User:WingerWillard/sandbox/Process heat

Article name is... Process Heat

Process heat is the transfer of thermal energy used in the manufacturing of industrial and consumer products from raw materials. Industrial heating processes include, but are not limited to: fluid heating, smelting, drying, calcining, melting, curing and forming, and heat treating and reheating. Furnaces, ovens, dryers, heaters, and kilns are all examples of process heating systems. Process heating systems can consume either fuel, steam, electricity, or a combination of these.

Industrial processes have varying temperature requirements, with the majority being under 400°C (752° F). In high-temperature processes, heat is often transferred through conduction or convection, while low-temperature processes utilize radiative heat transfer. Renewable technologies utilizing solar, geothermal, and biomass energy are becoming more common in pre-heating and heating processes under 400°C Nevertheless, fossil fuels continue to meet most of the demands of the process heating sector, especially those over 400°C.

In the US, process heat consumes about 36% of the industrial sector's energy, while the industrial sector as a whole consumes roughly one-third of the country's energy. 37% of worldwide energy consumption and 24% of ${\ce {CO2}}$ emissions come from the industrial sector.

Renewable Energy Sources

Solar

Solar technologies are used to generate heat, and are combined with steam, electric, or hybrid process heating systems in the industrial sector. Non-tracking collectors are flat while concentrated solar collectors concentrate large amounts of sunlight onto small areas.

Flat-plate solar collectors are non-tracking collectors which absorb heat energy through radiation. They are usually made of copper tubes, an absorber plate, glazed cover glass, and thermal insulation. Sunlight travels through the glass and strikes the black absorber plate, and thermal energy is transferred to a fluid that circulates throughout the collector. The transparent layer and absorbent plate trap heat and minimize convective heat loss. The fluid, usually a freeze-resistant water-glycol mixture, transfers heat outside the system. Durable and with an inexpensive upfront cost, they are most efficient at temperatures from 10 to 65°C (50-150°F).

Unglazed collectors are similar to flat-plate solar collectors but lack a glazing which traps heat. Thus, they are much less efficient and are more limited in their temperature range, but are relatively inexpensive.

Evacuated tube solar collectors are more efficient in cloudy climates, consisting of lines of thin copper tubes which maximize surface area exposed to the sun. The vacuum within the glass enclosure surrounding the copper tubes reduce convective heat loss. More expensive and efficient than flat-plate solar collectors, they can be used within a wide range of temperatures up to about 200°C.

Parabolic trough collectors are concentrated solar collectors which consist of parabolic mirrors that concentrate sunlight towards an absorber tube. A heat transfer fluid circulates through the tube and transfers the heat away. They are capable of producing temperatures up to 400°C, but the systems are expensive and complex. PTCs can be used for electricity generation and producing extremely hot fluids for a variety of chemical processes.

Photovoltaic heaters are used to convert sunlight to DC electricity, which is converted to thermal energy.

Thermal energy storage (TES) systems can store solar energy as heat in other mediums during the day, to provide a constant supply of heat when sunlight is variable. Systems include:

• Sensible Heat Storage: The heat is stored as energy in a medium, which fluctuates in temperature to release or store energy. Common mediums include hot water, steam, and thermal reservoirs filled with pebbles or gravel.
• Latent Heat Storage: The heat is stored in a phase change material (PCM), which undergoes a phase transformation at a constant temperature to store or release energy. Strong PCMs include salts and metallic alloys, because they exhibit a large change in enthalpy and a low change in density. PCMs are not currently used by industry, but are under research.
• Thermochemical Storage: The heat is stored/released by breaking chemical bonds.

Geothermal

Since temperature is relatively constant a few feet beneath the Earth's surface, geothermal heat pumps can provide a year-round supply of heat for industrial processes. Currently, they are only economically competitive in areas with naturally high geothermal activity, such as Iceland. Worldwide, 1.6% of geothermal energy is used in industrial applications for a total of about 16,390 TJ per year, an increase of 56.8% since 2015.

Direct use geothermal systems utilize groundwater heated via natural processes. Hot groundwater is pumped from the source and used either directly for a process, or with a heat exchanger. Cold water circulates back underground, and the water is reheated.

Deep geothermal systems use wells over a mile beneath the Earth's surface. Hot water or steam is pumped from deep underground and is delivered to the industrial heat process. Once the heat is transferred, cool water is pumped back dispersed underground.

Biomass

As the oldest energy source used by humans, biomass accounts for 10-12% of the world's energy and is a common fuel source in process heating operations. Biomass encompasses all organic material that can be burned to produce heat or electricity. Woody biomass (wood and forestry byproducts) are often used for cooking or space heating, whereas liquid or gas biofuels are used in other sectors. If high efficiency combustion processes are combined with renewable harvesting, biomass fuels are carbon neutral. In developing countries, biomass accounts for about one-third of all energy consumed, and is mostly used for cooking and space heating. The US has a 8000 MWe capacity for heat and electricity, while in Europe, biomass is mostly used for heating.

The combustion of wood follows the net equation:

${\ce {CnHmOp + (n + m/4 - p/2)O2 -> nCO2 + (m/2)H2O + heat}}$

The four steps of combustion are:

1. Heating/drying: Physically bound water must evaporate leave the organic material before a reaction can take place. This takes place up to 200°C, and the water can either leave with the flue gas or be reduced to to H2 gas.
2. Pyrolytic decomposition: Between 200 and 600°C, a set of decomposition reactions takes place which breaks down the hemicellulose, lignine, and cellulose in the biomass. Volatile compounds, such as methane, carbon dioxide, and hydrogen gas are formed, along with fixed carbon.
3. Gasification: Takes place between 700 and 1500°C. During this stage, air or pure oxygen oxidizes pure carbon and converts it to gas. One example of this: ${\ce {C + O2 -> CO2}}$
4. Oxidation: All gaseous products are oxidized to ${\ce {CO2}}$ and ${\ce {H2O}}$. This process is exothermic, producing heat.

The heat produced by formation of carbon dioxide and water during the oxidation stage of combustion is used for cooking or space heating, or transferred elsewhere via a thermal energy system. Biomass itself can also act as a TES system and store solar energy. The energy flux density is also high compared to other renewable sources. Modern biomass heating plants can supply tens of megawatts of power while maintaining a 90% conversion efficiency. However, only conversion rates of 25-30% are possible when using biomass for electricity production.

Industrial biomass combustion systems have thermal capacities up to around 3500 kW. They are usually equipped with an automatic ventilators which can adjust combustion gases to minimize particulate pollution. A heat exchanger is located near the top of the unit, which transfers heat from the flue gas to water.

Process Heating Systems

Process heating systems are used heat transfer operations, such as fluid heating and smelting. These operations allow specific unit processes to take place.

Types of heating systems

Type of fuel consumed	Description	Examples of equipment used	Percent (%) of energy consumed in US manufacturing sector
Fuel	Solid, liquid, or gas fuel is combusted and heats the material directly through convection, or indirectly, through radiant heating.	Furnaces, kilns, high-temperature generators.	66%
Steam	Steam-based systems store heat as latent heat, and transfer it directly or indirectly (through heat exchangers). They are used for heating processes under 204°C (400°F).	Boilers, fluid heating systems	29%
Electricity	Electric current is applied to heat the material directly through resistance heating, or indirectly, through induction heating.	Electric arc furnaces, infrared radiation, induction heating systems.	5%

Examples of unit processes include distillation, boiling, pasteurization, and sterilization.

External links

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