In many sectors, the road map to net zero is clear: replace fossil fuels with renewable energy, and carry on as before. But what do you do when the chemical process at the heart of your business produces carbon dioxide as an unavoidable side product, and requires massive amounts of heat? Will North reports.

Quicklime is a key product for the building industry, with a range of other applications. The process of turning lime into quicklime is one that can be summed up in a simple formula: CaCO3 → CaO + CO2. It’s a process that has been used by humans since prehistory. Calcium carbonate is heated, to release carbon dioxide, leaving behind calcium oxide. 

In this process, the carbon dioxide is always released. For a Roman engineer, preparing materials for a new aqueduct in a pre-industrial world, producing quicklime for lime mortar would not have contributed significantly to climate change. But today, with carbon emissions having a very real impact, and processed minerals required for almost everything we build, the emissions caused by burning fuel, and by the calcination process, must be addressed.

This will be done in two ways. First, mineral processors must reduce—and eventually eliminate—the carbon produced from burning fossil fuels. Next, they must find ways to capture and store, and potentially use, the carbon produced by calcination. The development of carbon capture and storage networks is one that the sector—and the UK government—must address. But already this year, they have shown how energy-related emissions can be reduced and eliminated.

Now we’re cooking with hydrogen

At Tunstead, in Derbyshire, Tarmac recently ran a trial that aims to address the first of these issues: the energy requirements of a continual processing operation, which keeps kilns burning at temperatures over 900C. Part of the UK Industrial Fuel Switching programme, this trial was conducted with the support of the Minerals Processing Association and the UK Department of Business, Energy and Industrial Strategy (which has subsequently been reorganised into two departments, the Department for Energy Security and Net Zero, and the Department for Business and Trade).

Tarmac lime plant manager Andy Flanagan explains the process used at a plant like Tunstead. “This is one of the oldest chemical processes in the world. We heat up crushed limestone—calcium carbonate—to above 950 degrees in a kiln. The calcium carbonate releases CO2, leaving the calcium oxide, or quicklime. This can then be further processed to produce calcium hydroxide, or ground into powders. 

“These products produced are required in many key critical industries within the country, such as water treatment—both potable and effluent; pharmaceuticals; scrubbing of the flue gases from energy-from-waste plants; massively into the construction industry—soil stabilisation, aerated blocks; steel and glass industries and paper industries: too many to list.”

Lime enters the kiln on a conveyor, and fossil fuels are burnt, providing the heat needed to produce calcium oxide. 

“For a lime operation, it’s a continuous process,” Flanagan explains. “To produce calcium oxide, we need to drive off the CO2, and depending on the lime quality, 1000 kilograms of calcium carbonate—limestone—will produce 560 kilograms of calcium oxide, the finished product, quicklime. Therefore, the remaining 440 kilograms is co2, which is produced and goes, unfortunately, up the stack. Which is why we’re really working heavily on how we capture the process co2.”

The release of carbon dioxide through the chemical process of producing quicklime is unavoidable. But the other source of carbon emissions—fuel—can be addressed. And that’s what Tarmac and its partners at the Mineral Products Association, or MPA, first set out to address with the recent trial, which was backed by the UK government.

Diana Casey is director of cement and lime, and energy and climate change, at the Mineral Products Association. She has worked to develop the sectors carbon-free roadmap. “The government has been keen to understand—or to accelerate—fuel switching across different industrial sectors. And they had some funding to support trials and feasibility studies, and then some demonstrations of different fuel switching options.”

The MPA received funding for three projects: a feasibility study, looking at fuel switching in the cement sector. And then, leading on from that, two studies, one in cement and one in lime, testing that fuel mix.

“On the cement side, we actually split our demonstration into two. So we tested hydrogen and waste biomass in a cement kiln,” says Casey. “And then we tested the use of plasma or electrical heating in a cement calciner—these are different parts of the cement manufacturing plant. And then on the lime side, we tested the use of hydrogen to replace natural gas.

“The aim of the trial was to make an incremental switch from natural gas to hydrogen. And it was looking at testing not just the impact on the lime product quality, but also what the impact on the kiln system might be. We also used it to understand more about the regulatory constraints around the use of hydrogen and also how the hydrogen can be delivered to the site and the costs involved in that.”

The sector relies heavily on natural gas. Removing this from the process would have a significant impact on its emissions. But it will not be able to switch over to 100% hydrogen overnight. Generation and distribution capacity will be built over time. So kilns must be able to accept increasing concentrations of hydrogen, mixed in with natural gas, over a period of years.

“Within Tarmac, we’re natural gas fired,” says Flanagan. “In the Buxton area for hundreds— even thousands—of years, lime was produced by burning wood and coal. The Tunstead and Hindlow sites were converted to natural gas, as this infrastructure was developed across the country in the 1970s. At that point, moving from coal to natural gas was a great environmental improvement. We didn’t have black plumes of smoke coming out of the stacks from the lime production in the area. But now we need to move to the next stage.

“Tarmac volunteered to carry out the hydrogen trial on one of its shaft kilns in Tunstead. We determined that if we could successfully burn a blend—or 100% hydrogen—on the shaft kiln, then we will be able to transfer those innovations to other kiln types.”

A winning idea

Fergus Harradence is Deputy Director for infrastructure, construction, and rail in the department for business and trade. He says the government wanted to support this trial, and the wider transition to low carbon fuels in industry.

“The construction products sector, which includes mineral products and mineral processing has a turnover of around £55bn a year, or about 1.2% of all of UK GDP. It’s a sector that employs about 300,000 people across over 20,000 firms. And it’s one that makes a significant contribution to the regional economy across England, but also in Scotland, Wales and Northern Ireland.

“It’s a really important foundation sector for the economy,” Harradence explains, “Because without the products that it produces, we can’t build and maintain the infrastructure and aims that the UK needs.”

But it’s also a major contributor to carbon emissions. “Globally, cement production accounts for around 7% of all carbon emissions,” says Harradnece. “It is the most significant source of industrial emissions. And it’s vital that we keep working in order to reduce and ultimately to eliminate that.”

One component of the UK government’s support for reducing emissions is the industrial fuel switching competition, which helped fund the hydrogen lime kiln demonstration at Tunstead, as well as projects looking at cement production.

“It’s a £55 million competition run by the Department for Energy Security and Net Zero, which aims to accelerate the development of innovative clean energy technologies and processes for a range of industrial applications,” says Harradence. “It’s trying to encourage switching from high carbon fuels—diesel, natural gas, for example—to alternatives such as hydrogen electricity, that’s been generated from renewable sources, or biomass. 

“It’s a two phase competition. The first phase supported 21 feasibility studies aimed at identifying the most promising potential innovations in this area. And the second phase, which will run between this year and 2025, aims to support the most promising projects to develop further and move from the states of effectively successful experiments,  technologies that are not that far removed from the market.”

Turning up the heat

The trial would assess how lime kilns performed—and any impacts on their maintenance and lifespan—if hydrogen replaced fossil fuels, either partially or completely.

“A lot of work went into the initial looking at the project scope, and we set off on a journey, looking at the various blends that we’re working with, [that BEIS believes] the country would maybe put into the network,” says Flanagan. “We started at 6% energy—which is 20% hydrogen by volume. We mixed the hydrogen with the natural gas that we were currently burning, turning down the natural gas fuel, and then increasing the hydrogen fuel, so that we got the correct energy per kilogramme of limestone in the kiln.”

While kilns, and the bricks that line them, are designed to withstand immense temperatures over many years of continuous use, they are not indestructible. Flanagan and his team would gradually increase the hydrogen blend, to test the impact on the kiln.

“Hydrogen’s got a higher flame temperature than natural gas,” he explains. “When we are applying the heat in the burning zone, for the separation of the CO2, that increases the risk of actual fusion of the limestone. 

“One of the concerns we had was that this would cause fusion and ultimately block the kiln and could have catastrophic effects. So that had to be measured and evaluated during the HAZOP process.”

Less catastrophic, but still important, impacts were also considered. Tarmac needed to consider the impact on the kiln’s refractory lining. “We look to run the kiln for 10 years—from re-lining—so it was important to discover whether the higher flame temperature would increase the speed of wear, due to the higher temperatures breaking down the chemicals in the refractory bricks differently, compared to natural gas.”

One of the big green claims for hydrogen is that, when burnt, it gives off only water. That’s great for drivers and pedestrians, as well as the planet. It’s not such a good thing when you are managing and maintaining a processing plant.

“We needed to measure the impact of that within the exhaust systems and the bag filters for corrosion due to the increased moisture in the waste gas,” says Flanagan.

Nothing to see here

As Flanagan and his colleagues gradually increased the hydrogen composition of the fuel used for the kiln, they saw what they wanted to see: nothing.

“How this was measured as a real success, was that it was one of the projects that you didn’t want to see change,” says Flanagan. “We wanted to see consistency with natural gas operations. And, certainly through the blended trials, we saw very little change.” 

Casey confirms this. “The outcome of the project was that at low levels of hydrogen substitution—so around 20%, by volume—there was limited impact on the kiln operation, the product or any emissions to air,” the MPA director explains. “We did notice that as those volumes of hydrogen increased, there were some challenges in terms of the kiln operation. We have to be careful with the sintering process, and kiln blockages were also a little bit of a threat. But these weren’t insurmountable issues. I think it was a really successful trial and demonstrates that hydrogen is viable for the lime sector in future.”

Despite these challenges, Flanagan reckons that Tarmac is ready to move to initial use of a 20% by volume, or 6% by energy, hydrogen/natural gas mix. And, during the trial, the plant manager was even able to—briefly—run the kiln on 100% hydrogen. 

The trial is a key stepping stone on the UK’s path to net zero, says Harradence. “This was a high quality project with very considerable potential, not just for Tarmac, but also for wider use across the industry, potentially into other sectors. It could ultimately help us to create the hydrogen economy by creating a market demand for hydrogen in the UK.”