Reducing Cost of Production in Cement Plant BY Nohman Mahmud
One of the major factor contributing to the success of an enterprise / organization is the Cost of Production reduction / optimization. The route to which demand Operational Excellence, Optimization of processes, Eliminating / reducing wastage’s, rework, scrap and establishing ,operating ,optimizing and bench-marking KPI’s & SMART Targets.
As far as cement plant operations is concerned the Cost of Production may be divided into following Major Heads A) Raw Materials Cost B) Fuel Cost C) Power Cost D) Cost of Quality E) Maintenance Cost F) Packaging Material Cost G) Human Resources Cost H)Marketing / Sales Department Cost I ) Financial Cost
Out of the above cost heads; will focus primarily on the relevant technical Cost Heads i.e. A to D .
A) Optimizing Raw Materials Cost:
First step in the optimization of Raw Materials cost is to benchmark and identify that which of the existing raw materials is most cheaper and technically viable to be effectively used for cement manufacturing; are there some alternative or waste raw materials available which can be used for the production of cement manufacturing process. In case suitable waste or alternative raw materials are available than mostly the cost of those raw materials will be less than the typical raw materials used for cement manufacturing process.
Total cost analysis should be done not only with regard to the cost of materials (Purchased or Factory Quarried / Mined); but should also include the impact of that particular raw materials on the individual process especially with reference to grinding energy requirements and thermal energy requirements.
a) In some areas the Limestone contain variable MgO content , so instead of dumping / land filling the high MgO limestone ; the high MgO limestone can be reserved for the cement grinding stage as it is not the combined MgO which causes expansion and is only free crystalline Hard Burnt MgO which is produced during pyroprocessing which is not good from quality point of view .However since there is a limit on the total MgO content in different international standard hence total MgO must be reasonably low than the max allowed limit as per International standards.
b) Some limestone deposits contain high Alkalis; which instead of dumping / land filling can be balanced by using high Sulfur low cost fuels (Resulting in decrease of production cost) or by small usage of gypsum (helps in Pyroprocessing by modifying the liquid phase properties) during the raw milling stage. In case the limestone is having high Sulfur it can be balanced of by using high alkali content of the raw materials; or by slightly using high oxygen during the pyroprocessing .
c) Raw Materials moisture content not only have impact on heat consumption but also on the Waste Heat Recovery Generation (if such plant is installed). More the moisture content in the raw materials more heat will be required to dry it out and hence high will be the specific heat consumption for that plant along with low waste power generation. The design of Preheater stages is often based on the amount of moisture present in raw materials; with high moisture the number of preheater stages will be decreased from the conventional five or six stage preheater. Considering the moisture content in the raw materials; increase in cyclone stages in preheater will reduce the specific heat consumption. As a generalization increasing the cyclone stages from 4 to 5 to 6 will result in specific heat saving of 25 K Cal/Kg of Clinker / stage with the resultant decrease of Preheater Exit gas temperature which means less heat is available for raw materials drying and waste heat generation. However, it must be understood that first step should be specific heat reduction by optimization and any heat left should be used for WHRPP generation and not the other way that pyroprocessing should be inefficient and heat should be lost for producing waste heat .
B) Optimizing Fuel Cost
Normally the highest cost as far as integrated cement plant is concerned is the cost of fuel. Fuel cost can be optimized by analyzing the types of fuels available which may include the conventional fuel like Coal, Natural Gas or AGO / LPFO or use of Bio Fuels, Alternative or waste fuels like Chicken Waste, Bagass, Rice Husk, Carbon Black, TDF, RDF etc and then selecting the most appropriate and cheaper one. Detail analysis is required and not just the Rs / KCal value which is often considered as the sole criteria for fuel selection.
a) One aspect which needs to be remembered during fuel selection especially when using solid fuels is the amount of Ash, higher the Ash in fuel, higher will be the LSF required in Kiln Feed / Raw Meal to get the desired LSF /C3S in clinker for producing the same strength required in cement as per the strength class / type of cement produced. High LSF means high heat consumption or cost of production; since the highest energy requiring reaction during cement manufacturing process is the calcination which is elimination or removal of CO2 from raw materials primarily limestone.
b) Second critical aspect of fuel selection is the sulfur content; which is also mostly the limiting factor in fuel selection, higher the sulfur in fuel, higher the alkalis required to balance it out in order to avoid the buildup / coatings in the pyroprocessing. Moreover, in cement standards, there is limit on total SO3 in cement, higher sulfur input from pyroprocessing means less gypsum can be added to reach that max level, in case gypsum is cheaper than clinker than high sulfur from fuel means less addition of gypsum which means high cost of production not just due to materials price but also since gypsum is much soft in comparison with clinker and hence resulting in low cement mill output. For example, pet coke use at high substitution percentage can increase the clinker SO3 content and thereby increases the cement mill power consumption.
c) Another important aspect of fuel selection specially in solid fuels is the Volatile Matter; less the volatile matter, higher the fineness required to burn it out meaning higher coal mill power consumption. Example Coal which is mostly having high volatile matter in comparison with Pet Coke can be grinded to less finesses without any issue in burning process however when pet coke is used much higher level of fineness is required in order to avoid burning process meaning high coal mill energy consumption.
d) One more parameter to be considered while evaluating coals /solid fuels is the hardness; often expressed using Hard Grove Index (HGI) value. Higher the HGI easier will be the coal to grind; hence less power consumption and cost saving.
e) One practical point needs to be understood while using organic / bio fuels like Chicken Waste / Bagasse etc is that they lose their calorific value over period of time; hence needs to be consumed on First In First Out basis (FIFO).
f) Sometimes additional benefit while using alternative / waste fuels can be realized for example while using TDF; in addition to calorific value, iron can be added from the wires inside TDF resulting in laterite or iron ore saving.
g) While using any fuel with very high chloride , alkali or sulfur content (Especially Chloride) a part of hot gas has to be bypassed to prevent excessive build ups; and bypassing the hot gas means increase of specific heat consumption and increase of raw meal to clinker factor ; resulting in high cost of production.
h) The use of biomass fuels with their higher moisture content and lower Net/Gross CV ratio results in more heat loss via steam resulting in heat penalty meaning higher cost of production.
i) Fuel cost optimization can be done through indirect way i.e. by optimizing the Raw Meal Parameters (LSF /SM /AM / Presence or absence of Quartz / Calcite), Physical Properties of Raw Meal (Fineness / Residue on 90 or 200 Microns), Burning Conditions (Oxidizing / Reducing), Burner orientation as well as momentum , optimization of secondary / tertiary air temperatures as well as optimization of cooler operation.
j) Fuel burning should always be carried out in oxidizing conditions because burning of fuel in oxidizing conditions ; the maximum fuel potential is realized. Complete oxidation of carbon produces 94 Kcal / Mole while incomplete oxidation of carbon produces 53 K Cal / Mole; indicating more or less just half of the heat potential is realized in the reducing conditions.
k) Different types of Mineralizers / Fluxes can be used to reduce the overall thermal energy requirements like CaF2 / Laterite / Bauxite / Iron Ore / Gypsum etc.
C) Optimizing Power Cost:
The critical principle in optimizing power cost is by ensuring the suitable raw materials and production of good quality in process materials like clinker to ensure minimum grinding energy requirement. Elimination / Reduction or Replacement of harder to grind materials / abrasive materials with softer materials will result in grinding energy saving. Suitable lab trials are required to ensure that which material will use more energy to grind which can be later on correlated with the field results in actual industrial use.
a) For example, as far as Siliceous or Argillaceous materials are concerned Silica can come from number of raw materials like Clay, Shale, Slate, Sand Stone, Silica Sand. However, all have different characteristics like Abrasiveness, Hardness and Combinability behavior inside the kiln; hence resulting in different raw mill output, kiln operational parameters as well as power consumption.
b) Similarly, limestone can have different quality; which needs to be addressed and utilized on the basis of balancing with argillaceous materials. Very high purity limestone will be hard to grind as well as combine in comparison with moderate purity limestone; Marl will be even more easy to grind as well as combine in the kiln.
c) Suitable production parameters are needed to be established and bench marked to ensure that minimum grinding energy is required during either of the grinding / crushing stages. Reactive clinker produced as a result of finely grinded raw meal produced within oxidizing conditions using short high energy intensive flame / high momentum and then quickly quenched / cooled will produce clinker which will be easier to grind in the cement milling stage; hence less power consumption.
d) Grinding power savings can be realized by using appropriate additives to replace clinker (Ex Limestone, Pozzolonas, Fly Ash etc) will result into energy savings and hence reduction of cost of production.
e) Optimization of Sulphates (Gypsum) will also result in energy saving during cement milling stages, since gypsum is much easier to grind in comparison with clinker.
f) Grinding aids can also be used to minimize the grinding energy especially during the cement milling stage, however detail cost / benefit analysis is required to establish tangible benefits.
D) Optimizing Cost of Quality
One of the important but less familiar factor contributing to the cost of production (Often Hidden) is the Cost of Quality; however, this concept is less known to the cement fraternity hence require proper elaboration. Cost of quality (COQ) is defined as a methodology that allows an organization to determine the extent to which its resources are used for activities that prevent poor quality, that appraise the quality of the organization’s products or services, and that result from internal and external failures.
Cost of Quality is divided into Cost of Good Quality & Cost of Poor quality. A)Appraisal costs B) Prevention Costs. C) Internal failure costs D) External failure costs. Appraisal and Prevention Cost are included into Cost of Good Quality while Internal and External Failure Cost are included in Cost of Poor Quality.
Appraisal costs are associated with measuring and monitoring activities related to quality. They could include verification, quality audits and supplier assessment and ratings.
Prevention costs are incurred to prevent or avoid quality problems. These costs are associated with the design, implementation, and maintenance of the quality management systems. They include Quality Planning, Quality Assurance & Training and Development.
Internal and External Failure Costs are bad costs of quality and these need to be eliminated / reduced. Internal Failure cost include waste generation, scrap generation, Rework / Rectification of faults. External Failure Cost include Repairs and Service costs, warranty claims, Complaints, Returns, Rejections & deratings / recycling of product.
Production of good quality products conforming to the requirements of customers / standard requirements with minimum waste, rework and customer complaints at the economically and commercially viable cost is the ideal target of the cement plant QA / QC ,R & D and Production Department.
Focusing and investing more into Appraisal and Prevention cost will eliminate or reduce the non-value adding processes which lead to Internal / External Failures hence reducing the bad cost of quality / cost of production.—–See the previous article of undersigned indicating how to practically focus on this subject.
Summary and Conclusion :
Cement Plant cost of production can be reduced / optimized by carefully studying the Raw Materials & Fuels available and than selecting and using the best among the available options ; in addition to Raw Materials and Fuel identifying the Process KPI’s and than Operating and optimizing those KPI’s will result into cost savings.