Peter Mullinger

Chapter 13 – Economic Evaluation

Chapter 13 demonstrates how to resolve the financial performance of a furnace with the engineering and operating information that forms the basis of this book. The underlying principles and methodology of financial accounting practices are described, explaining the distinctions between capital, revenue, cost and profit. Financial analysis through the use of ratios is reviewed. The processes of project costing are described including the use of scaling factors and Lang factors. The various commonly used investment analysis tools are presented, including pay-back, return on investment, net present value discounting and internal rate of return, A worked example of a steel reheating furnace is used to explore a number of engineering options to improve its performance, and to show how the financial case for each is built up and compared.

Chapter 4 – An introduction to heat transfer in furnaces

Publisher Summary This chapter explores the different processes through which heat is transferred in a furnace, namely, conduction, convection, radiation, and electrical heating. The primary objective of a furnace is to transfer thermal energy to the product. In a solid, the flow of heat by conduction is the result of the transfer of vibrational energy from one molecule to the next, and in fluids it occurs in addition as a result of the transfer of kinetic energy. Under transient conduction conditions, the temperature gradient through the system varies with time. Heat transfer by convection is attributable to macroscopic motion of a fluid and is thus confined to liquids and gases. Convective heat transfer is in reality the conduction of heat through a flowing fluid to a fixed surface, whereby the conductivity is defined by a convective heat transfer coefficient. All materials radiate thermal energy in the form of electromagnetic waves. When radiation falls on a surface it may be reflected, transmitted, or absorbed. The fraction of energy that is absorbed is manifested as heat. The physical laws governing thermal radiation are well established but are insufficient to describe technical processes quantitatively. Heat can be transferred by other parts of the electromagnetic spectrum, particularly in the long wave region, in and beyond the infrared. The primary source of energy for heating in this region is electricity. If there is a temperature difference between two parts of a system, then heat will be transferred by one or more of three methods.

Future Trends and Concluding Remarks

Chapter 15 considers the future trends in furnace control, materials of construction, furnace design, furnace applications and emission limits. It is considered unlikely that there will be any revolutionary change in furnace designs owing their long evolution to the current situation. Limitation of CO 2 emissions is seen as one of the principle influences in the immediate future together, with new applications requiring specialised furnaces. The technologies being developed for carbon capture and sequestration (CCS) and their potential impact on future furnace operations are discussed.

Selected Examples of Real Furnace Applications

Chapter 14 describes application of the methodologies described in the preceding chapters to several real furnace situations. The examples have been chosen to show a variety of applications and range from the adaption of an existing furnace to new duties, such as change to a difficult waste fuel, to completely new designs, such as a new reforming furnace for fuel cell applications.

Furnace Design Methods

Chapter 12 describes practical furnace design methods, including both the process design and the subsequent mechanical design but concentrates on the former. The design constraints imposed by economics, legislation physical laws, codes and standards etc. are discussed and the importance of choosing an appropriate design early in the process is explained together with an example of how the cost of design changes escalate, the later they occur.

Chapter 9 – Furnace Efficiency

Chapter 9 defines how the term efficiency is variously used in relation to thermal performance, combustion, and productivity. Simple analytical equations are given to calculate the relative contributions of thermal sinks, viz. product, flue gases, structure losses etc., from performance data. The methodology required to conduct mass and energy balances on a system is explained. The measurements required and the instrumentation used are described. The construction of mass and energy balances is demonstrated through a worked example. The efficiency of conversion of energy sources is discussed using an energy triangle. The concepts of high and low grade heat are introduced, Equipment design for the recovery of heat from furnace process streams, e.g. product or flue gases, is explained, including recuperative and regenerative heat exchanges. The final part of the chapter shows how information from mass and energy balances form the basis for efficiency improvement calculations.

Chapter 6 – Combustion and heat transfer modelling

Publisher Summary This chapter provides insight into the various models that are used to partially simulate a full-scale system. Modeling is termed as partial because it is not possible to satisfy all the scaling criteria or algorithms required for a complete model. Selecting the significant variables correctly is often termed the “art of modeling,” and requires the modeler to be skilled in both an understanding of the design objectives, as well as the mechanics of the modeling process. It is advisable to use more than one modeling technique to reduce the risk of inadvertently overlooking a significant variable. Two different categories of models are available for the designers to choose from; physical modeling and mathematical modeling. The mechanics of physical modeling involves the construction of a geometrically scaled model, maintaining faithfully the internal dimensional scale in a clear plastic simulation. In mathematical modeling furnace designs can be very simply divided into two basic shapes, rectilinear boxes or right cylinders. In each case their major axis can be horizontally or vertically aligned. This generalization is particularly true for the radiant sections of at least 80% of all industrial furnaces. Different scaling parameters are also discussed in the chapter, which are important to the application of any model. The complete process model of a furnace is not yet a universal reality, but the application of good models will always yield better solutions than guesswork or extrapolation.

Chapter 12 – Furnace design methods

Publisher Summary This chapter provides insight into the designing methods of furnaces in a bid to assist future furnace designers to focus on and achieve the objective, i.e. fuel efficient, low cost design. A new furnace is a major investment, with a typical design life of 20 years and an operating life of probably twice that, so decisions made during the design stage will have implications for the furnace owner(s) for decades. Cost of fuel, amount of capital, and even the region where the furnace will be used are factors to be kept in mind during the designing stage. This chapter limits the discussion to the process design of furnaces and how the furnace is integrated with the rest of the process. Wide ranges of chemical and physical processes are undertaken in furnaces and it is impossible to cover them all. In any case it is important to undertake a thorough analysis of the physical and chemical processes to understand the ideal rates of heating and cooling to achieve the ideal or optimum product. To achieve a cost-effective design process, it is critically important that the conceptual design is undertaken thoroughly. Time, intellectual effort, and especially critical thinking spent on this part of the design process are repaid many times over by cost savings later. In reality, design usually involves a number of iterations where the preliminary design is changed as a consequence of feedback from the results of the preliminary efforts, and/or the designers become aware of other possibilities.

Chapter 9 – Furnace efficiency

Publisher Summary This chapter presents an analysis of the efficiencies and inefficiencies of a system, which can be used to identify where the greatest opportunity for gains can be derived. It can also be used to benchmark the performance of one system against another. The thermal efficiency of any furnace system is defined as the useful energy derived from the system relative to the energy input. However, it is not necessary to measure the energy that is lost through the walls of the furnace, etc. to calculate the efficiency. It is only necessary to know the useful energy out, which is usually relatively easy to determine, as it will be directly related to the production rate. The efficiency of a furnace system depends on each step by which the energy is transferred from the chemical fuel energy and the electrical energy to the process energy contained in the end product. The total efficiency is dependent on the efficiency of each operation in the process, such as pump, preheater, furnace, and cooler. Examination of furnace performance data can help to usefully identify the relative contributions of thermal quantities. If the production rate is plotted against thermal input then, allowing for a reasonable degree of scatter owing to other influences, a straight line relationship is usually observed.

Chapter 10 – Emissions and environmental impact

Publisher Summary This chapter focuses on the emissions and the environmental impact of a furnace. Most fuels consist of carbon and hydrogen with small quantities of sulfur, chlorine, phosphorous and nitrogen, etc., together with traces of metals. These are released into the atmosphere during the process of combustion. A further source of emissions comes from the mineral matter contained primarily in solid fuels but also in heavy fuel oil fractions and some waste liquids. These minerals are usually converted to metallic oxides and emitted with the flue gases as fly ash, or deposited in the furnace as residual ash. Carbon, nitrogen, and sulfur each have more than one oxide but the two oxides of carbon have a special importance, since the concentration of CO present in the final flue gas is a very good indicator of combustion performance, while CO2 has been identified as a primary cause of global warming. Particulate emissions arise from ash in the fuel (coal and oil), and in the case of direct contact processes also from the feedstock/product. Typical of these contact processes are cement and lime manufacture, alumina calcination, ferrous and nonferrous metals production, and glass manufacture. The operation of almost any furnace will be subject to either regulatory or voluntary environmental control by local, national, and sometimes international bodies. Whereas traditionally monitoring of furnace operations was reported regularly to a designated agency with local knowledge to meet local standards, newer controls are based on integrated pollution and prevention legislation covering whole industry sectors, both nationally and internationally.