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FEATURE: Carbon re-engineered – five genius engineering tricks cutting emissions

Pádraig Belton

Dr Jonathan Wilson and Professor Graham Hargrave working on their low-emissions diesel technology at Loughborough University
Dr Jonathan Wilson and Professor Graham Hargrave working on their low-emissions diesel technology at Loughborough University

Far above the Pacific Ocean on the north flank of a Hawaiian volcano, the Mauna Loa Observatory started collecting data on atmospheric carbon dioxide in 1958.

Then it was recorded as 315 parts per million (ppm), already higher than the 280ppm global average for the time before the Industrial Revolution. On 9 May 2013, the daily average measured at Mauna Loa rose above 400ppm for the first time on record. The year 2017 saw a new high of 405ppm. 

Then, in 2018, more carbon was emitted into the atmosphere than ever before, thanks in part to a 4.7% rise in China, 2.5% increase in the US and 6.3% in India. The total figure was 37.1bn tonnes, 2.7% more than in 2017.

The 185 countries that have ratified the 2015 Paris climate-change agreement have committed to try to limit global warming to 1.5ºC above pre-Industrial Revolution levels. We are already at around 1ºC. The difference between 1.5º and 2ºC would be a planet with coral reefs and summer Arctic sea ice, and one without. But limiting global warming to 1.5ºC means almost halving CO2 emissions by 2030, and reaching net zero by about 2050. Particular focus is needed on high-emission material production. A reduction this large will keep the best engineers busy. Thankfully, they have already started. 

Concrete changes

After water, concrete is the second most consumed substance in the world. Making it emits more CO2 than any single country, apart from China and the US. In 2015, the world made 4.1bn tonnes of concrete. In doing so, it also made 2.8bn tonnes of CO2 – 8% of that year’s total global carbon emissions, according to the Dutch environmental agency.

As its principal ingredient, concrete typically uses inexpensive ($126 per tonne) Portland cement and 65% lime, which accounts for two-thirds of cement’s CO2 emissions. Turning its precursor limestone – calcium carbonate, CaCO3 – into lime – calcium oxide, CaO – releases CO2. The remaining carbon emissions come from heating kilns – normally to about 2,050ºC. But using a different cement chemistry, with clay as a silica source in place of limestone, can cut by 30% the amount of lime needed to make a batch of concrete.

A carefully considered approach will use raw materials found near cement manufacturing facilities and mean formulating a process that uses existing equipment, says Nick DeCristofaro, chief technology officer at Solidia Technologies in New Jersey. Making cement is “intrinsically a very inexpensive, low-margin process,” so a substitute technology has to be low-cost too.

Solidia cement also reportedly requires less heat in the kiln to fuse – ‘only’ about 1,200ºC. The Solidia concrete mixture is formed by adding the particulate cement matter and water to a rotating chamber, where it creates a soup that is transferred into a vibratory press and then poured into moulds. 

The formed slabs then move to an enclosure filled with CO2 to set. By a nice trick of chemistry, Solidia concrete hardens by addition and absorption of CO2 instead of water. The last hardening stage absorbs 240kg of CO2 for each tonne of cement, says DeCristofaro.

The most efficient Portland cement plant in the world emits 810kg of CO2 for each tonne of cement. Solidia produces 560kg at the plant but then absorbs 240kg, bringing the net carbon emission to 320kg. “Pretty dramatic, if I say so myself,” says DeCristofaro.

Paper cuts 

Paper production not only accounts for 1% of CO2 emissions – it consumes trees, the ‘lungs of the Earth’. On average, 10-15 trees will sequester a tonne of CO2 within 30 years of planting, according to Carbon Neutral, an Australian organisation that plants trees to offset emissions. 

So, to save the trees, Anders Ankarlid, head of Stockholm’s A Good Company, started making notebooks and other stationery from a paper substitute made from stone and plastic.

The manufacturing process involves making a compound of limestone and polyethylene, adding heat, then using an extruder to form the compound into a sheet. Additives are injected to correct the colour. 

Adapting a plastic extruder to work with a more dense limestone compound has been the chief challenge so far, says Ankarlid.  

Limestone is “abundant in almost every part of the world, with economical costs,” says Takanori Tada from TBM, a Tokyo company that makes a paper substitute called Limex. About 40% of paper’s CO2 emissions can be reduced by using limestone and plastic instead, he says.

The limestone content of the paper is now 50-60%, with the rest made up of high-density polyethylene resin, says Tada. “Our goal is to achieve 70% or 80%” limestone and to substitute bio-based resins for the petrochemical-derived polyethylene, he says.

Fuelling progress

While the thin whine of electric cars has gradually become a more familiar sound in recent years, the chugging and clattering of heavy-duty diesel engines is likely to remain for decades to come. Tesla and its competitor Nikola claim ranges above 500 miles (805km) for their upcoming electric lorries, but they are still unreleased and operators are likely to face prohibitive costs for some time. 

With the right technology, however, diesel engines can have “ultra-low” emissions, says Graham Hargrave, an engineering professor at Loughborough University. 

Diesel “inherently offers a 20% CO2 saving over gasoline, from the fundamental physics of the combustion process,” says Jonathan Wilson, a postdoctoral engineering fellow who works with Hargrave. 

Other emissions – nitrogen oxides (NOx) and particulate matter – can make diesel an unappealing alternative, however. In 2015 the government estimated that NOx and particulates produced by diesel engines led to 52,000 additional deaths in the country each year.

To tackle the issue, NOx can be reduced into nitrogen and water using ammonia. Selective catalytic reduction systems fitted on most new diesel engines use the ammonia-containing AdBlue, made with 32.5% urea and 67.5% deionised water, to perform this reaction.

AdBlue only functions well at exhaust temperatures above 250ºC, however, buses and construction vehicles on stop-start journeys may never reach that temperature. “In reality most of the time it’s turned off,” says Wilson.

So the Loughborough team devised an answer, taking waste heat to pre-convert AdBlue into an ammonia-rich solution in a chamber located by vehicles’ exhausts.

“Essentially this does all the hard work of turning urea into ammonia for us,” says Wilson. “So it is possible to decompose NOx at temperatures down to 60ºC – we have a system that can push you to near 100% NOx conversion efficiency.”

They say the technology can be retrofitted on to existing vehicles easily, such as a prototype retrofitted on a two-litre diesel estate.

“I hope we’ll start seeing it in the heavy-duty market in under two years,” says Wilson. By reducing their life-limiting NOx emissions, fleets of heavy vehicles can take advantage of the lower CO2 emissions of diesel.

Updated aluminium

Before 1886, aluminium was scarce – Napoleon III apparently gave his favourite guests gifts of the low-weight, high-strength metal, while others received gold.

Then, on opposite sides of the Atlantic, engineers Charles Martin Hall and Paul Héroult simultaneously developed a large-scale production method. The method, which hasn’t changed since, produces aluminium by using electrolysis to strip the oxygen off aluminium oxide.

The process uses carbon as an anode and a white mineral called cryolite as an electrolyte, destroying the carbon anode in the process and releasing CO2. Smelting aluminium accounts for 0.8-1% of industrial carbon emissions.

Rio Tinto and Alcoa, two aluminium producers, have been working on developing a different anode that will release oxygen instead of CO2. The two companies hope to have the technology ready for retrofits or new smelters by 2024, says Alcoa spokesman Jim Beck. 

Historically, as much as 10kg of CO2 were released from the carbon anode to make 1kg of aluminium. Step-by-step improvements have reduced that to about 1.5kg, says Halvor Molland, senior vice-president of Norsk Hydro in Oslo.

There is a second carbon cost from smelting aluminium, however – it uses an enormous amount of energy. The world average is 14.1kWh/kg of aluminium, with about 3% of the world’s electricity supply going to aluminium extraction. In Australia, one of the world’s largest aluminium producers, that figure is 12%. If you are getting your electricity from coal-fired power plants in China, it could release 15-18kg of CO2 to produce that 1kg of aluminium, says Molland. 

Last year, Norsk Hydro smelted the first aluminium at its new Karmøy pilot plant in Norway. The plant’s HAL4e Ultra cells cut the energy consumed in making aluminium to 11.5-11.8kWh/kg. 

The technology works partly by shrinking the distance between the anode and the cathode. Other tweaks involved modifying the distribution of current through the cathode and making the busbar, which distributes the high-current power, more efficient.

The theoretical minimum energy needed to make a kilogram of aluminium is “in the range of 7kWh, and we’ve been set a target to work towards 10kWh/kg,” says Molland. “The closer you get, of course, the harder each step will be.”

Green steel

In 1950, 189m tonnes of steel were produced worldwide. By 2017, the figure was 1.7bn tonnes. 

Halfway through this century, annual steel production will be 2.2bn tonnes thanks to increasing demand in Africa and Asia, says Mårten Görnerup, chief executive of Hybrit Development in Stockholm. “The world needs steel,” he says. But in 2017, producing a tonne of steel released 1.83 tonnes of CO2, according to the World Steel Association.

Molten iron ore, from which steel is made, is high in carbon. Blasting oxygen on to the ore in a furnace lowers its carbon content and produces low-carbon steel. Predictably, though, the carbon and oxygen escape into the atmosphere as carbon monoxide and CO2.

This “is an extremely efficient process and has been around for 1,000 years maybe, and it’s now approaching its theoretical minimum – there’s not really that much to do if you’d like to increase efficiency there,” says Görnerup. But, he says, there is a different way to make steel.

At lower temperatures, of 800ºC to 1,200ºC, hydrogen can be used to create direct reduced iron, which can be melted to make steel. The only waste product is water. “The big advantage is we have no CO2,” says Görnerup.

The process does use lots of power. Replacing all of Sweden’s existing steel production would use 15TWh per year, 10% of the country’s annual electricity production. “It’s a substantial need but it’s not undoable,” says Görnerup.

One advantage is that the iron comes out as a solid instead of a liquid, so you can easily transport it, meaning you can locate the process where there’s an abundance of natural energy to fuel hydrogen production. Hybrit is building a pilot plant close to iron ore fields in the northern Swedish town of Luleå, to experiment with directly reducing iron.

From 2020 to 2024, the pilot facility will make one tonne of steel per hour – “very small in steelmaking terms, pretty large in laboratory-scale terms” – while finding ways to make the process more efficient. The process for the next few years will be “stop, evaluate, tweak, then you run away,” says Görnerup. By 2025, it is hoped to have the technology constantly running in ‘operational mode’. 

Currently, Hybrit’s production costs are 20-30% higher than normal steelmaking, but Görnerup says these will fall as green electricity becomes cheaper.


Read part two, "Carbon, re-engineered – can Climeworks capture 1% of annual carbon emissions?"

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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