Understanding combustion systems in asphalt plants

Marcelo Zubaran from Ciber provides an overview of how combustion systems and technology work in the production of asphalt mixes and the choice of fuel used in the process.Marcelo Zubaran from Ciber provides an overview of how combustion systems and technology work in the production of asphalt mixes and the choice of fuel used in the process.

In order to achieve optimum material adhesiveness, easy transportation, fast spreading and later compaction, the production of asphalt mixes needs to be hot, and the appropriate technology and choice of process can influence that result.

The main roles of the plant are to feed the rock materials and the asphalt cement, dry and heat the aggregates, filter the gases coming from the drying system, mix the materials and transport them to a storage silo or directly to a truck.

For Marcelo Zubaran, Application Engineer and Product Specialist for asphalt plant manufacturer Ciber Equipamentos Rodoviários (Ciber), one of the most important processes during asphalt production is drying the aggregates.

“The complete removal of the moisture from the aggregates – surface and pores – and the final heating temperature are factors that influence the adhesiveness with the asphalt cement, the minimum criterion of asphalt mix quality,” Mr. Zubaran explains.

He says the atomisation and combustion processes that occur in the plant, influenced by the plant equipment technology and the fuel used, contribute to the success of the thermal system and, consequently, helps produce a mix with the expected quality and optimisation of fuel consumption.

Combustion technology in asphalt plants

During production, aggregates are dried and heated in a drum with a burner on the opposite side of the aggregates’ inlet. The liquid fuel injected into the burner is atomised – broken down into very small particles in order to increase the superficial surface area of the material and improve burning. This happens when the fuel is mixed with oxygen inside the burner.

“The atomisation is influenced mainly by the geometry of the burner, by the fluid’s pressure and by the liquid fuel temperature. But, the gases (liquefied petroleum gas and natural gas) are already vaporised and therefore do not require this process,” Mr. Zubaran explains.

He says combustion can be defined as any relatively rapid chemical reaction in gas phase that releases a significant amount of energy in the form of heat. “The primary components for this reaction are oxygen and fuel. After atomisation, a spark or pilot flame causes ignition of the main flame. After the start of the reaction, the flame must be self-sustained with the addition of more oxygen, besides the one used for atomisation in order to comply with the stoichiometric ratio of 13 volumes of air to one of fuel,” Mr. Zubaran says.

“The air for atomisation corresponds to approximately 30 per cent of all air for combustion and the remaining 70 per cent can be sucked out of the environment through an exhauster (burners known as open fire) or it can be supplied mechanically by the plant without sucking out the air from the environment (total air technology).”

Fuels used

Burner heat in asphalt production is obtained by burning fuels and transforming chemical energy into thermal energy. The fuels used can be liquid or gaseous, with the liquids being divided into light and heavy oils, according to Mr. Zubaran.

“Light oils are petroleum-based fuel oils, which have a viscosity suitable for atomisation or spraying at room temperature. Diesel is an example of light oil,” he says.

Heavy oils – fuel oils derived from oil or from rocks that need heating for burning ¬– include the likes of shale oil, BTE (low sulfur content), BPF (low flash point), oil-based waste derived from petroleum or vegetable waste.

“The main gas used in production is propane gas and natural gas,” Mr. Zubaran says.

“The first is a mix of propane and butane gases, being a byproduct of petroleum distillation. Natural gas, on the other hand, is composed of methane in a higher amount. This last gas is a mix of light hydrocarbons found underground in petroleum deposits by accumulation in porous rocks, being the result of organic matter degradation.”

Depending on the type of gas used, they can require special burners and pressure regulation systems.

Mr. Zubaran says the application of gas as fuel in asphalt plant processes can provide benefits during maintenance.

“This is because it does not obstruct the atomising nozzles of the burners and that they are already vaporised and rule out the possibility of generating poorly atomised fuel, which can damage bag filters maintenance and influence consumption.”

While there is a variety of fuels to choose from, Mr. Zubaran says there are a few parameters relevant for the qualification of a fuel’s use.

“The first is its atomisation temperature, relevant for liquid fuels. Heating depends on the viscosity curve versus the temperature curve. The viscosity for burning should be below 21 at the critical solution temperature,” he says.

The second is the amount of sulfur present in the fuel, defining the byproducts resulting from burning and thus affecting the formation of pollutant gases.

The third is the lower calorific value (LCV), the sum of energy being released in the form of heat during combustion. “The higher the LCV, the greater the efficiency of the thermal system and the lower the fuel consumption,” Mr. Zubaran adds.

Relationship between fuels and consumption during production

One of the most relevant costs in the production of hot asphalt mixes is the fuel, and the fuel property that has the highest impact in consumption is the calorific value.

Mr. Zubaran says in regards to the properties of the asphalt mix, the aggregates’ moisture considerably affects consumption, as it reflects in the amount of water that must be evaporated in the dryer during the drying process.

“In a tangible way, the choice of the best fuel depends on local availability and cost.”

He says the relative consumption, according to the graph, is an important factor and should be weighted with the cost of fuel acquisition.

”The impact on plant maintenance should also be weighted, but in an intangible way,” he adds.

“In general, the use of gases reflects less maintenance than light oils, which in turn tend to generate less maintenance than heavy oils.”

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