Switching to greener fuels

A project

In the UK, an R&D project with these research objectives was co-ordinated by the Mineral Products Association (MPA), involving cement plants and leading technical service providers, to mitigate CO2 emissions generated by 100% combustion.1 In addition to hydrogen, dried pulverised sewage sludge pellets, meat and bone meal (MBM), glycerine and wood chips were considered as being the most frequently available biomass-sourced fuels. For the net-zero CO2 emission trials mostly existing equipment, with minor modifications to the burner ports (diverting alternative fuel handling and feeding equipment to and from the calciner and kiln), was implemented. For the calciner, a novel approach utilising thermal input from a plasma torch, combined with the co-firing of wood pellets, was employed. Initially, a 1 MW plasma torch was considered for the calciner’s 100% carbon neutral operation. However, following a feasibility study, the plasma input was reduced to 100 kW due to the lack of ‘off the shelf’ availability and the prohibitively high costs associated with short-term calciner trials. The lower power rating was about 1% of the calciner thermal input. However, the trial encountered problems and it operated for only 30 minutes. As a result, the effect of the fuel on the loss on ignition (LOI) of the hot meal could not be assessed. Nonetheless, during its operation, neither hot-spots nor operational fluctuations were observed.

For wood pellet feeding, trials were conducted to evaluate both pneumatic and gravitational dosing options . The trials found that the pellets conveyed on the inclined belt worked without blockages and dosing fluctuations, but the silo extraction system required some adjustments. The pneumatic trial was less successful due to conveying blockages within the transport line, even at lower feed rates. The MI-CFD modelling results concluded that the wood pellets were well suspended with a good burnout and that the small plasma torch, sourced for the trial when operated, had no impact on the process. In a follow-up study, plasma torch was replaced with H₂ so that H₂ was co-fired in both the kiln as well as in the calciner.

In addition to H₂ co-firing in the calciner, improvements to the plasma torch were investigated. Following its limited operation in the calciner, i.e., developing a plasma charge assisted biomass burner that can be safely operated under higher temperature and particle laden environments prevailing in calciners. Due to the cost and availability issues with the higher-rated plasma torch, it was decided to develop a technique in which a small portion of the fuel stream is directed over a plasma charge located inside the burner pre-combustor. In that way, the resulting ionised, semi-burnt and higher temperature fuel fraction would be subsequently re-entrained with the rest of the biomass, thereby accelerating its devolatilisation and combustion reactions.

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Read the article online at: https://www.worldcement.com/special-reports/11102024/switching-to-greener-fuels/