Thermal Desorption of Contaminants from Coal using Spouted Bed Reactor

Thermal Desorption of Contaminants from Coal using Spouted Bed Reactor

Problem

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.

Figure 1. (a) Schematic and (b) experimental system of the 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.

Figure 2. Pilot-scale Spouted Bed Reactor (8’ x 10’ x 16’), manufactured by ACT

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.

Battery Cooling Under Constraints

HiK™ Battery Frame

Battery Thermal Management is a well-documented design challenge due to high profile failures ranging from cell phones to aircraft. Proper thermal management not only assures avoidance of catastrophic failure, but can also extend the operating life of the batteries and surrounding components. In many harsh environment applications it is difficult to avoid high ambient temperatures, making thermal design even more crucial.  Advanced Cooling Technologies Inc. (ACT), is well-versed with battery thermal management and has created solutions that include passive (natural convection) cooling, active pumped liquid cooling, pumped two-phase cooling and forced convection designs.

One challenging design included an array of battery cells cooled via air. The customer had significant geometric constraints which led to a thin frame doubling as the battery containment device and heat spreader. The only location for a fin stack was at one edge of the heat spreader making thermal conductivity of the frame particularly important. After multiple design discussions, ACT and the customer determined the optimal solution to achieve a high thermal conductivity, light weight and cost effective design was to embed heat pipes into the frame.

The heat sink was along one of the long edges of the frame. Running heat pipes from top to bottom allowed for the most effective thermal design. The heat pipe design was such that the evaporator length would be nearly the entire distance, taking heat from a portion of each of the adjacent battery cells. The condenser end would have a high heat flux zone as only the tip would make contact with the heat sink. For these operating conditions, ACT ran performance curves for copper-water heat pipes and determined that two (2) heat pipes having a 2.1mm OD could be placed in each web of the frame. These heat pipes provide ample heat transport while fitting within the geometry of the battery frame. This approach also allowed for quick installation as there was no complex bending or flattening required

The final design consisted of eighteen (18) heat pipes and was capable of transferring over 200 W at nominal operating temperature. The overall thermal conductivity of the frame dramatically increased and was able to maintain a safe operating temperature of each battery. This solution was also shock and vibration tolerant.

Advanced Cooling Technologies has the expertise to provide custom solutions for your battery thermal challenges.  Contact one of our thermal engineers today. 

LED Case Study – The Remote Sink

In many lighting applications the LED device must fit in a fixed space to accommodate a variety of customer requirements, which often do not account for thermal management considerations.  A common example is luminaire design, where the ceiling or wall fixtures are based on pre-existing designs using non-LED technologies.  These designs commonly have both restricted space for heat dissipation through conduction, and limited air flow to remove heat via convection.   In cases where there is space to remotely dissipate the heat, heat pipes can be used to transport the heat from the device to a heat sink located elsewhere.  This is called the remote sink.

Figure 1. Heat pipes transfer heat from the LED to a remote sink, with very small temperature drops.

The remote sink solution has a heat pipe in direct contact with the LED device at one end, which serves as the evaporator. At the other end the heat pipe is connected to the heat sink, the condenser.  A sketch of a conceptual design can be seen in Figure 1.  Here two heat pipes are in direct contact with both the LED at the bottom and heat dissipating fins at the top. A wall or other enclosure can be placed in between the LED and heat sink to separate the two.

Aluminum has a thermal conductivity of about 180 W/m K, while the thermal conductivity of copper is only 400 W/m K.  In contrast, the effective conductivity of a heat pipe can range from 10,000 to 100,000 W/m K.   This high effective thermal conductivity allows the heat sinks to be located remotely from the LED.