ANALYZING CEMENT SAMPLES WITH AN OXYGEN BOMB CALORIMETER SYSTEM
Isothermal calorimeters have been known to test the reaction of cement on hydration. Determining the heat of hydration of cement is important and traditionally, the heat of hydration has been determined by measuring the heat of solution. Typically, samples are mixed with water, where the cement hydration process can continuously be monitored over time. The heat flow is used recorded and will reflect the cement hydration process and different phases of the complex process can be determined. The addition of mixtures will change the shape of the heat flow curve and the mixture affect can be quantified. The integrated heat flow over time will give the extent of hydration. Using isothermal calorimetry, the heat of hydration is measured by monitoring the heat flow from the specimen while both the specimen and the surrounding environment are maintained at the same temperature.
The DDS Calorimeters range can be used to study the reaction of cement pastes as well as the temperature during the reaction. It can be used for quality control in cement plants, for optimization of additives to give cement a certain property, as well as a general research tool for the cement laboratory.
Some other applications in the cement industry include :
- Research and development of new formulations in the area of additives and mixtures
- Setting time and premature stiffening of cement
- The influence of sulfate carrier content
- Quality control and production control of cement
- Cement research and concrete production control
Cement is made by heating limestone (calcium carbonate), with small quantities of other materials in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate for form calcium oxide, or quicklime, which is then blended with other materials that have been included in the mix. The resulting hard substance is then ground into a powder to make cement. The manufacture of cement includes mining and crushing of the raw material (limestone, clay, and sand), drying and grinding in a mill, burning in a rotary tube furnace, and finally pulverization of the cement. The oxygen bomb calorimeter is used in the analysis of the raw materials and the burning process of fossil fuels and waste materials.
A cement plant can consume 3-6 GJ of fuel per ton of cement produced, depending on the raw materials and the process used. Most cement kilns use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as secondary fuels, replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Secondary fuel can contain all types of polymers, paper, wood and low fractions of metal. Also recycled wood or parts of automobile tyres might be used as secondary fuel. Before using it as a fuel in the production of cement, several chemical analyses are necessary as quality control. One of this is the determination of the calorific value. When accepting waste materials as substitutes for raw materials and/or fuels in the cement production process, it is important to evaluate the contribution of the waste products to the process.
To determine the fuel requirement during production, the gross and net calorific values of the fuels utilized must be identified. The gross calorific value is determined in a bomb calorimeter according to DIN 51900 in an oxygen atmosphere and at a pressure of 30 bar; a bomb vessel is inserted into the combustion chamber that can assumed isothermal or adiabatic. The fuel to be examined is put into the bomb vessel, ignited, and combusted. The gross calorific value can be determined by measuring the heat released during this combustion process.
To minimize the cost of the production of cement and to reduce the use of fossil energy more and more cement plants utilize secondary fuels. Several chemical analyses for quality control, like the calorific value, must be carried out.
The most important uses of cement are as an ingredient in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.
The use of wastes in cement production can be interesting for two main reasons :
- Natural mineral resources may be substituted if these minerals are also contained in the ash component of the waste
- Fossil fuels may be substituted in case the autonomous incineration of the waste under the process conditions occurs at a sufficiently high combustion temperature.
The calorific value of waste can be used to differentiate between waste elimination and waste recovery in the cement industry. This differentiation is important for a process manager when considering the acceptance of waste, but also as a regulatory tool. The chemical composition of the waste is calculated to find the total contribution to the process. A complicating factor hindering the practical, user-friendly application is that one always needs to calculate the combustion temperatures from the composition of the materials considered.
The use of alternative fuels and raw material is common practice in the cement industry. Over 1 billion tyres are sold worldwide each year, which causes around 1 billion tyres to be scrapped. Despite in increase in the service life of tyres, volumes are growing constantly because of the increased vehicle use worldwide.
Tyres have a complex composition. Depending on their size and intended use they may vary in design, construction and total weight. The main component of a tyre is rubber compound and the compositions of the tyres produced by different manufacturers are very similar. Tyres are generally homogeneous. They are 100% recyclable and a valuable resource. Many countries with environmental legislation refer in their waste management practice to tyres in the waste management hierarchy. This sets the priorities of what to do with scrap tyres. This is : recycle, re-use, production of crumbs and strips, de-polymerisation, energy recovery and disposal.
In cement plants, tyres are used as a secondary fuel source. In order to calculate the benefits from substitution of primary fuels the following considerations have to be taken into account : the Calorific value, and the Biomass/CO2 value. The calorific content value of tyres is between some 6450 Cal/Kg and 8000 Cal/Kg. To substitute 1 ton of coal around 0.76 – 0.95 ton of scrap tyres is needed. Compared to pet coke the substitution factor is slightly more. Around 1.03 – 1.27 ton of scrap tyres is needed to substitute 1 ton of pet coke.
Hence the energetic utilization of tyres has clearly a better CO2 balance than the incineration of fossil fuels as the emission factor for coal is 96 ton CO2/TJ.
The use of tyres as a fuel in cement production influence kiln operation and clinker composition.
The use of alternative fuels to replace conventional fuels, in particular coal, is a widespread practice and can contribute to improving the global warming impact and total environmental footprint of the cement industry. The DDS Calorimeter range plays a small part in this process to determine the best possible materials to use.
The use of alternative fuels to displace coal reduces reliance on fossil fuels, reduces emissions of carbon dioxide and other pollutants, and contributes to long-term cost savings for cement plants. Further, due to their high burning temperatures, cement kilns are well-suited for accepting and efficiently utilizing a wide range of wastes that can present a disposal challenge.
The use of agricultural biomass residues in cement manufacturing is less common in industrialized countries and appears to be concentrated in more rural developing regions. The type of biomass utilized by cement plants is highly variable, and is based on the crops that are locally grown. For example, rice husk, corn stover, hazelnut shells, coconut husks, coffee pods, and palm nut shells which are burned in cement kilns. These substitutes are used as an alternative fuel for cement manufacturing. There is a wide range in the calorific values reported in literature for agricultural biomass. The quantity of agricultural biomass residues that are necessary to replace one ton of coal depends on the residue’s energy value and water content. Approximately 2 tons of biomass residue is required to replace 1 ton of coal. All fuel types have unique combustion characteristics that cement plant operators must adapt to in order for successful kiln operation; biomass is no exception. The relatively low calorific value of biomass can cause flame instability but this is overcome with lower substitution rates and the ability to adjust air flow and flame shape. Biomass is prone to change with time, thus care must be taken to use the material before it begins to break down. New biomass should be rotated into the bottom of storage facilities such that the oldest material is injected into the kiln first.
CHEMICAL AND HAZARDOUS WASTE
Cement plants have been utilizing certain approved hazardous wastes as an alternative fuel since the 1970’s. This includes materials such as spent solvent, obsolete pesticides, paint residues, and anode wastes. Because of the potential for chemical and hazardous wastes to contribute to unwanted emissions, adherence to proper storage and handling protocols is critical for cement kiln operators. There are some hazardous wastes that are presently deemed unsuitable for co-processing in cement kilns including electronic waste, whole batteries, explosives, radioactive waste, mineral acids and corrosives. These materials could result in levels or air emissions and pollutants in the clinker that are unsafe for public health and the environment. Waste fuels are blended together in ratios to match the calorific value of the fossil fuel used at the plant. This approach helps to avoid over-heating in the kiln and minimizes the need for other operating adjustments. In comparison to biomass, chemical and hazardous wastes generally have much higher calorific values. The quantity of chemical and hazardous wastes that are necessary to replace one ton of coal depends on the material’s energy value (calculated using an oxygen bomb calorimeter, like the CAL3K), and water content.
THE USE OF ALTERNATIVE FUELS IN CEMENT KILNS
The types of fuels that can be used as alternative fuels in cement manufacturing (focusing on energy and environmental considerations) include : agricultural biomass, non-agricultural biomass, chemical and hazardous waste, petroleum-based fuels, and miscellaneous alternative fuels.
Non-agricultural biomass includes animal byproducts like fat, meat and bone meal. Other varieties of non-agricultural biomass include sewage sludge, paper sludge, waste paper, and sawdust. The use of sewage sludge in cement manufacturing is a recent trend, due to wastewater treatment plants, bio solids increase and limited landfill space. Similar to agricultural biomass, there is a wide range in the calorific values reported for non-agricultural biomass-derived waste fuels. The range in calorific values of sewage sludge is enormous and depends on the characteristics of the wastewater that it derives from, and the treatment the sludge receives. Relative to other fuel substitutes such as petroleum-based wastes and some chemical and hazardous wastes, biomass has a low calorific value. The carbon neutrality of biomass is one incentive for using biomass; however, it requires enormous volumes of biomass to realize substantial conventional fuel offsets. The quantity of non-agricultural biomass residues that are necessary to replace one ton of coal depends on the residue's energy value and water content. A range of 1.6 and 10.3 tons of biomass residue is required to replace 1 ton of coal.
Globally, approximately 30% of waste-based fuels are derived from petroleum products including tyres, waste oils, rubber, plastics, petroleum coke (petcoke), and asphalt. Among these fuels, tyres and waste oils are the most common. Petroleum-based waste fuels have high calorific values. The quantity of petroleum-based wastes that are necessary to replace one ton of coal depends on the material’s energy value and water content. Between 1.3 – 1.8 tons of chemical and hazardous waste is required to replace 1 ton of coal.
These are a variety of miscellaneous waste fuels such as automobile shredder residue, carpet residue, textiles, wax residue, landfill gas, and municipal solids waste that are burned in cement kilns. Fluff, the term for non-recoverable components of end-of-life vehicles, is typically about 20% by weight of the vehicle and is an amalgam of rubber, plastic, wood, paper, dirt, fabric and ferrous and non-ferrous metal pieces. These materials end up on landfills and is fired in cement kilns as an alternative fuel. The quantity of miscellaneous waste that are necessary to replace one ton of coal depends on the material’s energy value and water content. Approx. 0.9 – 2.3 tons of miscellaneous water is required to replace 1 ton of coal.
The DDS range of Oxygen Bomb Calorimeter Systems can be used to measure the calorific value of the above mentioned materials to calculate how it will burn in a kiln during the manufacturing of cement. It can also be used to calculate the efficiency of alternative fuels used in the cement manufacturing process and it’s effectiveness during the burning process. The bomb calorimeter is used to calculate the calorific value of the waste products used as alternative fuel, playing a small but important part in the process of cement manufacturing in reducing waste products in an Eco-friendly and environmentally friendly way.
THE CAL3K-F CALORIMETER
We recommend using the CAL3K-F Oxygen Bomb Calorimeter to measure the calorific value of the above mentioned samples as a low sample throughput would be required. The system is more affordable for low sample throughput and expandable when a higher sample throughput would be required, making it more affordable for smaller laboratories and testing facilities.Learn More about the CAL3K-F Calorimeter
The CAL3K-F system is robust and suitable to handle the burning of the following samples :
- Coal and Oil Samples
- Petroleum Coke
- Fuel Oils
- Wood / Recycled Wood
- Metal (small quantities)
- Automotive parts & Tyres
- Food & Nutrition by-products : rice husks, corn stover, hazelnut shells, coconut husks, coffee pods, palm nut shells
- Animal by-products : fat, meat and bonemeal
- Biomass products : sewage sludge, paper sludge, waste paper, sawdust
- Chemical / Hazardous Waste : spent solvent, obsolete pesticides, paint residues, anode wastes
- Petroleum based fuels : tyres, waste oils, rubber, plastics, petroleum coke, asphalt
- Other fuels : automobile shredder residue, textiles, wax residue, landfill waste, solid waste.