Coal naturally contains mercury and mercury compounds, which are extremely toxic in certain chemical forms. When coal is burned, mercury is released into the environment, making coal-burning power plants one of the largest anthropogenic sources of mercury to the air. In order to remove mercury compounds from coal, thermal desorption systems are utilized to increase the volatility of contaminants and remove them from solid materials. Spouted bed reactors are a possible solution in reducing moisture levels and increasing particles mixing during the thermal desorption process. Typically, spouted bed reactors have been used in various physical operations such as drying, due to high superficial inlet velocity leading to vigorous movement of particles in the spout region.
The spouted bed reactor utilizes gas injected vertically upwards through a single central orifice into a bed of solid particles. In the central “spout” region, the particles are entrained by a central jet which generates a spouting flow pattern. When the particles reach the “fountain” region above the bed, they return to the sides and land on the bed surface. In the annulus region on the sides of the bed, fluid percolates outwards and upwards, counter-current to the movement of the particles. The vigorous particle movement and the high gas velocity in the spout region avoid slugging, segregation, and the formation of agglomerates in the bed. Thus, spouted beds perform well in the treatment of large, sticky, or highly irregular particles with a wide range of size distributions. Figure 1 shows a schematic of the standard conical spouted bed reactor.
Solution – Thermal Desorption
ACT investigated the benefits of utilizing a spouted bed reactor for the thermal desorption of mercury, sulfur, arsenic, and lead in coal.
Thermal desorption removal effect on different coal types when using air at 300 ℃
- Bituminous: thermal desorption can remove 65% of Mercury, 28% of Sulfur, 43% of Arsenic, and 45% of Lead content.
- Lignite: thermal desorption can remove 68% of Mercury, 39.3% of Arsenic, and 54.6% of Lead content.
- Anthracite: thermal desorption can only remove 25% of the Mercury content from the coal, and is not as effective as some of the other materials due to high moisture content and irregular shape.
ACT performed reactive computational fluid dynamics (CFD) analysis to extend the design of our custom spouted bed reactor from lab-scale to pilot-scale, and identified the optimal operating conditions for thermal desorption of coal. Consequently, a pilot-scale spouted bed reactor was manufactured by ACT and assembled at Lehigh University’s Energy Research Center. As shown in Figure 2, The system has an overall size of 8ft × 10ft × 16ft (W × L × H), is made of stainless steel and carbon steel and is capable of thermal desorption at 500 lb/hr loading rate. The current design employs a batch process, where the thermal desorption of each batch takes one hour. However, automated valve and sensor options are available for future implementation of the technology to enable the automated process.
Implementing into Industry
ACT’s spouted bed system can be implemented into a standard power plant, based on NETL’s Cost and Performance Baseline for Fossil Energy Plants, Case B12A (NETL, 2015), with a nominal net output of 550 MWe. A plant-scale spouted bed system will be equivalent to 150-pilot-scale reactor systems. They can be integrated into the flue gas stream between the economizer and the air preheater. The cost of running the plant scale spouted bed reactor system becomes the difference in power output, which is 10.46 MWe. The 150-spouted bed system will also be capable of thermal desorption of 75,000 lb/hr (500 lb/hr in each reactor), which is 18% of the current power plant’s coal mass flow rate of 395,053 lb/hr. As a result, coal power plants integrated with spouted bed reactors can utilize coal with higher Mercury content. This higher Hg-content coal can be up to 75,000 lb/hr, based on the processing capacity of ACT’s thermal desorption system. Since the contaminated coal feedstock is usually less expensive, the coal power plant can adjust to market dynamics with the potential to improve the economy.