Institute for Energy Process Engineering and Fuel Technology > Research > Reduction of Energy Consumption in High Temperature Processes Introduction

Reduction of Energy Consumption in High Temperature Processes Introduction

Introduction

Despite the expansion of renewable energy usage, to this day about 80% of total primary energy of the Federal Republic of Germany will be met by fossil fuels (Figure 2.7.1). The reduction of primary energy from fossil fuels from 4-5% within the last 10 years has had a correspondingly small effect on CO2 emissions (Figure 2.7.2). Total energy consumption can be divided into four areas:

  • Conversion into "electric energy" (power: fossil and biogenic fuels, wind, solar, fuelcells, etc.)
  • Conversion into "heat" (heat engine, combined heat and power, combined heat, cooling and power)
  • Conversion into fuels for transport (fossil and biogenic fuels, wind to hydrogen, etc.)
  • Conversion and material handling in production processes

The first three areas relate to the supply of energy and deal exclusively with the conversion of one form of energy into another. They are currently the focus of public interest. Much less attention is paid to those areas where both the aspects of energy conversion and the treatment of substance are to be considered equally. This use of energy is just as important in the course of the energy chain as the supply. Potential reduction of energy use in production processes is still much more important than the use of currently available renewable energy sources.

 

 

 

Figure 2.7.1: Primary energy consumption since 1998

 

Figure 2.7.2: CO2-Emmisions since 1998

 

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Process Analysis and Modeling

One must first realize that energy is mainly converted into other forms through combustion processes (fossil and sometimes renewable fuels) and that this conversion occurs directly. For example, electricity is predominantly created by burning of fossil fuels in power plants.

A process analysis begins with a discussion about the main variables that affect it (Figure 2.7.5)

  • for ignition of gaseous, liquid and solid substances (commodities, fuels, residues, etc.)
  • to heat and mass transfer processes in the materials to be treated,
  • sectioning the processes into blocks, etc.

One must then consider which of the known material handling apparatus may be of use for a particular process, such as:

  • Combustion chambers,
  • Shaft furnances,
  • Metallurgical melting vessels,
  • Rotary kilns,
  • Fluidized-beds,
  • Shaft ovens.
  • etc.


These materials processing equipment are described collectively with the term "industrial furnaces". Industrial furnaces are plants which treat high temperature materials.

Finally, one must mathematically model the respective, complete process to identify the potential for process improvements and opportunity for optimization. Figure 2.7.5 gives an impression of some of the influences to be observed. 

 

Figure 2.7.5: Primary factors inuencing thermal treatment processes

An example of a simplified process analysis is shown in Figure 2.7.6. It depicts the essential building blocks, and their dependence on one another, that are necessary in the first step of recovering precious metals from liquid and solid (dust) residues created by oxidation (combustion).

Researchers are concerned, for example, with recovering Molybdenum compounds in liquid residues found in the chemical industry (catalyst residues, etc.) or recovering vanadium in the ash-like residues formed during petroleum coke combustion, etc. The main device is a "combustion chamber" separated into a stirred tank (SR) and plug flow (PF) - followed by heat transfer and emission control. The main factors affecting combustion such as temperature, under-or over stoichiometric reactants, residence time along the pathway, etc. can than be determined. The product (recycled) is obtained in the exhaust gas purification process. Figure 2.7.7 shows an example of such a process control scheme.

 

 

Figure 2.7.6: Process control schematic in a furnace with gaseous, liquid, and powder
feedstocks

 

Figure 2.7.7: Technical pilot-scale furnace

 

During the combustion stage, various forms of metal oxides are created due to the varying combustion conditions. With the increasing depletion of scarce resources, more people will embark on this path of resource recovery.

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Process improvement with "traditional methods" (heat recovery, etc.)

The energy input must be kept as low as possible. Traditional methods of heat recovery are considered first. Only a few are presented below:

  • Fuel preheating (e.g. when using low caloric fuels and residues from production facilities (Figure 2.7.8),
  • Oxygen enrichment, in order to reach the required process temperatures with less effort (example in Figure 2.7.9),
  • Charge preheating. This step is denoted, for example, as scrap preheating, etc. In the following text, the significance of this step is discussed in context with the complete process.

It is important to realise that the above listed methods were rediscovered in the current political discussion on energy efficiency improvements. For example, ten years ago in the field of classical thermal waste treatment hardly anyone concerned themselves with energy usage, although it was consistently pointed out in the energy process engineering discipline. In the mean time, the slogan "waste to energy" was coined, and old, well known processes presented themselves as new possibilities for improvement.

 

 

Figure 2.7.8: Maximum eciency of heat recovery when preheating air and fuel

 

 

Figure 2.7.9: Fuel savings by the use of oxygen enrichment

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Process improvement through energy substitution

The classic cupola furnace is given here as an example of an energy substitution that is carried out in several steps. The furnance is operated with coke and is often used for melting metal, lumpy substances. It can also be used for melting sludge briquettes, etc. (Figure 2.7.10).

 

Figure 2.7.10: Use of a test furnace

 

In this case blast furnace gas is not anymore produced. Without stored enthalpy leaving the reactor with this gas, the required inlet fuel is less. This leads to an increase in energy supplied. However, an increase in the energy feed to the furnace leads to a reduction in the temperature (Figure 2.7.12). This makes for a particularly difficult process optimization problem.

 

 

Figure 2.7.11: Schematic diagram of a natural gas operated cupola furnace (without coke)

 

 

Figure 2.7.12: Parameter combinations of thermal capacity and oxygen enrichment of
combustion air

 

From this figure, one can see that by increasing the oxygen concentration as a response to the increased fuel feed counters the reduction in temperature. The process simulation reflected this behavior spot on in operational practice. One can now try in a second step to substitute the natural gas with a lean gas (waste gas, blast furnace gas, etc.), as shown in Figure 2.7.13.

 

 

Figure 2.7.13: Schematic representation of a cupola furnace, divided into different zones

 

In addition to the furnace depicted in Figure 2.7.11, Figure 2.7.13 shows how the exhaust gas can be transferred and combusted with coke-oven gas in a secondary combustion process used to preheat the combustion air and fuel used in the primary combustion. In this case, the primary combustion is also fired with coke-oven gas. Figure 2.7.14 shows an operating diagram for a process with additional (circuits) used to recover heat from discharged exhaust gases.

 

 

Figure 2.7.14: (KLKO) operating diagram

Figure 2.7.14 shows us that by using the previously discussed methods, one could almost reach the operating area of natural gas when burning coke-oven gas. The value and accordingly the energy substitution ratio, or energy exchange ratio, depend not only on the calorific value, but also decisively on the plant arrangement. This is clarified in Figure 2.7.15. This figure shows an example of a process with a temperature of 1500°C, where it is more beneficial to preheat the air and fuel with a low-calorific gas than by heating with a higher-calorific value fuel without preheating.

 

 

Figure 2.7.15: Energy exchange ratio as a function of gas temperature for different plant
arrangements and operating procedures

If it is so, that a cupola furnace can be operated with lean gas, one can imagine using still a further, third step. A second cupola furnace can be fired using the coke-oven gas generated in the first oven so that the entire system approaches the, "zero waste principle".

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Process Improvement through Process Coupling

Steel scrap is nowadays preheated using electric arc furnaces. It is melted and heated up for the casting process and for metallurgical treatment. According to the size of the furnaces (e.g. 50 MW needed for classical case-related power plants) for which this performance can be obtained. If we could combine the parts power plant and furnace in a single aggregate, (i.e. by the latter is designed as a direct-fired oven) we could prevent ambient heat losses (condensation, cooling tower) during electrical energy production.

 

 

Figure 2.7.16: Direct red furnace for the melting of steel scrap (pilot scale)

 

To prevent loss on ignition, caused by oxidation of the exhaust gas produced from the direct combustion, the exhaust gas must exit the combustion chamber immediately after the molten mass is created. It must then be over heated in a separate vessel. This can be used with an electric arc furnace with a significant decrease in the energy expense compared to the conventional variant.The aforementioned merging of a power plant and an electric arc furnace with the direct-fired shaft furnace is in the experimental stage. In industry, it has been tried often to combine the conventional method (a furnace with electrodes) with other processes to reduce the consumption of energy. Thereby, an electric arc furnace vessel will be combined with a shaft furnace for scrap metal preheating (Figure 2.7.17). The exiting exhaust gas would then heat the scrap before it falls (flows) into the molten bath. The heat recovered in this way replaces the electrical energy need at a ratio of one to one. This results in a reduction of the fossil-fuel derived energy input in the accompanying power plant at a reciprocal value of the power plant's efficiency (38\% equals a 2.6 fold reduction).Unfortunately, as the scrap is being heated and the exhaust gas cooled during this preheating procedure, impurities in the scrap are removed by the exhaust gas. Therefore the exhaust gas must be burned in an externally fired post-combustion vessel. Thus, a portion of the energy saved in the power plant must be used here. Figure 2.7.18 shows the three building blocks of this system: the "furnace vessel", the "shaft", and the "post-combustion chamber".

 

 Figure 2.7.17: Single shaft furnace with a separate combustion chamber for postcombustion
of the exhaust gas.

However, it should be noted, that depending on technical boundary conditions, coupling the system with an external post-combustion stage may result in the overall process seeming no longer worthwhile. Studies are currently underway to develop more advanced methods of scrap processing (scrap cleaning) to avoid the mentioned post combustion stage. The aim is to leave as much energy as possible in the process itself. Overall it must be stressed, that careful observation is needed in regard to mass and energy balances of the selected subsystem, as well as entire system boundary when combining processes. This is because it is only possible to provide a stable value with "rising balances".Currently in the world of recycling, the definition of the system boundaries is frequently disclaimed, sometimes leading quickly to false conclusions (misleading efficiency considerations).

Figure 2.7.18: Classication of the shaft furnace in individual modules

Figure 2.7.19: Flow diagram of an electric arc furnace

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Process Improvement through "Process Intensification"

As an example of improvements through process intensification the heat recovery process of coke production ist concidered, see figure 2.7.20 and 2.7.21. The heat recovery coke process is charaterized by a complete utilization of the coke-oven gas produced during the process.

 

Figure 2.7.20: Schematic diagram of the heat recovery process

Figure 2.7.21: Construction of the heat recovery process

An empty furnance, see figure 2.7.21, is heated up to a temperature of around 1100°C. Than a mixture of coking-coals is plant as "compressed packets" into the hot oven. Due the heat radiation from the furnance walls, the coal is gradually heated up and the process of coking begins. The gases produced in the coking process are mixed with air (Figure 2.7.20) and are burned under fuel rich conditions in the space above the coal bed. The combustion products heat up the surrounding surfaces eliminating the need for an external power supply. This gas produced by the partial combustion is fed from the upper chamber to the lower chamber via downcomers and further combusted with a second air. The coke/coal bed rests on the furnace heat. This bed is heated from below and above, so the reaction fronts exist. The coke reaction is finished when the reaction fronts meet, hence the coal is completely reacted to coke. Hot, combustion products produced during the process can be used in waste heat boilers to generate electricity.This coke making process is known for long. However, to date there exists only experimental experience but no process analysis, and no optimization guidelines. It is just now again becoming relevant, because it bypasses the usual by-products (tars, etc.) and their associated processing and environmental concerns produced by traditional coking methods. This process creates "only" hot, burned exhaust gases which do not have to go through gas cleaning.

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Summary

From the few shown examples, the following is important to notice, that the points of view

  • Process analysis and modeling
  • Process optimization using classical methods,
  • Process optimization using energy substitution,
  • Process optimization using process coupling and intensification

are importend methodologies for improving the use of energy resources in industrial processes. These methodologies have been often forgotten in the current discussion on energy efficient societies.


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Contact:Dr.-Ing. M. Mancini
 

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