Recycling energy

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The manufacturing and electric-power industries, by and large, capture only a small portion of the potential energy in the fuel they burn, and then discard the rest as waste energy. Many cost-effective approaches are available to recycle these waste streams, generating incremental electricity and thermal energy without increasing pollution or burning additional fossil fuel. Recycled energy's unused potential may be society's best-kept secret.

Recycling waste energy can take two approaches. In the first case, power plants are sited at an industrial facility that produces a stream of waste energy, such as gas that is normally flared, hot exhaust or high-pressure gas or steam that must be decompressed back to atmospheric pressure. These plants, known in the literature as 'bottoming cycle cogeneration plants', convert the waste energy streams into electricity. The resulting electricity is typically sold back to the industrial host for use on site, thus avoiding the need for transmission and distribution wires and avoiding the losses associated with transmitting the same power over great distances. In the second case, explained in more detail below, power plants burn fuel to generate electricity and then recycle the inevitable waste heat to replace the supply of thermal energy from a separate boiler. These local power plant facilities convert 33-45 per cent of the fuel's potential energy to electricity, just like their larger centralized brothers. But instead of venting the remaining 55-67 per cent of the energy, these plants recycle the heat in order to supply the host with steam, hot water or process heat. These cogeneration units provide two energy services - heat and power - with one fire, saving money and pollution emissions.

Recycling industrial energy waste

Various industrial waste energy streams can be recycled into useful heat and power. These streams include hot exhaust, low-grade fuels and high-pressure steam and gas. High temperature exhaust can produce steam to drive turbine generators and produce electricity. Hot exhaust is available from coke ovens, glass furnaces, petrochemical processes and steel reheat furnaces. In other cases, presently flared flue gas from blast furnaces, refineries or chemical processes can be burned in boilers to produce steam. All pressurized gases, including steam, have the potential to generate electricity via backpressure (decompression) turbines. Industrial and commercial boiler plants produce high-pressure steam for distribution and then typically reduce pressure at points of use by means of valves. Nearly every college and university campus, as well as most industrial complexes, could produce some fuel-free electricity from such steam-pressure drops. Other opportunities abound. Consider the gas transmission companies that currently expend energy to compress natural gas, but then reduce that pressure with valves to feed local distribution systems. Recycling this discarded energy could supply 6500 megawatts, roughly equivalent to the output of nine coal-fired power plants.

Industrial energy recycling is well proven, with roughly 9900 megawatts in operation in the US. But this is only 10 per cent of the 95,000 megawatts of potential identified in a recent study for the US Environmental Protection Agency (Bailey and Worrell, 2004). Recycling waste energy could have produced 19 per cent of US electricity in 2003, displacing a quarter of the fossil fuel that was burned to generate electricity. In 2003, 22.9 per cent of US electricity was produced with renewable and nuclear energy, and recycling industrial energy would have raised the non-fossil total to 42 per cent.

At a smelter on the southern shore of Lake Michigan, more than 250 individual ovens bake metallurgical coal to produce blast furnace coke - expanded lumps of nearly pure carbon. The energy-recycling plant at this smelter converts the energy in the coke oven exhaust to produce 90,000 kilowatts and 500,000 pounds oflow-pressure steam for Mittal's Inland Ispat steel complex. In 2004, this single plant generated more clean power than we estimate was produced by all of the solar collectors throughout the world.1 The capital cost for the energy-recycling plant was US$165 million, compared with over US$5 billion invested in the world's solar photovoltaic arrays. Each dollar of investment in this energy-recycling plant produced 33 times more clean energy than a dollar invested in solar collectors and also produced 3.6 times more clean power than a dollar invested in wind generation and associated wires. These comparisons are not intended to disparage renewable energy, but to show the relative value of recycled energy.

Mittal Steel enjoys significant economic benefits from recycling wasted energy. Producing the same steam with natural gas and purchasing the same amount of electricity from the grid would have cost more than US$110 million per year at October 2005 prices. This project, therefore, illustrates how energy recycling satisfies both sides of the global warming debate, simultaneously reducing energy costs and CO2 emissions.

Combined heat and power plants

The second way to recycle energy is to use waste heat from electric generation to provide thermal energy for heating, cooling and industrial processes. In 2003, the US power industry consumed 33 quadrillion British Thermal Units (quads) of raw energy in thermal power plants (excluding renewable energy) to deliver 11 quads of electricity, an average 33 per cent efficiency. Combined heat and power (CHP) plants sited near thermal users can achieve up to 90 per cent overall efficiency and can easily recycle half of the waste energy, doubling the conventional powersystem efficiency. The US energy recycling potential is thus roughly 11 quads, or 13 per cent, of total US fossil energy consumption. This potential saving is in addition to the industrial energy-recycling potential noted above.

Roughly 92 per cent of the world's electricity is produced at remote generation plants, which discard, on average, two thirds of their input energy. To recycle the byproduct waste heat, a power plant must be located near thermal energy users because steam and hot water cannot be transmitted long distances without prohibitive losses. CHP plants, sited near thermal users, utilize all of the technologies and fuels used by central generation plants, but produce significantly more useful energy from each unit of fuel. The capacity of a single CHP plant ranges up to 700,000 kilowatts.

The World Survey of Decentralized Energy for 2005 by the World Alliance for Decentralized Energy (2005) found that 7.5 per cent of worldwide electric generation was provided by CHP plants, but noted a great disparity among countries. The US and Canada generated respectively 7.2 per cent and 9.9 per cent of their power with CHP plants, while some countries generated between 30 per cent and 52 per cent of their power with more efficient CHP plants. Three US states reported no CHP plants, while California and Hawaii produced over 20 per cent of their power using CHP. Differences between countries in CHP use shown in Figure 20.1, as well as differences among US states, reflect local power industry governance. The actual amount of heat recycled using CHP plants is not captured in these macro data, which only report total electric production, not heat recovered and utilized.

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