The energy transition in the cement sector is accelerating under general public pressure to fight climate change. Technological advancements such as alternative fuels, waste heat recovery, lower clinker factor, the reduction in the cost of renewable energy, advances in electrical storage systems, and the approaches to carbon capture and storage / usage (CCS/U) in the cement industry will continue to present opportunities for improvement. Could green hydrogen also provide answers...?
The rising proportion of renewable energy being used in a number of markets is to be welcomed in the context of our collective efforts to mitigate climate change. However, as the world transitions to a low-CO2 (i.e. a higher renewables) future, there remain issues regarding storage of the energy generated by such methods. At present the peaks and troughs associated with renewable energy, most notably solar energy and wind power, are compensated for by feeding back to national power grids in which renewables are only part of the mix. Current energy storage solutions can handle the fluctuations from renewables, but greater capacities will be needed as the proportion of renewables increases. Storage times, i.e. the discharge capacity of the system, will have to increase from the current 2 - 4 hours to days, weeks or months. Discharge times, currently around 10 hours for large battery solutions, will also need to be lengthened to fully harness the potential of renewable sources.
Significant research therefore continues into effective storage solutions, which will be crucial in the effort to decarbonise our economy and, hence, the cement sector. To date answers to the intermediate storage problem have included:
- Batteries of various sizes and chemistries;
- Gravity storage: A heavy object (or water) is winched (/ pumped) upwards using electrical power and then lowered in a controlled fashion to regenerate power when required;
- Thermal batteries: Heat is transferred to high-capacity concrete blocks, ceramic blocks, pebbles or aluminium ingots;
- Kinetic storage, including flywheels.
Hydrogen as a storage solution
Hydrogen (H2) is the simplest and lightest chemical element. Due to its high reactivity, it is not found on earth in its molecular form, but as its simplest oxide, water (H2O) or as methane (CH4), plus a myriad of more complex chemicals. Molecular hydrogen can, however, be generated from water via electrolysis, with molecular oxygen (O2) also produced:
2 H2O (+ electrical energy) -> 2 H2 + O2
Both hydrogen and oxygen are extremely reactive and can potentially be used as fuels / oxidants in rockets, fuel cells and other applications:
2 H2 + O2 -> 2 H2O (+ heat energy)
However, electrolysis of water is very energy intensive. This means that electrolysis using fossil-derived electricity is unsustainable. In contrast, hydrogen generated from renewable electricity would, despite being used as a thermal fuel but without CO2 emissions, represent a sustainable way to store and move energy. This is termed ‘green hydrogen.’ Indeed, 2019 saw a big push in this area, as well as with hydrogen generated from existing industrial processes. Some nascent technologies include thermal splitting of water using solar concentrators
or nuclear energy.
As well as the ability to be stored for prolonged periods, the biggest advantage of hydrogen is that it has an existing infrastructure. 10Mt of hydrogen was produced in the US in 2019, mostly from fossil fuel fired steam / methane reformers (which produce carbon monoxide (CO) and hydrogen). This produces large amounts of CO2. The alkaline electrolysis process mentioned above and shown in more detail in Figure 1 is also used.
Even with proven technology available, research continues with one option being to replace the water with steam. This would open up the possible use of waste heat from industrial processes, including cement plants, in order to increase the current conversion efficiency of 70%. If we assume that the conversion efficiency of a fuel cell is about 70%, then the ‘round-trip’ efficiency of the electrolyser/fuel cell pair is around 50%. The overall efficiency of an electrolyser / combustion process depends on the exact parameters of the combustion equipment used and the proportion of hydrogen in the fuel mix.
Firing cement kilns with hydrogen
The use of green hydrogen in the cement industry is soon to be a reality, with a multinational trialling its use at a plant in Europe as part of its target to achieve 100% fossil fuels substitution. Hydrogen as a fuel to displace all or part of the fossil fuels used could soon be a reality. This approach is gaining most traction in Europe where natural gas is fairly expensive and there are costs associated with emitting CO2. Mixing green hydrogen with natural gas is seen as a way to decarbonise electricity and heat production. Already large OEMs are offering gas turbines that can fire a mix of hydrogen and natural gas and are working toward 100% hydrogen combustors. As with oxygen, hydrogen can be mixed with natural gas at up to 10%, with very little impact on the combustion equipment.
Managing hydrogen
Of course precautions should be taken when handling hydrogen. It is very flammable and also very buoyant. This means that it escapes easily and quickly. Material for pipes and valves has to be specially selected to resist hydrogen-induced embrittlement. Major burner OEMs already have some experience firing pure hydrogen or mixtures. At any concentration it is a fairly easy-to-handle fuel, as long as the high flame propagation speed is considered and temperature peak issues are properly understood and dealt with. For rotary kilns this can lead to damage to the refractories and often higher NOx levels, depending on the location of the hydrogen injection and the local oxygen concentration.
In fact, when gas was produced from coal in the 19th Century, the resultant ‘town gas’ contained about 40% hydrogen. Today, coke oven gas and non-condensible gas (50 - 60% H2) are routinely fired through the main burners of rotary kilns used to process lime mud in pulp and paper plants and in the lime kilns used at steel plants.
Don’t forget the oxygen
While in electrolysers the focus is on hydrogen production, one important advantage of co-locating such electrolysers within cement or lime plants is the ‘free’ oxygen that would be available (See Figure 3). Indeed, some cement manufacturers currently pay a premium to receive oxygen in tanks from third parties due to the myriad benefits it brings to the process: A shorter and more stable flame; higher quality clinker; greater use of alternative fuels (including the use of lower quality alternative fuels); increases in production rate (or lower electrical use by the ID fan at the same rate, and, finally; reduction in CO2 emissions. When oxygen is currently vented at some electrolysers for lack of an immediate practical use, this is a clear opporunity.
Concluding remarks
While great strides have been taken with regard to reducing the clinker factor, increasing the use of alternative fuels, raising process efficiency and improving other parameters in the production process, the path to fully decarbonising the cement industry continues to be long and incremental. However, it is clear that there is a way forward that uses existing mature technologies and green hydrogen generated from renewable sources. This can offer opportunities to both store and regenerate electrical power and also to directly fire the cement making process.
Hydrogen economy data
Density: 1kg H2 = 11Nm3
Mass energy density: 1kg H2 = 3.2kg of petrol
Volumetric energy density: 1Nm3 H2 = 0.25L of petrol
1MW electrolyser produces:
H2: 200Nm3/hr / 18kg/hr / 0.43t/day
O2: 100Nm3/hr / 144kg/hr / 2.45t/day
Conversion efficiency: ~70%
Water use: Production of 1kg of H2 requires at least 10L of demineralised water.
Use of hydrogen in the cement sector
1. VICAT is studying the possibility of recovering the CO2 emitted through the use of hydrogen to convert it into a usable bio-fuel or chemical intermediate. It sees this as part of a wider scheme to decarbonise not only its clinker production process but also a fleet of trucks, which it has already ordered (See Figure 3). Waste heat from cement production can be used to increase the yield of hydrogen. Finally, the plant will use the oxygen generated to boost efficiency in its kiln. The use of hydrogen directly as a fuel in kiln and/or calciner will also be investigated, with the aim of zero fossil fuel use in the future.
2. The Hanson (HeidelbergCement) Ribblesdale plant in the UK announced in February 2020 that it would trial the use of hydrogen in its kiln, along with biomass. The results will be shared across the cement sector.
3. Lafarge Zementwerke, OMV, Verbund and Borealis signed a memorandum of understanding in June 2020 to plan and build a full-scale unit at a cement plant in Austria to capture CO2 and process it with hydrogen into synthetic fuels, plastics or other chemicals
