Abstract
Carbon dioxide is one of the major greenhouse gases and its massive emissions have caused serious impact on the global climate environment. Carbon reduction from coal-fired power plants is the key to achieving Hong Kong’s carbon neutrality target. Carbon dioxide capture technology is of great significance as a possible underwriting technology. The main carbon capture technologies are absorption separation, adsorption separation and membrane separation. This paper introduces the current development status, application research progress and future development trend of carbon capture technology; summarises domestic and international carbon capture demonstration projects; compares the advantages and disadvantages of carbon capture technology and proposes application suggestions applicable to coal-fired power plants in Hong Kong.
Introduction
The serious consequences of climate change are recognised worldwide, making the reduction of greenhouse gas emissions a global issue. in September 2020, China announced its goal of achieving carbon neutrality by 2060, and in the same year Hong Kong officially announced its carbon neutrality target for 2050. The implementation pathway is detailed in the Hong Kong Climate Action Blueprint 2050 published in 2021, with a commitment of about HK$240 billion to implement various climate change mitigation and adaptation measures over the next 15 to 20 years. As power generation accounts for about two-thirds of Hong Kong’s carbon emissions, the Government has identified net-zero power generation as one of its key carbon reduction strategies[1]. CO2 emissions from the power generation side are mainly due to coal-fired power stations, gas-fired power stations, oil-fired and combined cycle gas turbines. Hong Kong is supplied with electricity by CLP Power Hong Kong Limited and Hongkong Electric Company Limited. The HEC’s 2021 Sustainability Report shows that the share of new energy generation is only 0.5%[2]; the 2021 CLP Sustainability Report shows that non-fossil energy generation also accounts for only 21.7% of total electricity generation[3]. In view of the above, carbon capture, utilisation and storage (CCS) technologies are an indispensable option for reducing emissions from power plants, which will also help to reduce the carbon intensity of future power systems. By 2021, a total of 135 carbon capture and storage facilities will be built worldwide, with 27 in operation and a total capture capacity of 36.6 Mtpa[4]. This paper provides a systematic overview of the technical routes, basic principles, advantages and disadvantages and development directions of the currently operating projects, and proposes a technical route applicable to power plants in Hong Kong.
1 CCUS technology
Carbon Capture Utilisation and Storage (CCUS) technology is one of the effective ways to reduce CO2 emissions. The technology aims to capture CO2 from large industrial sources and store it in a suitable geological structure or convert it into a useful product.
1.1 CO2 capture technologies
CO2 capture technologies can be classified according to the location of capture, as shown in the diagram, into pre-combustion capture, in-combustion capture and post-combustion capture, and according to different processes, into absorption, adsorption, membrane separation, etc. According to the Global Carbon Capture and Storage Institute, the vast majority of carbon capture projects in operation in power stations around the world use post-combustion chemical absorption. This method is considered to be the most suitable carbon capture technology for low concentration flue gas in power and other industries, and is also one of the most mature and widely used carbon capture technologies in the commercial demonstration stage. Membrane capture technology is also in the industrial experimental stage and is not yet in long-term operation. Other capture technologies can be seen in Figure 1 [5].
Figure 1. Methods and techniques of CO2 capture
1.1.1 Chemical absorption
Chemical absorption uses a weakly alkaline absorbent to react chemically with CO2 at low temperatures (~40 °C) and under heated (~120 °C) conditions, a reversible reaction occurs to release CO2 [6], the process is shown in Figure 2. Liquid amine absorption is currently the most mature and the only carbon capture technology that has achieved large-scale commercial application. The essence of the process is that the acidic gas reacts with an alkaline absorbent in a reversible chemical reaction to form unstable salts such as carbonates, bicarbonates or carbamates that can decompose and release CO2 for the purpose of carbon capture and recycling. Common absorbents include organic amines, ammonia, carbonates, etc. [7]. Chemical absorption methods are generally based on organic amine absorbents. Early absorbents were mainly MEA composite amine absorbers; in recent years, there has been a joint development of multiple systems, with MDEA, DEA, diethylenetriamine DETA, phenothiazine PZ, AMP and other composite amine absorbers as the mainstay of research, with the aim of achieving a further reduction in energy consumption for capture.
