Fossil Free drives carbon negativity

Lime mud reburning in the Kraft process used in pulp/paper/board production is a major source to CO₂-emissions. 

The lime kiln is often the last big consumer of fossil fuels in the mill. In Sweden many lime kilns have been converted to bio-fuels, which instead means that great amounts of biomass are consumed.  

Thus there is great potential from a sustainability perspective to replacing the lime kiln in Kraft pulp mills with a more efficient process. Using gasplasma produced from fossil free and climate neutral electric power is such a possibility, eliminating the use of fuels as well as enabling negative emissions, by facilitated capturing of carbon dioxide with biogenic origin from the lime.

The Paris Agreement

The threat to our climate posed by global warming is a key issue of our time.  IPCC has identified CO₂-emissions from fossil fuels as the biggest threat to the earth’s climate and it will be necessary to phase out fossil fuels by 2050 in order to keep the global temperature increase to within +2°C. In parallel with this, it will also be necessary to introduce new technologies to capture and sequester CO₂ if these goals are to be met.

Recently, there is increasing focus on climate neutral technology to reduce fossil carbon emissions and to reach the goals of the Paris Agreement. Sweden has unilaterally decided to achieve no net emissions of greenhouse gases to the atmosphere by 2045, and after that to achieve negative emissions.

The Swedish pulp and paper industry has high emissions of CO₂. As recently as 2017, Swedish pulp and paper mills emitted close to 23 Megatons of CO₂, of which about 0,5 Megatons of CO₂ stemmed from fossil fuels. It is close to half of Swedens total emissions of 53 Megatons of fossil CO₂. (Source: Swedish Environmental Protection Agency).

Traditional lime mud reburning 

Today’s lime burning is performed in fueled rotary kilns. This traditional technology represents a major investment for the mill with significant demands in terms of process control with long residence time and considerable maintenance needs.  

The Plasma technology opportunity demonstrated

In high temperature processes in metallurgy, such as electro ovens, the use of electricity is well established due to its efficiency. With 40 years of experience, ScanArc Plasma Technologies AB has developed and implemented plasma technology in industrial processes who need hot gas produced without fossil fuels or others. A plasma torch is used to convert electric power from an electric arc to ignite a gas into a plasma state at 3 000°C – 5 000°C. 

ScanArc’s plasma technology is named non-transferred because the electrodes and the arc are enclosed in the torch, unlike arc furnaces where the second electrode is for example the treated charge in the oven.

With ScanArcs high performance plasma technology, 85-90% of the electric power is converted into useful heat. With torch effect of 0,1 – 8 MW and the possibility to multiply the number of torches a very broad effect register is covered.

Calcination with plasma

Feed system and plasma generator technology from the steel industry can be applied for the burning of fine-grained lime in the Kraft pulp process.  The reactor must be modified so that the burnt lime is not melted and burnt lime dust is separated from the emitted carbon dioxide gas at 900°C before the lime is cooled.

LimeArc technology (also previously known as Elmesa) is an innovative technology which can replace or complement the conventional rotary kiln for reburning lime mud in Kraft pulp mills. In the LimeArc’s technology, calcination takes place in a plasma reactor where small lime particles are suspended in a gas instead of tumbling around in the bottom of traditional rotary kiln type calcination ovens. 

The LimeArc concept comprises two parts which can be implemented together or independently.

1. Calcination (reburning) of lime mud with electrically generated plasma, which is being verified in lab- and pilot tests.
2. Dry hygroscopic slaking of quick lime (slaking reburned lime mud with water vapour), which has been tested in laboratory and pilot scale in 2006 and 2008, respectively. Dry slaking enables recovery of more heat and at higher temperatures as process steam, which means that fuel is conserved in the boilers of the pulp mill.

Advantages:

• CO₂ from the lime mud is achieved at high concentration and can be captured or converted into other products.
• No fuel is used, which reduces the emissions from the mill.  If biofuels were used they are now made available to biorefinery, instead of being combusted.
• Gas volume from the process is reduced to 15% compared to fueled limekilns.
• Faster calcination, requiring seconds as compared to hours in traditional processes.
• Process control improved.
• Reactor volume is one hundreth of that for conventional rotary kiln processes
• Reduced energy cost and consumption.
• A third of spent energy can be recovered as process steam which saves fuel to the boilers.
• Reduced investment required.

In a typical Kraft pulp mill in one year, the LimeArc process:

• Replaces 20 000 m³ oil which eliminates 50 000 tons of fossil CO₂ …
• … or releases 40 000 tons of biofuels which instead can be processed to yield more valuable products
• Delivers 90 000 tons of highly concentrated biogenic CO₂ from lime-burning which can be converted into for example 80 000 m³ electro-methanol in a biorefinery

Carbon dioxide collecting

An electrified limestone calcination process where the process heat is generated by a renewably-powered plasma torch has great potential to provide a process gas consisting of 100 percent carbon dioxide (CO₂), which in turn facilitates the capture and purification of CO₂ from the process gases.  Implementation of the LimeArc technology together with CCS in the pulp and paper industry’s calcination process would lead to large negative CO₂ emissions in Sweden in the future, on the order 1.75 Megaton/year.

This offers an opportunity to collect and effectively reduce carbon dioxide at a cost of about one third of that of collecting it from exhaust gases diluted with combustion fumes in a fuel fired kiln.  Furthermore, the lime in a Kraft pulp process chemical recovery is recycled, and the carbon dioxide emitted from the lime-burning originates from wood pulp, not from fossil carbon. Thus, it offers a carbon dioxide sink which is available also in a fossil fuel-free mill. 

Adding production capacity

LimeArc plasma calcination could be used either as a booster module or as full scale equipment together with vapour slaking and drying. 

A first application is thought to be a module complementing an existing fuel fired rotary kiln as a cost effective alternative when 20-30 % increased capacity is needed.  It is known that many mills could benefit from increased capacity, although in most cases this would require heavy investment. Typically, to date, higher capacity has been achieved by installing an external flash drier at the kiln.  When this is insufficient, a competitive alternative could be to install a LimeArc booster calcination module. If a flash dryer is already installed, this could be the only alternative to a major investment in a new kiln.

Implementing the LimeArc process can have considerable advantages compared to a new fuel-fired rotary kiln, in terms of environmental impact, energy consumption and overall process economy. The investment cost of a full scale plasma reactor combined with vapour slaker and dryer was calculated to 120 MSEK compared to about 300 MSEK for a conventional kiln. The yearly total cost for energy and capital was calculated to be 50 MSEK compared to 77 MSEK. Both figures are based on the KTH report from 2003.

Chemical recovery in the Kraft pulp process

Quick lime is an intermediate chemical in preparation of the Kraft pulp cooking liquor (white liquor), and is recovered by calcination of lime mud achieved from the causticization process. Calcination is performed at a temperature of about 1000 °C.

Quick lime is mixed into green liquor and slaked with a generation of heat. The green liquor is then causticized and converted into white liquor. Precipitated lime mud is then separated from the liquor, washed, de-watered, dried and reburned again. In summary, the following chemical reactions occur:

• Calcination: (lime mud ➔ quick lime) CaCO_3(s) + heat ➔ CaO_(s) + CO_2(g)
• Slaking: (quick lime ➔ hydrated lime) CaO_(s) + H₂O_(l,g) ➔ Ca(OH)_2(s,l) + heat
• Causticization: (hydrated lime ➔ calcium carbonate) Ca(OH)_2(s,l) + Na₂CO_3(l) ➔ CaCO_3(s) + 2NaOH_(l) 

About 8 % of the carbon from wood pulp used in the Kraft pulp process is absorbed as carbonate in the lime and thus available for recovery in the exhaust gas from lime-burning.

Because the carbon atom in the CO₂-molecule originates from biomass (i.e. pulpwood), the major part of the CO₂-emissions from the lime kilns are biogenic. To generate the high temperature required for the calcination reaction and the following sintering process, the existing lime kilns are fueled.  Depending on local circumstances either bio-fuels (e.g. wood pellets, tall oil, etc.) or fossil fuel oils are used. About 2/3 of the CO₂ emissions originate from calcination and 1/3 from the combustion process.  Currently, 11.7 Megatons of pulp are produced in Sweden annually of which an estimated 75% are produced using the Kraft pulp process.  Approximately 0.25 tons quick lime is required to produce 1 ton of kraft pulp, which results in an emission of approximately 0.20 ton of biogenic CO₂ from calcination alone.  This means that the total emissions of biogenic CO₂ from calcination in Swedish lime kilns today is about 1.75 Megaton/year providing a great opportunity for large negative CO₂ emissions if this CO₂ is captured and stored.  However, this breakthrough can only be made through investment in a new technology like LimeArc’s.

In the transformation toward a complete mill for effective CO₂-capturing – either through oxyfuel combustion, that is combustion of black liquor in the recovery boiler in a mixture of oxygen and carbon dioxide instead of air, or pressurized oxygen-blasted black liquor gasification – the calcination requirement will increase, causing an additionally increased need for CO₂-capturing from the mills calcination process.