Introduction
Hydrothermal carbonisation (HTC) is a process of converting biomass into carbonaceous materials by heating it in water under high pressure and moderate temperature. Although HTC is mainly used for biomass, there is potential for its application to coal to reduce CO₂ emissions. Let us look at how the HTC process affects the reduction of greenhouse gas emissions and what benefits and challenges are associated with its implementation.
Coal Stabilisation in the Hydrothermal Carbonisation Process
HTC converts coal into more stable forms, which can reduce CO₂ emissions during combustion. Stabilisation involves changing the structure of coal, reducing its reactivity during thermal processes. This process mimics the natural formation of lignite, but in a much shorter time, often within a few hours instead of millions of years. As a result, stabilised materials can be used more efficiently in the power industry, especially in power plants, where minimising heat loss and reducing emissions are of key importance.
Production of Eco-friendly Solid Fuels with Higher Calorific Value
The HTC process enables the production of solid fuels with a higher calorific value and lower moisture content. These fuels are characterised by better combustion quality, which translates into higher energy efficiency and lower CO₂ emissions. Additionally, HTC fuels can be used as coal substitutes in industrial sectors such as cement plants or steelworks, contributing to a significant reduction in the carbon footprint. By eliminating moisture and improving the chemical structure, these fuels are more environmentally friendly compared to traditional fossil fuels.
Reduction of Chemical Contaminants in Hydrothermal Carbonisation Products
Thanks to HTC, it is possible to effectively remove contaminants such as sulphur and nitrogen, which are responsible for SO₂ and NOₓ emissions. This process improves the quality of the resulting fuels, reducing their environmental impact. The removal of sulphur and nitrogen also helps to minimise corrosion in industrial installations, which lowers infrastructure maintenance costs. Indirectly, this contributes to lower CO₂ emissions by improving combustion efficiency and reducing the impact of pollutants on the atmosphere.
Practical Applications of Hydrothermal Carbonisation Products
Carbonaceous materials produced during the HTC process can be used in a wide range of applications. As CO₂ adsorbents, they are used to capture carbon dioxide from the atmosphere, which is particularly important in CCS (Carbon Capture and Storage) technology. In soil science, HTC products support carbon sequestration, improving soil structure and its water retention capacity. This enables long-term storage of CO₂ in terrestrial ecosystems, further reducing its amount in the atmosphere. Furthermore, these materials can be used as components in the chemical and materials industries, opening up new opportunities for innovative applications.
Key Challenges and Limitations of Hydrothermal Carbonisation
Energy Efficiency
The HTC process requires significant energy inputs for heating and maintaining high pressure, which can limit its effectiveness in reducing CO₂ emissions, especially when the energy comes from non-renewable sources. To maximise the benefits, it is necessary to use renewable energy sources, such as solar, wind or geothermal energy, to power HTC installations. Furthermore, the development of more energy-efficient reactors and the optimisation of process parameters can contribute to a further reduction in energy costs.
Industrial Scale
Large-scale implementation of HTC requires significant investment in infrastructure and adapting the technology to various types of feedstock, such as different types of biomass or industrial waste. Biomass with a high lignin content, such as wood and its waste, is particularly beneficial for the HTC process due to its energy efficiency. On the other hand, industrial waste with a high heavy metal content can be problematic, as it contaminates the final carbon products and requires additional purification processes. Optimising process parameters for specific types of biomass allows for maximising benefits and minimising limitations. Currently, one of the greatest challenges is developing scalable HTC reactors capable of processing thousands of tonnes of material annually while maintaining process efficiency. Logistical issues, such as the transport and storage of feedstocks and final products, can also affect the profitability of the technology. Cooperation between the public and private sectors, as well as financial support in the form of subsidies or tax relief, can play a key role in accelerating the commercialisation of the technology.
Operating and Maintenance Costs
The HTC process requires high-quality components, such as high-pressure sealing systems, water pumps, and heat exchangers, which are prone to wear due to high pressure and temperature. Regular maintenance and replacement of these components increase operating costs, representing a significant budget burden.
Material Limitations
Not all types of biomass and coal are equally effective in the HTC process. Chemical impurities in feedstocks can affect the quality of the final products and the efficiency of the process itself. This requires careful selection of raw materials, which can be a challenge for large industrial installations.
Carbon Footprint of Infrastructure
The production and installation of HTC systems, including reactor construction and transport, generates its own carbon footprint. To make the technology more environmentally viable, infrastructure-related emissions must be minimised through the use of more sustainable building materials and efficient logistical processes.
Socio-Economic Impact
The adoption of HTC technology may require changes in local waste management systems and industrial infrastructure, which can cause social resistance or economic difficulties in regions with limited financial resources. Public education and appropriate implementation strategies can help overcome these barriers.
Lack of Standardisation
The lack of uniform standards for the design and implementation of HTC installations hinders their widespread adoption. Standardisation could lower costs and increase the efficiency of the technology.
Summary
Hydrothermal carbonisation of coal offers potential benefits in reducing CO₂ emissions through carbon stabilisation, the production of lower-emission fuels, and the potential for carbon sequestration. HTC products can be used in various sectors, from energy to agriculture, supporting sustainable development and the circular economy. However, fully exploiting this potential requires further research and process optimisation. With the support of science and technological innovation, HTC can play a key role in the fight against climate change.
