Rationalising cement plant design
The typical cycle
In a typical project plan of a cement plant, it is generally accepted that the time between the placement of the first major purchase order and first excavation at site is at least six months, provided the customer places an EPC order on a major cement machinery supplier having adequate resources for carrying out simultaneous engineering on multiple streams. If, on the other hand, the customer opts to place individual equipment orders on OEMs, this time period could stretch to anywhere between eight to eleven months. In the equipment ordering mode, it is the customer who bears the responsibility of coordination between the OEMs, the consultant and the contractors at site.
The purpose of this article is to stimulate some out-of-the-box thinking to evaluate and check if even the accepted six months between the first purchase order and first excavation, can be reduced further. We shall not attempt this in the case of equipment ordering mode as the intense coordination, required between customer, OEMs and the consultant for even the first release of construction drawings, shall automatically ensure delays.
Let us start with the future sizes of cement plants. Today, most of the large limestone deposits in India have been either tied up or already exploited. The existing deposits are far away from urban centres, which are the large consumers of cement. It is therefore likely that the days of 10,000 tpd plus cement plants are over, and that the plants of the future are likely to be in the 5,000 to 7,000 tpd range. This size will help in exploiting smaller deposits, which are also likely to be more accessible from a logistics point of view. While this article does not rule out the large plants completely, it is our contention that the majority of the plants of the future are likely to be in the 5,000 to 7,000 tpd range.
With the need to cover the stockpiles, all the material storages will have to be either silos or longitudinal storages. The storage capacities of these stockpiles in terms of number of days for various materials like limestone, additives, clinker, coal and gypsum are also fairly standardised.
Most of the deposits today are located away from existing railway lines. Thus, the primary mode of transport of materials to and from the plant is likely to be road. Most plants are located on fairly flat ground and minor undulations in the ground are levelled off and the plinth levels of all the process buildings in the plant are usually considered at one level.
The systems ultimately selected by most customers are also fairly standard. Vertical roller mills for raw and coal grinding, reverse air bag houses for de-dusting of kiln gases, six stage pre-heaters, three support kilns, coolers, ESPs for cooler gases and clinker tanks for clinker storage, are the norm for most plants. The central portion of the plant, which is considered the heart of the cement plant stretching from the raw mill, through the blending silo, pre-heater, kiln, cooler up to the clinker silo, are usually laid out in a straight line.
Hence, one can see that the so-called ´variables´ in a cement plant generally lie within a narrow band of values, and can therefore be frozen at an optimum value. And this brings us to the central question of this article. Why are all cement plants today designed from first principles, going through the motions of obtaining equipment drawings from the OEMs, preparing general arrangement drawings of the process buildings and designing the civil structures to finally produce construction drawings?
It is suggested for the technology suppliers and the EPC contractors to get together and pre-engineer the plants to be ready with at least the general arrangement drawings and a portion of the civil design of the main process buildings. If the 3D models of the buildings are made ready, the site conditions, which vary from site to site, can be factored in and the required analysis can be done expeditiously.
Standard solutions to save time
The idea is to standardise and modularise the heart of the cement plant such that the final general arrangement drawings are ready before the customer orders the plant. It can be seen that, for a particular plant size, at the most, three sizes of raw mills will be sufficient to handle the variations in the grindability of different types of limestone. An additional fourth size means that the next plant size can be catered to. Three sizes each of blending and clinker silos will suffice to fulfil most customer requirements, for two plant capacities, 5,000 and 7,000 tpd. The cyclone pre-heater, kiln, cooler and coal mill sizes are usually decided once the rated capacity of the plant is frozen. The layout of the modules can be prepared in such a manner that it is possible to remove a module of one size and plug in one of another size without affecting the rest of the layout.
Thus, most of the engineering related to layouts can be completed earlier and about four to six weeks can be saved from the six months for engineering as mentioned earlier. This is possible only in an EPC mode of ordering. When the customer chooses to order the plant as individual equipment or even as packages, there is a certain amount of sequencing that gets inherently built in to the project management process. In this case, the customer becomes the hub and the rest of the stakeholders, the spokes. Every major activity between the stakeholders needs to be ratified or approved by the customer, thus inserting points of delay in the entire process. In EPC mode, this coordination between receipt of equipment drawings, preparation of general arrangement drawings and civil design happens internally with the customer being kept in the loop at all times. While the customer can interject whenever necessary, the mandatory customer approval, that is required in the other modes, does not hinder or delay the process. The customer thus manages the project by exception.
Another area that frequently remains unaddressed is rationalisation or streamlining. When the plants are ordered in package mode, the customer attempts to bring together the OEMs, the consultant and the civil and mechanical contractors, and hopes that between them, they will put together the most cost-effective plant. But if one looks at the post-order value drivers for the stakeholders, none of them have a genuine interest in reducing wasteful civil quantities. The OEMs and consultant are interested in reducing the man-hours they spend on the project, to cut their costs. The contractors are paid per cubic metre of concrete or tonne of steel and do not have a say in rationalising the layouts or civil designs. That is why, at the end of a project, most customers have a nagging In an EPC mode, the EPC contractor is simultaneously responsible for layouts and civil design. He is in a better position to bring about rationalisation of the plant. An added advantage is that site queries that usually crop up during construction are handled more expeditiously because the civil design is in-house. But that advantage is not relevant here.
If pre-engineering, as suggested above, is also carried out, the EPC contractor has more time to rationalise and can therefore do a better job. The customer needs to play a significant role in this process. Tempting as it may be, he needs to curb the natural tendency to make changes in the pre-engineered layouts. Changes to be incorporated will set back the process several steps and squander away the time gains expected from this approach.
If cement plant sizes can be standardised to some popular capacities, it is possible for EPC contractors to modularise the main process sections and pre-engineer some of the critical paths in the project in order to save valuable project implementation time.