The temperature of syngas leaving a gasifier can be as high as 1600°C. Recovering some of this heat improves the process efficiency. Heat recovery systems can reclaim 5–25% of the energy in the feed – a significant amount. Put another way, the use of a heat recovery system can increase the process efficiency by 5 percentage points.
The syngas cooler is one of the most crucial components in gasification plants. It operates with gas inlet temperatures of 400-1600°C and gas-side pressures up to 8 MPa. These operating parameters combined with harsh operating conditions, such as corrosive raw gas components (H2S, HCl, H2) and high dust loads, make for challenging design and material selection.
There are various types of hot syngas coolers, including: radiant coolers, convective coolers, direct quench coolers and any combination of these.
Radiant coolers work at high temperature (>700°C) and use radiant heat transfer to cool the hot syngas. Heat is recovered by generating high pressure (HP) steam.
Convective coolers consist of a set of tubes in a container. Heat is transferred by convection and conduction. Both fire tube boiler and water tube boiler designs have been used. Convective coolers generate HP and/or intermediate pressure (IP) steam.
In direct quench coolers, the hot raw syngas is rapidly cooled by injecting cooling medium into the syngas. Water or cooled syngas can be used as the cooling medium. A third option is chemical quench cooling, where the thermal energy from cooling the syngas is used to produce other valuable products.
Often, different types of syngas coolers are combined and various syngas cooling systems are in operation in existing IGCC plants. A radiant and convective cooling system design is suitable for cooling hot syngas with temperatures much higher than 900°C. The hot syngas from the gasifier enters a radiant cooler where it is cooled to around 800°C. It then flows into a convective cooler for further cooling. This concept has been successfully demonstrated at Cool Water and Polk IGCC plants.
Radiant and quench cooling uses a radiant cooler first then a full or a partial water quench cooler. It has been selected for the Edwardsport IGCC plant, but no operating experience of the syngas cooling system is available yet.
In partial quench and convective cooling, the syngas leaving the gasifier is cooled to a temperature of 800–900°C (a temperature well below the ash melting point of the coal) by mixing directly with cooled, dust-free recycled syngas of about 230–250°C. The syngas exiting the quench cooler is further cooled in a convective cooler. Both Shell’s SCGP and Uhde’s PREFLO gasification process employ the gas quench and convective cooling concept although the design details differ. Shell’s cooling system was used in the Buggenum IGCC plant (which closed in 2013), and Uhde’s syngas cooling concept operates at the Puertollano IGCC plant in Spain.
Entrained-flow, two-stage gasifiers employ chemical quenching in the second-stage gasification. The sensible heat in the syngas leaving the first stage of the gasifier is used in the endothermic gasification reactions in the second-stage of the gasifier. The syngas exits the gasifier at 1000–1100°C and enters the convective cooler. E-Gas, MHI and EAGLE gasifiers are examples of entrained-flow, two-stage gasifiers with different design details. The E-Gas gasification process is in operation in Wabash River IGCC (USA) and the MHI is at Nakoso IGCC plant (Japan). No major issues regarding these syngas cooling systems have been experienced.
In total quench cooling, water is sprayed into the quench chamber to cool the hot syngas. There is no heat recovery in this design and the raw gas exiting the quench cooler is saturated with moisture. This cooling design is widely used in gasification plants to produce chemicals. It is reliable and has a low capital cost but also results in low overall process efficiency.
Convective cooling only. Fluidised bed gasifiers operate at relatively low temperatures (900–1100°C) which are high enough to decompose most of the tar, oils, phenols, and other liquid by-products but lower than the ash fusion temperature so clinker formation is avoided. The hot syngas is usually cooled in a convective cooler. Dust deposition and/or caking, erosion and corrosion in the convective cooler can be minimised by optimised design and operating conditions. Several chemical plants in China operate U-Gas gasifiers (a type of bubbling fluidised bed gasifier) with convective coolers for high temperature syngas cooling. This gasification process has operated successfully on a wide range of fuels including low grade coal and waste coal.
The selection and actual design of a syngas cooling system is influenced by factors such as the type of gasifier employed, the characteristics of the coal feed, the overall gasification process application and costs. Over the past three decades, various high temperature syngas cooling system designs have been developed to meet varying technological challenges and process requirements. Operating experiences from commercial gasification plants provide valuable knowledge for improving and optimising the designs and operating conditions of future more efficient, more reliable syngas cooling and heat recovery systems.
The report High temperature syngas coolers by Dr Qian Zhu CCC/257, 60 pp, September 2015 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.