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INTRODUCTION
Concrete is currently the most extensively used construction material and the global consumption of concrete increases every year. As an essential component in concrete, cement consumption also increases annually. Global cement production grew from 594 Mt in 1970 to 2770 Mt in 2007 [1] [2]. Cement production is extremely energy-intensive and accounts for about 2% of the global primary energy consumption, or up to 5% of the total global industrial energy consumption [3]. Between the large quantities produced and the huge energy consumption, cement is responsible for significant amounts of CO2 being released into the atmosphere. The production of cement contributes to CO2 emissions through two sources: the decomposition of limestone and the combustion of fossil fuel. The CO2 emissions resulting from conversion of limestone into calcium oxide are fairly constant and equate to approximately 540 kg CO2 per tonne of clinker produced. Since multiple factors are involved (such as the thermal efficiency of the kilns), the CO2 emissions resulting from the combustion of fossil fuels fluctuate. In 2006 the global average gross CO2 emissions per tonne of clinker was 866 kg [4]. This value accounts for 5-8% of total human atmospheric CO2 emissions [5]. Recently many steps have been taken to combat CO2 emissions in the cement industry including improving energy efficiency of the kilns, replacing fossil fuel with renewable energy sources and substituting part of Portland cement with other cementitious materials [4]. While these actions have made progress in reducing CO2 emissions they still do not provide a completely sustainable solution.
CONCEPT
The aim of this project is to produce a binding material capable of replacing original Portland cement. This binder will have a reduced impact on the environment specifically regarding the release of CO2 into the atmosphere and the drain on limited resources. The objective is divided into two initial tasks:
1. Obtain a material comprised of hydraulic minerals from the ashes produced in the combustion of a blend of sustainable biomasses and/or waste-products.
2. Adapt the raw materials and the sintering process so that the combustion of biomasses and/or waste-products can be conducted in a way that also generates energy (i.e. using a fluidized bed combustor instead of a rotary kiln).
Through the combination of the above-mentioned tasks a CO2 neutral binder capable of replacing original Portland cement should theoretically be produced. The project will be conducted in a way that will develop a scientific basis for techniques to produce sustainable cement, or bio-cement.
CO2 NEUTRALITY
By replacing the traditional raw materials used in the production of Portland cement (limestone and clay) with biomass and waste products a bio-cement will be created that does not further deplete limited resources. Since biomass is renewable source of raw materials and energy, there is no concern over depleting limited supplies. Landscape mutilation resulting from the mining of limestone will also be avoided. Furthermore the burning of biomass and the resulting decomposition of CaCO3 into CaO does not contribute new CO2 into the
atmosphere, unlike the combustion of fossil fuels or the decomposition of limestone which release carbon that has been stored for millions of years. By replanting harvested biomass the emitted CO2 will be absorbed and returned for a new growth cycle. This cyclical process ensures that no “new” CO2 will be released into the atmosphere [6]. Since firing of the biomass will be done in conjunction with energy production, the ashes, or bio-cement, can be viewed as a by-product. In this regard all the energy invested in harvesting, processing and preparing the biomass will be associated with energy production and not bio-cement production. The utilization of the ashes also provides a solution to the disposal of the waste which would otherwise contribute to landfilling. The utilization of biomass, particularly those stemming from landscaping and agricultural residues, provides a solution for their disposal as well.
RAW MATERIALS
The initial task is to identify biomasses and waste-products apt to replace traditional raw materials used in the production of Portland cement. Since the raw mix will also be required to produce energy upon combustion it is also necessary that the biomass conforms to the requirements of fuel used in energy production. Often these two objectives are at odds with each other. A biomass rich in a desirable element may not always be the most suitable for energy conversion and conversely a fuel that combusts well and provides a large amount of energy may have no validity as kiln feed. With these two objectives in mind raw materials need to be chosen that accommodate one need without adversely effecting on the other.
REQUIREMENTS FOR CLINKER
Any materials may be used to produce Portland cement so long as they will give the proper chemical composition after burning [7]. The clinker minerals found in Portland cement, which we are attempting to replicate, are C3S, C2S, C2A and C4AF. The cementing action of Portland cement is derived largely from the chemical reaction between the clinker minerals C3S and C2S with water. Due to its favorable hydration characteristics, primary importance is placed upon C3S and, to a lesser extend, C2S. In order to form the clinker minerals C3S and C2S we need to obtain both calcium and silica from the biomass both of which need to be present is suitable quantities, proportions and forms [8]. In traditional cement the calcium is provided in the form of calcium carbonate obtained from limestone chalk marl or even seashells. The silica comes from clays and shales.
Generally mineralizers are also incorporated to facilitate the formation of calcium silicates at reduced temperatures. The standard mineralizers are aluminum oxide and iron oxide coming from bauxite and iron ore. In addition to calcium, silicon, aluminum, and iron trace amounts of minor elements (such as sodium, potassium, magnesium, and titanium) often enter into the crystal structures of the clinker minerals [9]. However raw materials with contain excessive amounts of sodium or potassium oxide are considered to be undesirable. These compounds are capable of reactions which can ultimately result in degradation of the concrete. Other oxides such as manganese oxide and phosphorus pentoxide are less common but should also be avoided because they can create problems during burning and degrade the overall quality of the cement produced [7].
REQUIREMENTS FOR ENERGY PRODUCTION
Selecting the best-suited biomass or fuel for energy production depends on a variety of factors all of which relate to the composition of the biomass. Biomass fuels are generally divided into 4 different primary classes: (1) wood and woody materials, (2) herbaceous and other annual growth materials, (3) agricultural by-products and residues, and (4) refuse-derived fuels (RDF) and waste or non-recyclable papers often mixed with plastics [10]. The structural compositions of hemicellulose, cellulose, and lignin, as well as the concentration and composition of inorganic materials account for the distinctions among the first three classes. The calorific value or the heating value is the measure of the energy content of a fuel and is a decisive factor in selecting an appropriate biomass. The heating values for common fuels are generally known but they can also be broadly predicted through characteristics of the biomass such as the ash concentration, the carbon concentration, and the amount of cellulose vs. lignin. Among biomasses a decrease in ash concentrations corresponds to an increase in the heating value.
Higher heating values are also associated with higher carbon contents. Since heating value correlates to the amount of oxygen required for complete combustion, lignin has a higher heating value than cellulose due to its lower degree of oxidation. When selecting a biomass it is critical to choose a fuel with a high enough calorific value to in order to achieve a sufficient net energy. The moisture content of a particular fuel is a limiting factor in biomass combustion due to large amount of energy necessary to evaporate the water which can severely affect the heating value. Generally, self-supporting combustion proceeds up to a moisture content of 65%, otherwise more energy is necessary to satisfy evaporation than that which is liberated during combustion. Biomass with larger hydrogen to carbon ratios (and similarly but lesser so oxygen to carbon ratios) are known to lose a larger portion of fuel during the pyrolysis stage of combustion [10]. With that in mind it is important to select biomasses with low ratios. Nevertheless biomasses still lose a significant portion of their mass in comparison to coal. The quantity of certain elements (such as Si, K, Na, S, Cl, P, Ca, Mg, Fe) should be reduced or when possible eliminated due to their involvement in reactions which lead to ash fouling and slagging. As their principal ash-forming constituents, herbaceous fuels contain silicon and potassium. The presence of silicon is necessary in the formation of clinker minerals but the combination of the two can present potentials ash deposition problems.
The alkali and the silica can react to form alkali silicates which will soften or melt at temperatures below 700˚C. While we cannot reduce the concentration of silicon beneath the minimum amount necessary to form suitable quantities of C3S and C2S, we can attempt to limit the presence of alkalis (which correlates well to cement requirements). Alkalis will also react with sulphur to form compounds that damage the combustor heat transfer surfaces. Potassium is the alkali to be most cautious of since it is the dominant source of alkalis in most biomass fuels. While all biomasses are subject to fouling behaviour the rate at which fouling can transpire is dependent upon the composition of the biomass and can be predicted with the alkali index. By comparing the quantity of alkali oxide in the fuel to the unit of fuel energy one can analyse a biomass’ propensity to foul. The lower the ratio the less likely a particular biomass will foul. While the alkali index does not completely predict fouling behaviour it acts as a suitable guide to aid in the selection of biomass and thus reduces the risk.
COMBINATION OF REQUIREMENTS
Finding a biomass rich in CaO or at least in the quantities necessary to form C3S can be problematic. While most biomasses contain significant quantities of Ca, even those used in energy production, the values pale in comparison to that of limestone. With this in mind our proposal is to form the bulk of the raw feed on woody biomass from maintenance of parks and roads. This fuel is already used in the biomass power plant in Cuijk (The Netherlands) for which the 80 MW bubbling fluidized bed combustor has been adapted. From this power plant we have received samples and the ash was found to contain 21,9 M-% Ca (and 12,5 M.-% Si). While significantly below the necessary requirement it is particularly high for a locally available biomass. One option to boost the CaO in the raw feed is to incorporate egg shells which are a by-product of food industry. Egg shells have limited potential in the production of energy but they are extremely rich in CaO (95%) and small quantities can make a significant impact on the overall CaO concentration. Another solution is to incorporate waste products from pulp and paper mills, particularly lime mud and green liquor dregs. While these materials are rich in Ca one must be cautious of the moisture content. Finding a biomass rich in silica is less problematic than it is for calcium. In fact there has been ample research conducted using the ashes of plant residues (such as rice husks and sugar cane bagasse) as secondary cementing materials due to their rich concentrations of SiO2 and their ability to act as a pozzolanic admixture [11] [12]. While these plants are very common in tropical regions, they are not typically grown in northwestern Europe. There are however non-tropical plants currently grown as energy crops which still have a significant concentration of silica, for example Miscanthus and switch grass [13] [14]. The goal is to combine the various biomasses to get a raw mix with 65% CaO, 22% SiO2, 6% Al2O3 and 3% Fe2O3. Based on these concentrations we should theoretically be able to create clinker so long as we achieve the necessary sintering temperature.
FIRING
Once a raw mix design is finalized it will be fired in a standard kiln and the resulting ash will be analyzed to determine the chemical and mineralogical composition. The raw feed will then be optimized until the desired clinker minerals are achieved and present in the ideal quantities. Subsequently the best raw feed mixes will be fed into the fluidized bed combustor. The resulting ashes will be analyzed and compared to those produced in the kiln. At this point both the firing method and the raw mix will be further adapted to optimize the resulting bio-cement. The concept of using a fluidized bed to manufacture Portland cement is not new. In 1962 Pyzel became the first person to use a fluidized bed for the sintering stage in cement production. Heertjes continued that work into the 70’s using a sprouting bed. He reported that the manufacture of clinker in a sprouting bed is a promising process [15]. Although, over the subsequent years, research regarding this form of production seems to have been abandoned as rotary kilns have been further optimized to be increasingly more energy efficient. Nevertheless the possibility of producing cement in a fluidized bed exists and over the last few decades the technology of fluidized beds has also advanced. One major concern which arises when replacing a rotary kiln with a fluidized bed combustor is the disparity in firing temperature. It is standard practice in energy production to utilize a combustion temperature in the range of 500-900˚C. Of course in the production of clinker it is necessary to achieve a much higher temperature in order to reach the sintering point. Even with the incorporation of mineralizers it will be difficult to fire at temperatures lower than 1450 ˚C and still get sufficient quantities of C3S.
PRODUCT DEVELOPMENT
Once the raw feed mix and firing method are optimized a variety of bio-cements for various applications will be produced at the laboratory scale. The goal is to develop a series of bio-cement-based products with characteristics and performance similar to currently available products. From these bio-cements the sustainability and durability will be tested and then compared to what is presently available on the market. A life cycle analysis will be performed on the bio-cements to verify the improved sustainability in comparison to original Portland cement. At this point any potential up-scaling issues will be addressed and the bio-cements’ ability to replace original Portland cement in the short and long term will be ascertained.
CONCLUSIONS
The goal of this paper is to present the project 'Towards the development of CO2 neutral cement'. At this time the project is in its initial and still largely theoretical stage, and aims for utilizing various biomasses as the raw materials to develop a cementitious binder, or bio-cement. The bio-cement will be produced concurrently with the production of energy, making this binder CO2 neutral. After the bio-cement is produced on the laboratory scale its sustainability will be quantified and a series of products will be developed.
ACKNOWLEDGEMENTS
We would like to acknowledge Technology foundation STW for financial support of this project STW 11338 'Towards the development of carbon dioxide neutral renewable cement'.
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