Reducing methane emissions

As the cement industry prepares to meet its sustainability objectives, this month’s Technical Forum focusses on the potential of methane as an alternative to coal in firing kilns. More than 70 per cent of kilns are still fired with coal or petcoke whereas only 10 per cent are currently using natural gas.

News in the first weeks of November 2021 was largely dominated by the Conference of the Parties (COP26) meeting in Glasgow, UK. The meeting had been postponed from 2020 due to the COVID-19 pandemic and was widely trailed as a last chance to save the Earth from climate change and global warming. Whether such hyperbole is useful, or whether what was agreed is sufficient, remains to be seen.

Nevertheless, some important commitments were made at the conference, two of which are discussed in this month’s Technical Forum:
1. More than 100 countries committed to reducing methane emissions by 30 per cent below 2020 levels by 2030.
2. Over US$130trn of capital was committed to net zero by the Glasgow Financial Alliance for Net Zero.

These might seem obscure topics to talk about in a cement technology-themed article, since there are no methane emissions from a cement kiln or the cement manufacturing process. Dr Clark’s interest in methane (natural gas) is its potential to replace coal or petcoke for firing cement kilns.

A coal alternative

There are, of course, numerous cement kilns around the world that are already fired with natural gas. Perhaps 10 per cent of the world’s kilns are fired with natural gas, but more than 70 per cent are still fired with coal or petcoke.

If we look at the fuel CO₂ factors in the GCCA Protocol we see a wide range of specific CO₂ factors for fossil fuels (see Table 1).

Table 1: CO2 factors in the GCCA protocol
Fuel	IPCC default (kg CO2/GJ)
Coal, anthracite, waste coal	96.0
Petcoke	92.8
(Ultra-) heavy fuel	77.4
Diesel oil	74.1
Natural gas (dry)	56.1
Oil shale	107
Lignite	101
Gasoline	69.3

Clearly there would be a CO₂ emissions saving in switching from coal firing to heavy fuel oil firing. It is unlikely to be an economic saving due to the cost of heavy fuel oil compared to coal in most parts of the world. However, there will be an even greater CO₂ emissions saving in switching from coal to natural gas firing. Natural gas may also be competitive in price to coal and, therefore, is an economically viable method of making significant reductions in CO₂ emissions.

If we consider a cement kiln with an average kiln thermal energy consumption of 3500MJ/t of clinker, then the default CO₂ from calcination is 525kg CO₂/t of clinker and thermal CO₂ is 336kg CO₂/t of clinker, if the kiln is fired by coal. So total CO₂ per tonne of clinker for an average, coal-fired kiln is 861kg CO₂/t of clinker:
Specific CO₂ emissions = 525 + 3500 x 96/1000 =  861kg CO₂/t clinker

If the same average kiln were fired with natural gas then the CO₂ from calcination remains the same at 525kg CO₂/t of clinker, but the thermal CO₂ is reduced to 196kg CO₂/t of clinker. The total CO₂/t of clinker for an average, natural gas fired kiln is 721kg, a saving of 16 per cent in total specific CO₂ emissions:
Specific CO₂ emissions = 525 + 3500 x 56.1/1000 = 721kg CO₂/t clinker

If 10 per cent of the world’s cement kilns could be switched from coal to natural gas, then 1.16Gt of CO₂ would be saved between 2020-50. This is more than that saved through the International Energy Agency’s (IEA) 2˚C scenario (2DS) projects for switching to alternative and biomass fuels between 2020-50. If 40 per cent of the world’s cement kilns could be switched from coal firing to natural gas firing, then around 4.64Gt of CO₂ would be saved in the 30-year period. This is more than the IEA’s 2DS scenario projects for carbon capture, storage and utilisation. To Dr Clark’s mind, switching cement kilns to firing natural gas is low-hanging fruit that should certainly be undertaken.

Putting methane to use

The excitement about the commitments made to reducing methane emissions in Glasgow was nothing to do with firing cement kilns with natural gas. It was because methane is a much more powerful greenhouse gas than CO₂. While it is 85x more powerful, it does not remain in the Earth’s atmosphere for as long as CO₂. Reducing methane emissions is seen to be the most effective way to slow global warming in the short term, ie, to 2050. Apparently, if methane emissions can be cut to 30 per cent below 2020 levels by 2030 then 0.2˚C of global warming by 2050 can be avoided.

This author’s specific interest can be traced back to his visits to the CIMERWA cement factory at Bugarama, southwest Rwanda, near the border with the Democratic Republic of Congo. There the precalciner kiln on the 0.6Mta cement factory is primarily fired with South African coal, which has to be hauled around 1200km from ports in Kenya or Tanzania. While visiting the factory, Dr Clark was fortunate enough to stay at a hotel overlooking the picturesque Lake Kivu some 15km from the factory.

Picturesque Lake Kivu may be, and peaceful as you watch the local fishermen catching their sambaza, but people also worry that it may be a “killer” lake. It is one of only three known lakes to be saturated with naturally-occurring CO₂ and methane. The other two lakes are Lakes Nyos and Monoun in Cameroon. Both Nyos and Monoun have suffered limnic eruptions, where a cloud of CO₂ is released from the lake. In 1984, 37 people were killed by the Lake Monoun eruption. In 1986 the eruption at Lake Nyos killed 1746 people. The CO₂, being heavier than air, filled the valleys within 15km of the lake, displacing the air and suffocating the inhabitants. Lake Kivu has never erupted, but around 2m people live in the lake basin, meaning a limnic eruption would be potentially catastrophic.   

Lake Kivu is estimated to contain 300bnm³ of CO₂ and 60bnm³ of methane. The lake lies in the volcanic Albertine Rift valley of Africa. Carbon dioxide enters the lake from the volcanic rock beneath, and is then converted into methane gas by bacteria and fermentation of biogenic sediments in the lake.

Some of the methane is extracted for electricity generation and there were hopes that methane could be made available from the lake to fire the CIMERWA kiln. A back-of-the-envelope calculation indicates that 40-50Mm³ per year of methane would be required to fully fire the CIMERWA kiln. Lake Kivu is estimated to have the capacity to provide 120-250Mm³ per year – much more than would be required for the CIMERWA kiln, leaving plenty left over for electricity generation.

The problem is raising the capital to safely extract the methane from Lake Kivu and transport it to the CIMERWA cement plant at Bugarama. This is where the US$130trn of capital committed to net zero comes in. What better project could there be to use up some of that capital? Not only would CO₂ emissions from the cement factory be dramatically reduced, but also the CO₂ emissions from hauling the coal all the way to Bugarama from Indian Ocean ports. Much better to come up with a plan to manage and use the gases in Lake Kivu than for a limnic eruption to take place, releasing vast quantities of greenhouse gases into the atmosphere and potentially killing hundreds of thousands of people. Not only could some of this US$130trn of capital be used to provide methane for the CIMERWA kiln but also capture the CO₂ that the lake is already storing. 

This article was first published in International Cement Review in January 2022.