Characterizing energy feed for the lime industry

In recent years the use of renewable feed such as biomass and bioenergy has been widely adopted worldwide to produce biofuels, biogas, biocatalyst, bio-composites, bioplastics, green chemical products, cellulose textiles, and the lime industry is becoming an important consumer of various biomass feed since this is abundantly available worldwide as a cheap and extremely important renewable energy source.

Biomass is a “carbon neutral” type of fuel that adapts to multiple process conditions, since it is not a traditional feed it must be thoroughly prepared, physically, and chemically characterized before being fired in a lime kiln. Cimprogetti has carried out internal technological laboratory tests to evaluate either the behavior of the calcination and the possible effects on atmospheric emissions.

Use of carbon neutral fuel allows “partial” 
decarbonization

Replace fossil fuels with renewable fuels to reduce operating costs

Diversify energy sources, hence the energy
suppliers ensuring flexibility and resilience
to the energy system

Align the interface between waste, product,
and chemicals to trusted feed as fuel

Lime industry prospective

Lime industries are indispensable for society, lime production represents industrial added value, therefore the lime industry is currently sensitive in new technologies related mainly to digitization, fuel and energy saving including better control of raw material, which should closely reach the ambition target on reducing greenhouse gas emissions to at least 55% considering the 1990 values.

Furthermore to become climate neutral by 2050 the lime industry should act with responsibility because several options are under evaluation  and choosing a shorter route economically feasible is not an easy strategical selection, but it will be a real opportunity to converge towards a clean and resilient energy system with a long-term perspective. Using renewable fuels such as biomass-derived fuels instead of fossil fuel is the first step on “the new route” to mitigating increases in atmospheric CO₂.

Renewable fuel

Renewables are forms of energy that respect the resources coming from the natural world. They do not pollute and do not run out, since they can regenerate at the end of the cycle.

Using renewable feed requires a wide range of knowledge about how to use it in our process and how it is impacting the quality for the multiple uses and purpose of quicklime and hydrated lime.

Biomass represents one of the selected renewable fuels, ranking it is not simple because it includes a wide spectrum of sources including all kinds of materials that were directly or indirectly derived from photosynthesis reactions such as vegetal matter and its derivates: wood fuel, wood derived-fuels, fuel crops and agricultural.

The process of biomass combustion involves several physical and chemical aspects of a certain complexity that during calcination in the regenerative kiln must be taken seriously into evaluation.

Sawdust fuel

In general, there are many industrial sources of sawdust, and the stream of such material should be controlled according to specific standards and methods, to predict the effect on calcination. With the use of biomass, it is therefore possible to reduce the CO₂ from the fuel used for every kg of CaO produced. Naturally, it is not possible to reduce the carbon dioxide caused by the chemical reaction of dissociation of limestone into CO₂ and CaO.

The level of carbon dioxide emitted per kilogram of calcium oxide produced is calculated by adding the amount of CO₂ produced by the calcination process to the amount of CO₂ produced by fuel combustion, as shown in Table 2.

Using renewable energy from wood biomass (sawdust), the amount of CO₂ emitted into the environment is considered, according to the Kyoto Protocol, to be equal to zero because CO₂ emissions during the burning of the sawdust is considered equal to the amount of CO₂ absorbed by trees during their growth (due to chlorophyllous photosynthesis).

If, for the sake of convention, it is accepted that CO₂ emissions arising from the use of biomass are equal to zero, the global reduction will be around -30%.

Fuel feed characterization

Suitable fuel & technical criteria must be defined for renewable feed or in general for alternative fuels and be considered to the pertaining process conditions. These criteria include chemical and mechanical proprieties such as:

Portions of non-combustible and combustible components

Portions of impurities and organic constituents

Elements and trace of minor elements which may affect the burnability and the pollutants emission

Moisture content which is becoming relevant for evaluating the energy demand

Fixed carbon content

Burnability

Volatile, which are also affecting safety assessments for plant configuration

Physical proprieties such as particle size distribution, bulk density

Cimprogetti technological solution

CIM-Firesolid is the solution implemented by Cimprogetti able to feed and dose various fuel to lime kilns.

This new system was designed by taking into consideration the following significant aspects:

Modularity as it must be inserted in existing systems since most of the project are in operational plant

Flexibility in being able to use different fuels since the energy resources are rapidly changing

Safety since most of the renewable fuel is characterized as “explosive powders”

Functionality on regenerative lime kiln

During the kiln “inversion” period a dome valve extracts the fuel dust from the silo and conveys it to the “loss-in-weight feeder”. A butterfly valve maintains the feeder pressurized while the fuel dust is being dosed and conveyed to the kiln. All the piping connected to the “loss-in-weight feeder” are interrupted by expansion bends to allow proper weighing. During the “loss-in-weight” tank filling time a venting system with bunker type bag filters allows the air relief opening two pneumatically operated valves. These valves are closed while the tank is pressurized, and the fuel dust is being dosed and transported to the kiln lances. A rotating paddle wheel placed inside the feeder ensures that the fuel dust is constantly and uniformly discharged into all rotary valves.

These variable speed rotary valves perform the dosing of the fuel dust which, in the whole combustion cycle, is uniformly fed to the kiln by means of a pneumatic transport system. Each rotary valve feeds two or four burning lances depending on the plant configuration. The fuel flow in each single line can be adjusted by changing the valve rotation speed by a frequency converter, whilst the whole dosing system throughput is regulated by changing all the motor speeds contemporarily.

Conclusion

Cimprogetti has been a complete full solution provider for the “Lime industry” since 1967. Based on interaction with different industrial players around the world, the company is tracing the “challenging” road to reach neutrality in 2050.

The reduction of CO₂ emissions from lime production will be a long journey, as the industry is defined as “hard to abate”.

As already mentioned, one third of the emissions of the lime production process comes from fuel. Thus, by switching fuels to renewable sources such as biomass would be an option in a short-term vision, to mitigate the total fuel contribution on CO₂ emission.

Energy efficiency measures would also provide some further emission reduction:

Using vertical lime kilns to replace the more energy intensive rotary kilns, although this would be capital expensive

Alternative sources of “green” energy must be taken in consideration such as “green” hydrogen, which is a clean-burning molecule that could become a zero-carbon substitute for fossil fuels in hard-to-abate sectors of the economy. The cost of producing hydrogen from renewable sources is primed to fall, but demand needs to be created to drive down costs, and a wide range of delivery infrastructures need to be built

Replace root blowers with new technology blowers to minimize the electrical energy in terms of kWh/t lime

Oxygen enrichment with the aim to concentrate carbon dioxide in the flue gas for utilization and storage

For the remaining two-thirds of the emissions, CCUS technologies are indispensable in a close-to-zero emissions scenario, the lime industry has many options and choosing a shorter route is not an easy selection still the research seems to be not mature enough for a large industry scale.

Finally, we need to find an application to allocate CO₂ production that will be another disruptive challenge for the “hard to abate” industry.

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