Recovering heat, losing emissions

Transport membrane condenser ups efficiency by 95 percent

By Chris Giron

August 12, 2011

Boiler-generated steam remains an efficient method of heat transfer for most manufacturing applications; however, a tremendous amount of energy and water can be lost. An advanced heat recovery system using transport membrane condenser technology enhances boiler performance, reduces water usage, decreases energy consumption, and lowers greenhouse gas emissions.

Heat recovery systems work to recover both sensible and latent heat, as well as water from the exhaust stream. A transport membrane condenser can increase fuel-to-steam energy efficiency by as much as 15 percent compared to earlier heat recovery systems.

Boiler-generated steam remains an effective method of heat transfer for most process and discrete manufacturing applications; however, a tremendous amount of energy and water can be lost. In natural gas combustion products, approximately 18 percent is water vapor, and if it is left to escape with the exhaust, it carries about 10 percent of the input energy with it. So a system can never exceed 90 percent efficiency if this moisture escapes.

This same condition exists in your home furnace or hot water tank, except that they send the condensed water to the drain instead of reusing it.

Fuel costs and environmental regulatory mandates have created a strong need for cost-effective solutions to enhance boiler performance to reduce use of fuel and other energy forms, decrease water consumption, and lower greenhouse gas (GHG) emissions.

Although the basic boiler itself has not changed much, a number of technological advances over the last 20 years provide a way for manufacturers using steam boilers to reduce variable fuel consumption through higher energy efficiencies and to operate in a more sustainable way.

One of these technological advances is heat recovery. Heat recovery systems work to recover both sensible and latent heat, as well as water from the exhaust stream (see Figure 1). This enhances boiler efficiency and preheats boiler feed water, which can be reallocated to other facility processes.

A boiler heat recovery technology that addresses these issues is a condensing economizer. Water passes through a condensing economizer on its way to the boiler. Sensible heat is recovered from the maximum boiler exhaust temperature, starting in the range of 400 degrees F, down to the condensing point of the flue gas, approximately 135 F (see Figure 2). At the condensing point, water droplets form in the condensing economizer and are recovered. The latent heat is recovered in the phase change of the water (as the vapor changes back into liquid).

By preheating the boiler feedwater with energy harnessed from the exhaust stack gases, economizers can potentially reclaim a lot of energy, thereby reducing the otherwise large emission footprint.

The Gas Technology Institute (GTI), a not-for-profit energy research organization, has developed and patented an advanced heat recovery system that uses transport membrane condenser technology (see Figure 3). This heat recovery technology can increase fuel-to-steam energy efficiency by as much as 15 percent (from 80 percent base boiler efficiency) compared to previous heat recovery systems (up to 95 percent fuel-to-steam efficiency). It also increases water capture by up to 20 percent, returning clean water to the system. Manufacturers can use this water in the boiler without the need for further treatment.

How Transport Membrane Condenser Works

Transport membrane condenser technology comprises nanoporous ceramic membrane tubes specifically designed to condense water vapor. The tubes capture, recover, and reuse sensible and latent waste heat, along with the water vapor from exhaust or flue gas. The clean water can be returned to the system. The nanoporous ceramic membrane tubes capture water via capillary condensation, which occurs more readily with nanoporous ceramic than with typical steel finned tube economizers. A partial vacuum on the tube ID helps to transport water through the tube wall.

A transport membrane condenser heat recovery system can be retrofitted on any clean fuel boiler to raise the efficiency by up to 95 percent. This technology thereby returns heat and clean water to the boiler system.

GTI engineers have calculated that the nationwide application of this technology could potentially save 600 trillion BTUs and eliminate 62,000 tons of NOx annually. At a natural gas price of $8 per million Btu, this would save U.S. industry $5.6 billion per year in energy costs. As significant, it could lower the amount of GHG emitted annually by 23 billion tons.

Gas Technology Institute (GTI) is a not-for-profit research and development organization, 1700 S. Mount Prospect Road, Des Plaines, IL 60018, 847-768-0500.