VDZ reports on a trial using hydrogen to reach 100% net-zero CO2 fuel emissions in the main burner.
The German cement sector is one of the most sustainable in the world, with stringent emission standards and high use of alternative fuels. Over the past 30 years, the cement industry was able to reduce the cement-specific CO2 emissions by more than 21%. As further efforts are needed in order to fulfil political decarbonisation objectives, in 2020 VDZ launched a Roadmap to net-zero CO2 cement production in Germany by 2050. The roadmap presents several CO2 reduction measures along the value chain, which includes among others three fuel-related targets:
- Increase thermal efficiency by 13%;
- Increase alternative fuel use to 90% (of which 35% biomass);
- Introduce up to 10% hydrogen into the fuel mix.
The introduction of hydrogen relies on CO2-free hydrogen production, which is presently scarce. Currently only 5% of hydrogen used in Germany was produced using water electrolysis ('green hydrogen') in 2021. It is expected that the amount of green hydrogen will increase in the future, while prices will fall. At a certain point, it will become appropriate to consider CO2-free hydrogen as an alternative fuel in cement production.
Introducing hydrogen into a cement plant has several effects. The advantages include a lower CO2 partial pressure, which reduces the temperature required for calcination and hence lowers the energy requirement. NOx generation is also expected to be lower as H2 doesn’t contain fuel nitrogen - in contrast to coal or most alternative fuels. The most significant disadvantages are the high costs and the still challenging H2 supply to the plant, given the amount of H2 required and lack of infrastructure such as pipelines. Supply over the fence or local production of H2 based on water electrolysis needs to be assessed case by case.
Fuel Switching Project
The Fuel Switching Project took place in the UK in 2021 and 2022 and was funded by the British Department for Business, Energy & Industrial Strategy (BEIS). It aimed to demonstrate the potential and technical viability of using 100% net zero fuels at the industrial scale, while maintaining clinker and cement quality. The practical use of hydrogen and biomass in the main burner was tested in 2021 at Hanson's Ribblesdale plant in the UK, which has a four-stage pre-heater kiln and an inline calciner.
The baseline fuel mix and the trial fuel mix are shown in Figure 1. The hydrogen was supplied at a rate of up to 340kg/hr. It is important to note that the inclusion of hydrogen allowed the use of greater proportions of biomass in the main burner (animal meal and glycerine, a byproduct from biodiesel production).
Trial equipment
The hydrogen-feeding station comprised two connected hydrogen tube trailers, each containing approximately 250kg of hydrogen at 230bar supplied by BOC (Linde Group). Two other trailers were connected on standby in order to assure a continuous feed of hydrogen to the kiln. Other unconnected tube trailers were ready to be connected as needed. The pressure was reduced in two stages to 10bar. There was a further reduction stage to <3bar to achieve the correct gas supply speed and volumes. The burner used had two concentric channels, one for H2 and one for cooling air. The operation lasted for around six hours.
CFD analysis
Cinar provided computational fluid dynamics (CFD) simulation for the Fuel Switching Project prior to the industrial trial. This predicted that the flame envelope would be larger and at a higher temperature with hydrogen than in the baseline case. It also showed that the temperature close to the burner tip would increase due to the very rapid ignition and combustion of hydrogen. The CFD analysis also looked at the impact of changing the speed of the fuel supply, which identified a suggested hydrogen burner tip velocity of 275m/s in order to create a sufficient burner momentum. The CFD also showed that the clinker bed temperature would - under these conditions - be the same as in the base case.
Industrial trials
While useful in setting out the main parameters for the trial and foreseeing possible impacts on the clinker burning process, CFD simulations are based on computational models and several assumptions. The initial trial results showed that the flame was bright and irradiated more similarly to the baseline scenario, despite the high feeding rate. This removed one of the main concerns, that the hydrogen-based flame would show lower heat radiation, which is a common issue with pure hydrogen flames. The flame was similar in length and there was no noticeable impact on the overall energy demand of the pyro-process as a whole.
CO and NOx levels both appeared to have increased moderately in volume percentage but this can be explained by the higher concentration of water vapour and lower concentrations of CO2 and O2 in the exhaust gas. The injection of hydrogen in the lance was pushed to its limits and a tip velocity of 900m/s was reached at maximum capacity. Such high injection velocity ensured an ignition far from the burner tip. There were some concerns with regard to noise at the burner platform due to supersonic tip velocities, but there were no audible changes to emphasise. The temperature profile in the kiln and coating formation was controlled in the central control room with the aid of a kiln shell scanner but there were no changes to report during the short trial.
Product quality
Clinker and cement samples from the baseline and trial fuel mixes were taken and analysed by VDZ using XRD, XRF and microscopy. These showed that the novel fuel mix did not adversely affect the mineralogy or performance of the clinker or cement. No changes to the clinker production have been noticed.
Costs
Following the trial, VDZ calculated the cost of using these very high volumes of hydrogen against other CO2 reduction measures, such as carbon capture and storage (CCS), which can cost Euro50-170/t depending on method, technology, necessary logistics and distance to the CO2 storage site.1 The calculation used a cost of approximately Euro17/kg for hydrogen supplied at the gate and including capex, which is well above market prices. The calculation shows that using hydrogen would cost Euro1560/t of abated CO2 at the hydrogen prices charged for the project. This does not take into account the fact that higher proportions of biomass could be consumed.
A gate hydrogen cost of Euro1.3/kg would be required to achieve a CO2 avoidance cost of Euro75/t. The figures presented above should be seen as indicative and valid for 2021 only. The assumptions and economics changed considerably with the rising prices of coal and natural gas in 2022, which presently might be more favourable for the substitution of coal by 'green' hydrogen than they were before.
Conclusions
Hydrogen is very expensive and, above all, most of it is not CO2-free at present. The Fuel Switching Project shows that its use is technically possible at very high volumes. The project achieved a 'net-zero' fuel mix in the main burner, with no major changes to flame characteristics, energy demand or product quality. The use of hydrogen as a fuel at scale may be a while away, but the Fuel Switching Project offers an exciting indication into the future.
Reference
1. VDZ, 'Decarbonising Cement and Concrete: A CO2 Roadmap for the German cement industry,' Dusseldorf, 2021. https://www.vdz-online.de/fileadmin/wissensportal/publikationen/zementindustrie/VDZ-Studie_Dekarbonisierung_von_Zement_und_Beton.pdf.
