From agricultural waste to green concrete and alternative energy

From agricultural waste to green concrete and alternative energy

First: Introduction:

The average total amount of concrete manufactured in some countries of the world is about 6.5 million tons, and it is responsible for the emission of approximately 8% of carbon dioxide gas, and the resulting phenomenon of global warming and climate change. Therefore, the world is moving towards reducing the carbon footprint in concrete, and finding appropriate solutions.

There are many modern ways to reduce the carbon footprint in concrete, such as modern carbon capture technology, which is one of the solutions that European countries rely on to transform into Carbon Neutrality countries by 2050.

One of the latest proposed methods that is still subject to research is to manufacture clinker at low temperatures to reduce the environmental footprint by about 30%.

European cement factories also offered in 2020 to manufacture mixed cements such as CEM II/C-M and CEM IV in which the percentage of clinker is reduced to 50 and 35%, respectively, and the percentages are supplemented with replacement materials. Such as: granulated blast furnace slag and/or limestone, glass waste, or types of ash obtained from burning various materials; The most prominent of which is burning coal in fly ash power plants, or ash resulting from burning wood or agricultural waste.

This article will discuss the definition and properties of ash resulting from burning the most important agricultural crop waste, which is rice crop waste, which is represented by rice husk (Photo No. 1) and straw rice (Photo No. 2), and it is known for its environmental damage when burned in open areas (Photo No. 3).

Despite the known environmental damage of open burning of rice crop residues, burning them using controlled means has many benefits. The energy released from burning is great and can generate electricity, and the remaining ash has effective cementing properties, and can partially replace clinker or cement in mortar and concrete. That is: when these wastes are burned under controlled conditions, the energy resulting from combustion is utilized, and the cement properties of the ash are exploited in the manufacture of green concrete.

Green concreteis known as concrete that contains a low percentage of clinker; To reduce carbon dioxide emissions resulting from the burning of limestone, which uses at least one type of cement replacement material, which has inherent hydraulic, pozzolanic or cementitious properties; That is: it can react with calcium hydroxide; To form water-based cement compounds, which are responsible for the adhesive property of cement, or with fillers; Like limestone, replacement materials are often the industrial or agricultural wastes mentioned above.

Manufacturing green concrete leads to reducing the percentage of emissions that harm the environment during the burning of limestone in cement factories, and it must be taken into account that the concrete maintains its performance quality and sustainability.

Second: Agricultural residues from the rice crop:

Rice represents one of the most important food grains for more than half of the world’s population, and nearly one-fifth of these peoples cultivate it, and it contributes c. M. A. By cultivating approximately 0.6% of the global total, and cultivating about 6 million tons, it represents more than 22% of the African continent’s production.

Cement factories are currently burning bales ofrice wastein the blast furnace; It benefits from the combustion energy resulting from these wastes, and from the ash that is a friend of clinker.

 However, the ash can be obtained individually and exploited as a partial replacement material for cement in green concrete.

The following are two studies on the characterization of rice husk and straw ash, and the behavior of cement resulting from replacing part of the cement with ash. To demonstrate the extent of its success in manufacturing green concrete, the article also includes an overview of exploiting the energy resulting from burning rice crop residues to generate electricity.

2-1 Rice husk rice husk

2-1-1 Ingredients of rice husk.

Rice husk consists of 80% organic materials and 20% ash. When the husk is completely burned in an abundance of oxygen, the percentage of silica in the ash reaches 97%.

Whereas silica is the main oxide that controls the cement property; Therefore, the ash has a high cement value, and the source of silica in the peel is the soil. When growing rice, the plant absorbs silica from the soil and accumulates it in its fine composition.

Since silica does not volatilize when burned; Therefore, it is concentrated in the ash, and turns it into a substance with an inherent cement property. That is: a substance that can react with calcium hydroxide; To be hydrolyzed cement compounds, which are responsible for the bonding property of cement.

Other minor elements in the ash are: aluminum, iron, alkalis (sodium and potassium) and magnesium.

Potassium plays an essential role in burning. It reacts with unburned carbon and forms a black layer on the ash, which is an undesirable reaction.

Photo No. 4 shows the forms of ash in the different stages of burning. It appears black when combustion is incomplete, and a black crust appears on its layers when potassium reacts with unburned carbon. It appears gray according to the percentage of unburned carbon present in it. It also appears white when devoid of carbon, and has the strongest cement properties.

2-1-2 Rice husk ash as a cementitious material.

The hardness of green concrete made by replacing part of the cement with rice husk ash depends greatly on the properties of the ash used, its softness and its silica content.

These properties are also affected by the burning temperature, time and medium of burning, the availability of oxygen necessary to complete combustion, and grinding methods.

As previously mentioned; Burning rice husk, which is rich in potassium, affects the completeness of combustion and forms a black layer on the ash. The lower the potassium percentage, the better the combustion conditions and the greater its pozzolanic cementitious activity, which contributes to increasing the cementitious property of mortar and concrete. The ash is characterized by its porous property and high surface area value.

In a master’s thesis conducted at the Faculty of Science, Helwan University, the thermal behavior of rice husk was studied laboratory for samples obtained from Gharbia Governorate. It was found that the maximum loss of its components by burning occurs up to 500°C. This was inferred from the two peaks that appeared in the differential thermal analysis shown in Figure 1, which appear at a temperature less than 500°C.

 

The peel was washed to get rid of potassium and the necessary quantities were slowly burned in the laboratory at 700 degrees Celsius. To ensure complete combustion.

The ash contained about 17% ash of the total peel used, and was distinguished by its gray color, indicating a weak percentage of unburned carbon, and not being negatively affected by the potassium reaction. Because its percentage decreased as a result of washing, the percentage of carbon in the ash reached 0.35%.

 

To determine the effect of ash on the behavior of cement, cement was replaced with proportions of up to 20% ash, and the compressive strength of the resulting pastes was measured. It was found that the compressive strength values of cubes containing 10% ash had improved significantly, as shown in Figure 3.

It was also noted that the mixture requires an increase in mixing water in the presence of ash (Figure 4), and that the mixing time is longer in the presence of 10% of ash (Figure 5).

 Studies have shown that the properties of green concrete based on replacing part of the cement with rice husk ash improve by increasing its softness.

2-3 Rice strawrice straw

2-3-1 Components of rice straw

The rice straw (Photo No. 5) consists of the same percentages of organic materials and ash mentioned in the husk, and the percentage of lignin is lower compared to its percentage in the husk, and the percentage of silica present in its ash is less than its percentage in the rice husk, and reaches about 70% when combustion is complete.

Other secondary elements, such as rice husk ash, are aluminum, iron, alkali, and magnesium.

Potassium also plays an essential role in burning; Because it interacts with unburned carbon, forms a black layer on the ash, and reduces its pozzolanic activity.

 

It was confirmed that there were no crystalline or precipitated salts and that the sample was active amorphous by analysis using an

 

 

 

2-3-2 Rice straw ash as cementitious material

By studying the thermal behavior in the laboratory of samples of rice straw obtained from Sharkia Governorate, it was found that the maximum loss of its components occurs by burning at lower temperatures than rice husk, and occurs at about 400°C.

 

In a master’s study conducted at the Faculty of Science, Helwan University, chopped straw was burned without washing at 500°C.

The percentage of ash produced was 20% of the total straw used.

The ash was gray in color and was scattered with black particles as a result of the potassium not being removed by washing (Photo No. 6).

X-ray diffraction analysis of the burnt ash sample shown in Figure 6 confirmed the presence of calcium carbonate, potassium chloride, and quater sand as crystalline materials alongside the active ash.

 

To identify the effect of ash on the behavior of cement, up to 20% of the cement was replaced with ash and the compressive strength was measured. As in the case of rice husk, it was found that the compressive strength of pulp cubes containing 10% ash improved significantly, and that the mixture needed to increase the mixing water in the presence of ash. Studies have also proven that the properties of green concrete based on replacing part of the cement with rice straw ash improve by increasing its softness.

2-4 Rice husk and straw as an energy source

The ratio of agricultural waste resulting from the rice crop is about 1:1,25:25, rice to straw to husk, respectively; Every ton of rice crop, which produces 290 kg of straw, can generate up to 100 kilowatt-hours of energy with a calorific value of 2400 kcal/kg.

Every ton of rice yield produces about 220 kg of husk. Each ton of husk generates from 410 to 570 kilowatt-hours of electricity with a calorific value of 3000 kcal/kg.

In general: burning straw is easier than burning chaff; Due to the low percentage of lignin in straw.

Denmark is a pioneer in burning straw and exploiting it as biomass for energy.

Conclusion: It is possible to use rice crop residues; To generate energy by gasification, and exploit the gas released to generate machines such as grinding machines; Because it provides the site with electrical energy, the resulting steam can be used for drying, reduces the use of coal fuel, and the ash is also used as a cement material used in the manufacture of green concrete.

Pictures Nos. (7 and 8) show the locations of using straw to generate energy. Picture No. 9 shows a sketch of converting straw into electrical energy.

It is worth noting: The colleges of engineering in the Arab Republic of Egypt have specialists in generating energy from biological masses, and they can be used to equip power plants, and to cooperate with professors of cement and concrete in the colleges of science and engineering. To use ash as a cementitious material in green concrete.

 

 

Paddy stubble bundles stored near a biomass power plant in Punjab. Photo by Manu Moudgil/Mongabay

 

References:

1) Decarbonated concrete-projects and initiatives of some European countries Belgium LEILAC project.

2) Carbon-free concrete an ecological innovation made in France.

3) FAO Rice Market Monitor – November 2013.

4) H.Y. Ghorab, M. Rizk, F. ElDirs, A. M. Abdel Fatah: Egyptian rice husk as cement replacement material Cement Wapno Beton Vol.3, 2016, pp.167.

5) A. M. Abdel Fatah: Egyptian Rice husk as cement replacement material. M.Sc.Chemistry Department Faculty of Science Helwan University. 2017.

6) M. E. S. Ahmed: Studies on the utilization of some biomass ashes as supplementary cementitious materials. M.Sc. Chemistry Department Faculty of Science Helwan University. 2018.

7) H.Y. Ghorab, M. Rizk, A. S. Meawad, M. El Sayed: Reporting the performance of the rice straw ash as cement replacement material Cement Wapno Beton no 2, 2018, pp. 107-114

8) S.M., Hafele, Y. Konboon, W. Wongboon, S.Amarante ans A.A. Maarifat, E.M. Pfeiffer, Effects and fate of biochar from rice residues in rice-based systems. Field Crops Res. 121, 3, 2011, pp. 430-440.