Furnaces
A furnace is a heat-generating appliance primarily designed to release heat safely and efficiently, often through the combustion of fossil fuels. These devices are versatile and can be used for various applications, including heating spaces, cooking, and generating steam for electricity. Different types of furnaces include water-wall furnaces, fire-tube furnaces, open-grate furnaces, kilns, blast furnaces, and solar furnaces, each with distinct operational mechanisms and purposes. Historically, furnaces have been crucial in the development of metallurgy and industrial processes, dating back at least 9,000 years to early kilns used for melting metals.
Water-wall furnaces utilize high temperatures to mold steel, while fire-tube furnaces typically create steam for heating and HVAC systems. Open-grate furnaces can combust various fuels, including renewable resources, making them adaptable for waste incineration. Solar furnaces harness sunlight to achieve extremely high temperatures, offering a pollution-free alternative for industrial applications. Overall, furnaces play a vital role in modern civilization, facilitating advancements in energy generation and material processing while also adapting to increasing environmental standards and renewable energy sources.
Furnaces
Summary: A furnace is an appliance designed to release heat in a controlled and safe manner, often (if not always) by means of the combustion of fossil fuels. In furnaces, the heat generated is transferred to a secondary medium, usually water.
Furnaces are heat-generating appliances used for heating, cooking, and boiling water to make steam for electric generation. Furnaces exist in several forms, but some of the main types are water-wall furnaces, fire-tube furnaces, open-grate furnaces, kilns, blast furnaces, and solar furnaces. The first furnaces were kilns; later, blast furnaces were invented to work and melt metal into useful shapes for plows and axes. At least 9,000 years ago, ancient humans were burning fuels, such as wood and peat, in clay- and rock-lined kilns to melt copper and make copper tools. When ancient humans recognized that manipulating the ratio of air to fuel (by forced introduction of air) could increase the temperatures of a kiln, a blast furnace was created. A higher air-to-fuel ratio could melt iron ore. This discovery led to the Iron Age and later to the Industrial Age. Without large furnaces for industry and power generation, modern civilization would not exist.
Water-Wall Furnaces
In a water-wall furnace, fossil fuels are burned to release the heat energy these fuels contain. The heat is released both as sensible heat and as thermal radiation. For example, if the fossil fuel burned is pulverized bituminous coal, the fire produced can have temperatures in the range of 3,000 to 3,500 degrees Fahrenheit. These temperatures can melt most steels. The water-wall furnace design was made to allow common steels to be molded at these high temperatures. The furnace walls that are exposed to the heat from the combustion are made of panels of tubing that can hold liquid water. The panels are connected so that the water inside the tubes can flow in response to both the heat from the combustion and the use of external pumps. The water that flows through these circuits of panels picks up heat from the combustion, mainly through convection. The heated water is able to remove enough energy to both cool the tubes below the metal’s melting point and heat the water so it can boil and be transformed to steam.
Most water-tube boilers are large boilers used by the electricity-generating industry and make high-pressure steam. Steam pressures greater than 3,000 pounds per square inch gauge and 100 degrees Fahrenheit can be generated in the tubes of large water-tube furnaces. The flue gas generated from the combustion flows through the furnace and is usually corrosive. The thickness and composition of the metal tubes in a water-tube furnace must be designed so they can cope with the corrosive flue gas. A corrosion allowance (extra metal) is usually added to surfaces that contact flue gas, or special alloys are used that resist corrosion. Recent combustion technology leads to staging fires (making combustion in two zones by having a rich lower zone and a leaner upper zone) in this type of furnace to reduce nitrous oxide emissions. Two-stage firing can reduce the temperature of coal fires to less than 2,800 degrees Fahrenheit, and at this temperature fewer nitrous oxide emissions are formed.
The US Department of Energy finalized energy efficiency standards for residential gas furnaces to cut utility costs and carbon and methane emissions in 2023. The standards would go into effect in 2028.
Fire-Tube Furnaces
In a fire-tube furnace, the combustion usually takes place on one end of the furnace in what is often called the firebox. In the firebox, a fossil fuel, usually oil or natural gas, is mixed with air and combusts. The firebox is usually lined with ceramic known as firebrick, and this ceramic protects the metal parts of the firebox from the damaging heat. The hot flue gas flows out of the firebox into tubes that are in bundles connected to the firebox. Inside the tubes is flue gas, and the outside of the tubes is usually in a tank or pressure vessel filled with water. The hot flue gas gives up its heat convectively, while moving inside the tubes to the water in the tank or pressure vessel. The water outside the tubes is usually heated until it makes low-pressure steam in this type of furnace. These furnaces are relatively small and are usually under 20 megawatts thermal, but large fire-tube boilers do exist. The main use for fire-tube boilers is to make steam for heating, ventilation, and air-conditioning (HVAC) purposes.
Open-Grate Furnaces
Large fireboxes are sometimes built to support a grate system for burning the fossil fuels that are placed in the furnace. The simplest example is a modern fireplace, but much larger systems can be built. These furnaces are often used to burn trash and refuse that other furnaces cannot combust. There are several subtypes, including shaker grates, which have mechanized moving grating, and fluidized bed furnaces, in which fuel heats to the point that it becomes fluid instead of solid. Grate furnaces can also burn coal that is too hard to pulverize, such as anthracite, and these furnaces can also burn coke coal. The advantage of this type of furnace is that almost any item that combusts exothermically can be used as fuel. The other advantage is that the fuel does not need to be liquefied or pulverized to combust. The fuel can sit on the grating for long periods of time in contact with combustion air, so the rate of combustion is not as critical as it is in water-wall furnaces, where the residence time of the hot gas in the furnace is relatively short. Grate furnaces can also readily burn renewable resources, such as grasses, tree leaves, and tree limbs, which were not typically used as fuel before the year 2000.
Solar Furnaces
Solar furnaces use a collection of mirrors to combine and collect the energy in solar radiation. Depending on the energy in the sunlight, these furnaces can generate very hot temperatures, up to 5,500 degrees Fahrenheit. If the mirror system for the solar furnace is not fixed but able to track the path of the sunlight, these furnaces can operate for several hours during daylight. Solar furnaces can be used for industrial purposes, such as metal casting, industrial research, and limited electric generation (daylight hours only). They are typically smaller size than fossil fuel furnaces, solar furnaces before the year 2000 were limited mainly to research. Because solar furnaces are nearly pollution-free, interest in them has been increasing.
Kiln and Blast Furnaces
A kiln is a container that can hold combustion, typically for cooking or hardening clay to pottery. If forced air is blown over a kiln, it becomes a blast furnace, which is typically used for metalworking and metal casting. These are the oldest of the furnace types and do not require steel or iron construction. Both can be used for electric generation, but usually the efficiency of combustion is lower than all other types of furnaces. Because of their lower thermal efficiency and relatively small size, these furnaces are not typically used to generate electricity.
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