of the clinker needs to be monitored by XRD or microscopy. If that is not possible the likely impact on the real mineralogy needs to be at least anticipated.

Impact of combustion conditions

The impact of combustion conditions is where the cement kiln process becomes truly dynamic. The combustion must be sufficiently intense to reach the required temperatures and temperature profile with the minimum amount of fuel and the minimum amount of excess air drawn into the kiln. Sub-optimal combustion quickly leads to excessive sulphur circulation in the kiln, leading to clogging of preheaters, formation of rings and balls in the kiln, and dusty and high free lime clinker. Dust recirculating from the cooler kiln further reduces combustion efficiency, driving ever greater sulphur recirculation and a downward spiral of process stability.

As the moderator writes he is returning from Latin America where a catalogue of preheater clogging, ring and ball formation issues have been solved by kiln main burner adjustments and accepting a higher free lime content of the clinker produced from the kiln.

Trying to achieve less than one per cent free lime in clinker is counter-productive in kilns with high sulphur inputs. Over-burning leads to sulphur circulation, and in turn leads to higher free lime clinker, resulting in more over-burning to try to reduce free lime. This causes even higher sulphur recirculation, and even higher free lime content of clinker in a vicious cycle, all ultimately leading to severely reduced clinker output, higher specific fuel consumption and kiln stoppages due to clogging, rings and balls.

These dynamic impacts of the combustion of the fuel in the kiln are almost always kiln specific and best illustrated by case studies. Those will be incorporated in the 2017 Cemtech Technical Workshop, but a common thread needs to be drawn through these case studies so that participants can relate them to their particular kilns and circumstances. There is almost nothing that happens in a particular cement kiln that has not happened somewhere else in the world.

Gas flow

The third of the progressive levels of optimisation in the 2016 Technical Workshop was the gas flow through the cement kiln process. This includes process air as well as emissions from areas such as the kiln.

Air flow

This subject starts with providing the preheated air required for the combustion of the fuels from the cooler of the kiln. That air combines with the carbon and hydrogen in the fuels to form the combustion product gases that are drawn through the process together with the carbon dioxide derived from the calcium carbonate in the kiln feed.

The dynamic nature of the cement kiln process now starts to become apparent. Air supply from the cooler affects many parts of the process, including:

• fuel combustion
• combustion product gases and volume
• temperature profile in the kiln
• clinker quality
• recirculation of volatile species
• pressure loss across the kiln system
• electricity drawn by the preheater induced draught fan
• ability of the induced draught fan to draw air into the kiln from the clinker cooler.

In the 2017 workshop these dynamic interactions are illustrated with case studies, but will also try to demonstrate the commonalities between the case studies.

Emissions

When considering the gas flow through the process we must consider the gases that exit the process, ie the emissions. The emissions from a cement kiln are both simple and complex, benign and noxious. This is a topic that could certainly consume an entire Technical Forum and probably an entire Technical Workshop.

Emissions from cement kilns is a topic of relevance to all cement producers, but the priority attached to emissions certainly changes from region to region. In Europe and North America it is a top priority and is becoming increasingly important in parts of Asia and Latin America. However, in other parts of Asia and Africa it is further down the list.

How to cope with these different priorities in the Cemtech Technical Workshops? Well, in truth there are far more potential topics to talk about than can be covered in a three-day workshop. In these Cemtech Technical Workshops we allow the participants to determine the priorities to a certain extent. In the Middle East and Africa the priorities might be primarily productivity and reliability. In Asia energy efficiency along with productivity. In Europe the same but with an increasing emphasis on environmental performance.

The Cemtech Technical Workshops try to cater to these differences by letting participants nominate their own priorities during these sessions.

Optimising profitability as the ultimate goal

Reasons for optimisation of the cement manufacturing process might be to improve plant productivity, resource consumption efficiency, productivity or environmental performance. Ultimately what needs to be optimised is the profitability of the cement factory and company.

The Cemtech Technical Workshops are technical – hence the name. However, the impact of optimisation on profitability must be the underlying reason for that optimisation. The workshop will always try to illustrate the potential impacts on profitability as its starting point.

We look forward to your participation in the updated Cemtech Technical Workshops in Dubai, UAE, 20-22 February 2017, or at any of the other Cemtech conference venues in southeast Asia and Europe to be held later in 2017.

Article first published in International Cement Review, January 2017.