Boilers Today

A boiler can be described as a conversion device for producing steam from fuel. Its basic features are decided by the fuels and the size by the amount of steam demanded by the turbine or plant it feeds. Over the decades, fuels used in boilers as well as the sizes of plants have steadily increased, making modern boilers far smarter and more complex than they were before. Besides, there are more combustion technologies today than in the past, increasing the variety of the boilers. Take a look at the boiler scene that prevailed some 50 years ago:

• The mean size of the boilers was smaller.

• They were feeding on fewer fuels that were relatively better in quality.

• Demands of multifuel firing and extreme boiler efficiencies were not necessary.

• Dynamics were more gentle, as the instrumentation was not sophisticated.

Coal was the primary fuel except in certain countries that later became mainly OPEC nations. The quality of coal was also generally much superior, as opencast mining was not so widely practiced. The oil situation was getting better as cheap oil promised to get cheaper and a lot of oil-based power generation was on the horizon. Natural gas (NG) was yet to come into common use. Gas turbine (GT) technology was in its infancy, and GT-based power, which was more expensive, was generated in small quantities as peak power or convenience power. There were no heat recovery steam generators (HRSGs) to talk about. Fluidized bed combustion (FBC) was yet to appear and pollution issues had not gathered momentum.

The utility market was ruled by pulverized fuel (PF)-fired boilers at the upper end and stoker-fired boilers at the lower end, with coal as the primary fuel. The industrial base was not as large, and the basic industries were yet to attain the present-day sizes, and hence smaller PF and larger stokers were meeting the process requirements as well. In OPEC areas and richer countries, oil firing was also extensively employed. Oil — and gas-fired boilers were very popular, and extensive modularization had taken place.

Self-sufficiency of each nation was the practice, in preference to globalization. Trade among countries was limited and carried on within certain groups, with many trade bar­riers among the groups. This limitation encouraged a profusion of manufacturers catering to limited defined markets. Naturally, the firms were smaller in size and scale, and there was a bewildering variety of shapes, sizes, and designs. The age of computers had not arrived yet, and the optimization of products was low.

All of this situation changed gradually in the 1980s and 1990s. Fuel issues underwent a tectonic change with changes in the power generation practices and boiler designs. This was followed by the oil shock of mid-1970s. Some genuine softening in oil prices that began to be seen in 1990s withered away, dashing all hopes of reviving oil as a primary fuel. This high oil price has led to the resurgence of NG because of better availability and

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Simultaneous creation of the required infrastructure. And dramatic growth and consoli­dation in the GT business.

• Bigger and better GTs, achieving progressively higher cycle efficiencies, leading to ever lower CO2 and NOX emissions, managed to eclipse coal for over a decade from the mid-1990s.

• Heat recovery steam generators, as a separate line of business, have now come to be established by a very different set of manufacturers who were small waste heat boilermakers in the former times.

• Extensive LNG facilities now serve markets that are far away from supply zones.

Despite a setback of a decade for coal due to the popularity of NG, coal has seen a very active period of technological growth.

• The quality of coals has declined over the years although the pollution limits have been progressively reduced, providing encouragement for FBC technologies to assert themselves for firing solid fuels.

• Lower NOX limits have changed the burner and furnace designs.

• Lower SOX limits have spurred the business of deSOx units for PF boilers using high-sulfur coals.

• Low CO2 demands from power plants have brought supercritical boilers into focus again.

• Fuel flexibility and environment friendliness of FBC boilers have heightened the inter­est in them worldwide to consume fuels that were hitherto considered unusable.

• Interest in renewable energies has been growing steadily, coupled with distributed power generation, making biofuels a small but important segment.

The utility market has undergone a total metamorphosis all over the world with the separation of generation and distribution and dismantling of tariffs and protections. This privatization effort has led to the growth of new entities called independent power pro­ducers (IPPs) with multinational locations, whose main objective is to generate power at the lowest cost. Naturally the premium is on the highest efficiency and reliability along with optimum fuel and operating flexibility. These new parameters have started driving the market, demanding boilers with better standards, higher efficiencies, and larger sizes, which are more environmentally compliant. Gold plating of specifications practiced by many rich utilities has started, yielding to superior functionality and standardization.

Meanwhile, there has been a fundamental shift in the business of basic industries from the advanced world to the developing nations. The industry has been built to world-class sizes to satiate the global demand with a great ecological awareness hitherto not prevalent. Naturally, the industrial boiler market has also started needing bigger and more efficient boilers with better fuel flexibility, and, in many cases, with multifuel capability.

• Stokers have been relegated to firing biofuels.

• Bubbling fluidized bed combustion (BFBC) boiler is the mainstay for the industry for the lower range (in the developing markets), whereas circulating fluidized bed (CFBC) is preferred for the higher range. Pulverized fuel-fired boiler is slowly exit­ing in favor of CFBC.

• The captive and cogeneration (cogen) power requirements have become sizable and comparable with, if not bigger than, the utility power plants that existed a

Couple of decades ago. They are served by, generally, PF boilers that are more efficient and environment friendly.

• In the hydrocarbon sector, the captive and cogen plants have assumed greater and greater capacities with the growth of the main plants and the availability of larger GTs.

• Cogeneration, as a concept, has also made rapid progress in traditional chemical and allied industries, driving up the steaming conditions and efficiencies of the boilers.

The situation of the past years was very placid. The fuels were limited, sizes smaller, applications restricted, instrumentation and control sluggish, water treatment still under active evolution, pollution controls almost nonexistent, efficiencies of both the boiler and the auxiliaries relatively lower, and finally the optimization of designs and operation was less than adequate with no computer support. With all these changes having taken place, the contemporary boilers are significant upgrades over the earlier models. Fluidized bed combustion boilers and HRSGs are new additions to the boiler family as they were not in existence 50 years ago. At the same time, there has been marginalization of the stok­ers with a new role for them. With the widening of energy needs and a greater variety of applications, boilers of all types and sizes are required.

The shrinkage of world economy in 1990s along with the dismantling of barriers and globalization of markets leading to the shift of business and industry to the Asian markets has seen severe attrition among equipment makers of all power gear, which has been very painful. As a matter of fact, the consolidation efforts in the industry are still going on. The overcapacity that prevailed earlier has now practically disappeared. The manufacturing base has substantially moved to Asia. However, the design and development activity con­tinues to stay with the United States and Europe within the firms that have survived. Many boiler companies have been made parts of EPC companies that supply power plants.

Have the boilers changed a great deal among all these changes of business, markets, manufacturers, fuels, technologies, etc? The answer is bound to be a mixed one. The boil­ers have undoubtedly changed in size, versatility, response, environmental compliance, and even combustion techniques. But they have undergone few changes in terms of the fundamentals or the building blocks. One can notice a change in the way they are orga­nized, but the blocks are almost the same. The concepts have evolved to a higher level, but the fundamentals have remained untouched.

This book, which goes into these details of boiler fundamentals and engineering and explains the current technological progress with various types of firing systems, should provide interesting and helpful reading to any serious power engineer.