Alternatives to natural gas for high calcium manufacturing: Hydrogen
This project aims to use of hydrogen as an alternative fuel for high calcium lime manufacturing. Natural gas systems are well established in the lime sector, both in terms of supply and process design and management. Alternative gas feeds will need to be considered not only for the possible impact on product quality, but also on operational processes, process engineering, health and safety, environmental management and workforce skills and competencies.
All lime is manufactured through high temperature kiln processes whereby calcium carbonate - from limestones or chalk - is heated to drive off carbon dioxide. This chemical reaction occurs at about 1,000oC and is known as calcination. The residence time of the stone in a kiln varies depending on the type of kiln and type of final product required, but can be anything between six hours and two days.
In the UK, high purity limes are required to service diverse markets, such as in mortars and renders, iron and steel manufacturing, soil stabilisation, emissions control, water and wastewater treatment, and pharmaceuticals and cosmetics. To meet the demands of these markets, UK lime is manufactured from high purity limestone. Natural gas is the preferred fuel as it introduces few impurities and is readily available through the gas transmission system. It also has lower carbon emissions when compared to solid fuel alternatives such as coal or lignite. There are no examples of replacement gaseous fuels for natural gas that would ensure maintain the quality of the high calcium lime product required in the UK.
In the UK, lime is manufactured using two types of gas-fired kiln; vertical shaft kilns and parallel flow regenerating (PFR) kilns. PFR kilns are widely considered to be the most energy efficient. Vertical shaft kilns use similar technology and processes but are less efficient. However, by their nature, vertical shaft kilns are more challenged by hydrogen fuel than parallel flow regenerating kilns, given counter-current nature of the heating and the limited fuel and air mixing in vertical shaft kilns and the importance of this mixing to product quality.
Key challenges to be addressed to convert vertical shaft and PFR kilns to hydrogen include:
- Gas density/calorific value, combustion stoichiometry, and flame speed and temperature and the impact on kiln performance and product quality
- NOx, other emissions to air and exhaust gas moisture content, including the formation of emission products in the kiln, and the impact on emissions control systems
- The long term embrittlement and degradation of materials in kiln systems, including damage to refractories.
As lime manufacturing is permitted under the Environmental Permitting Regulations, demonstration of hydrogen fuel in lime manufacturing offers an environmentally robust means to assess technology feasibility within the sector.
This project is funded by the Department for Business, Energy and Industrial Strategy (BEIS) Energy Innovation Programme. The project is managed by the Mineral Products Association (MPA) and the British Lime Association (BLA), and Tarmac are providing the demonstration site for the project.
By delivering projects through the BLA, the project outcomes can be shared widely across the sector. The BLA is a member of the European Lime Association (EuLA) and the International Lime Association (ILA) ensuring that UK technologies and best practice will have the widest possible reach.