CLEAN COAL COMBUSTION TECHNOLOGIES

Steam turbines can run on a variety of fuels but coal continues to remain a popular choice. However, the traditional coal-fired plants suffer from two major drawback: overall efficiency levels are low and pollution levels are high.

Growing environmental concerns and the need to improve conversion efficiency levels have led to the development of clean coal technologies. The most popular of these technologies are Fluidised Bed Combustion (FBC), Pressurised Fluidised Bed Combustion Combined Cycle (PFBC) and Integrated Gasification Combined Cycle (IGCC).

Improvement in overall performance of steam turbines for thermal power plants can be brought about largely through two kinds of advancement. Firstly,through improvement in mechanical efficiency by reducing aerodynamic and leakage losses as the steam expands through the turbine. Secondly, through improvement in thermodynamic efficiency by increasing the temperature and pressure at which heat is added to the power cycle.

Supercritical Technology

The steam temperature can be raised to levels as high as 580 to 600° C and pressure over 300 bar. Under these conditions, water enters a phase called "supercritical" with properties in between those of liquid and gas. This supercritical water can dissolve a variety of organic compounds and gases, and when hydrogen per-oxide and liquid oxygen are added, combustion is triggered. Turbines based on this principle are called Supercritical Turbines. These turbines offer outputs of over 500 MW. Some manufacturers are planning to commission steam turbines of 800-1,000 MW output in the next few years.

The supercritical turbines can burn low grade fossil fuels and can completely stop Oxides of Nitrogen (NOx) emissions and keep emissions of sulphur dioxide to a minimum. For example, lignite or brown coal has a high water content. So, it is normally not used for power generation. Yet, when lignite is added to water that has been heated to 600° C at a pressure of 300 bar, it will completely burn up in one minute while emitting no NOx and only 1 percent of its original sulphur content as SOx. This also eliminates the need for desulphurisation and denitrification equipment and soot collectors. Although large amounts of energy are required to create supercritical water, operating costs could be significantly different from existing power generating facilities because there would be no need to control gas emissions. The demand for cooling water is also reduced, almost proportionally to an increase in the efficiency.

Currently, supercritical power plants reach thermal efficiencies of just over 40 percent, although a few of the more plants have attained high efficiency upto 45 percent. A number of steam generator and turbine manufacturers around the world now claim that steam temperatures upto 700° C ("ultra" supercritical conditions) are possible which might raise plant efficiencies to over 50 percent, but by using expensive nickel-based alloys. Because supercritical water is corrosive, expensive nickel alloys must be used for the reaction equipment and power generators.

The main competition to supercritical system is from new gas turbine combined cycle plants which are now expedited to achieve an overall efficiency of 60 percent, making a huge difference in generating and life-cycle costs. However, the new gas turbines will release exhaust into waste heat recovery steam generator at temperatures above 600° C, thus necessitating the use of the high chromium steel and nickel alloys as used in the supercritical coal-fired plants.

The economic benefits of taking steam temperature above 635° C, the costs of nickel-based alloys are yet to be resolved. The extra costs of using nickel-based alloys can probably be compensated by reduction in the amount of material required through thinner tube walls and smaller overall dimensions of both plant and site requirements. Efforts are also afoot to develop materials which can withstand high temperatures and pressures to improve thermal efficiency.

However, increased live steam pressure may lower potential for improved performance due to auxiliary power consumption. In addition, increased pressure leads to a loss of thermal flexibility and this can also increase costs.

Fluidised Bed Combustion

During the seventies and also in eighties,, it appeared that conventional pulverised coal-fired power plants had reached a plateau in terms of thermal efficiency. The efficiency levels achieved were of the order of 40 percent in the US and the UK.The corresponding figures for India, however, were lower at 36 to 37 percent.

An alternative technology, Fluidised Bed Combustion (FBC), was developed to raise the efficiency levels. In this technology, high pressure air is blown through finely ground coal. The particles become entrained in the air and form a floating or fluidised bed. This bed behaves like a fluid in which the constituent particles move to and fro and collide with one another.

Fluidised bed can burn a variety of fuels-coal as well other non-conventional fuels like biomass, petro-coke, coal cleaning waste and wood. This bed contains only around 5 percent coal or fuel. The rest of the bed is primarily an inert material such as ash or sand.

The temperature in FBC is around 800-900° C compared with 1,300-1,500° C in Pulverised Coal Combution (PCC). Low temperature helps minimise the production of NO_x. With the addition of a sorbent into the bed (mostly limestone), much of the SO₂ formed can be captured. The other advantages of FBC are compactness, ability to burn low calorific values (as low as 1,800 kcal/kg) and production of ash which is less erosive. Moreover, in FBC, oil support is needed for 20-30 percent of the load versus 40-60 percent in PCC. FBC-based plants also have lower capital costs compared to PCC-based plants. The capital costs could be 8-15 percent lower.

FBCs are essentially of two types bubbling and circu-lating. While bubbling beds have low fluidisation veloci-ties to prevent solids from being elutraited, circulating beds employ high velocities to actually promote elutriation. Both these tech-nologies operate on atmos-pheric temperature. The circulating bed can remove 90-95 percent of the sulphur content from the coal while the bubbling bed can achieve 70-90 percent removal.

FBC thus offers an option for burning fuels economically, efficiently and in an environmentally acceptable way. Currently, size is the only limitation of this technology. While the maximum size of a PCC-based power plant unit could be 1,300 MW, FBC has achieved a maximum unit size of 250 MW.

According to some estimates, FBC represents only about 2 percent of the total coal fired capacity worldwide, but is of particular interest and significance for use of those coals which are difficult to mill and fire in PCC boilers.

Circulating Fluidised Bed Combustion (CFBC)

Unlike conventional PC-fired boiler, the CFBC boiler is capable of burning fuel with volatile content as low as 8 to 9 percent (e.g. anthracite coke, petroleum etc. with minimal carbon loss). Fuels with low ash-melting temperature such as wood, and bio-mass have been proved to be feedstocks in CFBC due to the low operating temperature of 850-900° C. CFBC boiler is not bound by the tight restrictions on ash content either. It can effectively burn fuels with ash content upto 70 percent (Fig. 7).

CFBC can successfully burn agricultural wastes, urban waste, wood, bio-mass, etc which are the low melting temperature as fuels. The low furnace temperature precludes the production of "thermal NOX" which appears above a temperature of 1200 to 1300° C. Besides, in a CFBC boiler, the lower bed is operated at near sub-stoichiometric conditions to minimise the oxidation of "fuel-bound nitrogen". The remainder of the combustion air is added higher up in the furnace to complete the combustion. With the staged-combustion about 90 percent of fuel-bound nitrogen is converted to elemental nitrogen ( N2) as main product.

Status of development of technology in India and World

In India, Bharat Heavy Electricals Limited (BHEL) has developed bubbling fluid bed boilers upto capacity rating of 150 tonne per hour for high ash coals and washery rejects. For units of capacity higher than 30 MW, circulating fluidised bed combustion (CFBC) technology is more economical for high ash coals and / or high sulfur coals. For higher capacity CFBC boilers, BHEL has entered into a technical collaboration agreement with M/s Lurgi Babcock Energy Technik, Germany to make boilers upto 200 MW. BHEL is currently executing an order for two units of Lignite fired CFBC boilers of 125 MWe each (390 tph steam flow) in Gujarat and has commissioned one coal fired unit of 30 MWe (175 tph) capacity in Maharashtra in 1996.

The first CFBC power plant of 110 MW at Nuclu. Colorado, USA is operating since 1990. Several such CFBC power plants are operating in Germany, UK, Canada and Japan using various kinds of coal and bio-mass fuels. The largest CFBC power plant is the 250 MWe unit in Gardane, France, commissioned in 1996. Presently, 350 MWe units are being constructed in Canada and Japan. CFBC is a mature technology with more than 300 CFBC boilers in operation world wide ranging from 5 MWe to 250 MWe. With line stone addition, 90 percent of the sulfur emission can be retained. With staged combustion and with relatively low combustion temperature of 850 / 900° C, NO₂ formation is about 300 to 400 mg/Nm³ only against 500 to 1000 mg/Nm³ in conventional PF fired boilers.

Pressurised Fluidised Bed Combustion Combined Cycle (PFBC)

A new type of fluidised bed design, the pressurised bed, was developed in the late eighties to further improve the efficiency levels in coal-fired plants.

In this concept, the conventional combustion chamber of the gas turbine is replaced by a pressurised fluidised bed combustor. The products of combustion pass through a hot gas cleaning system before entering the turbine. The heat of the exhaust gas from the gas turbine is utilised in the downstream steam turbine. This technology is called pressurised fluidised bed combustion combined cycle (PFBC) (Fig. 8).

The bed is operated at a pressure of between 5 bar and 20 bar and operating the plant at such low pressures allows some additional energy to be captured by venting the exhaust gases through a gas turbine which is then combined with the normal steam turbine to achieve plant efficiency levels of upto 50 percent. The steam turbine is the major source of power in PFBC, contributing about 80 percent of the total power output. The remaining 20 percent is produced in gas turbines.

PFBC plants are smaller in size than the atmospheric FBC and PCC plants and therefore have the advantage of siting in urban areas. The fuel consumption is about 10-15 percent lower than in PCC technology.

PFBC has been used only over the last few years. The development of this technology is dependent upon the compatibility of the hog gas clean-up system with the gas turbine inlet temperatures and maximum particulate size. Improvements on these two fronts would lead to greater acceptance of PFBC.

Status of PFBC Technology Development

The first demonstration plant of capacity of 130 MWe (+224 MW, co-generation) has been operating in Stockholm, Sweden since 1991 meeting all the stringent environmental conditions. Another demonstration plant of 80 MWe capacity is operating in Escatron, Spain using 36% ash black lignite. The third demonstration plant of 70 MWe at TIDD station, OHIO, USA was shut down in 1994 after a eight year demonstration period in which a large amount of useful data and experience were obtained. A 70 MWe demo plant has been operated at Wakamatsu from 1993 to 1996.

Presently, a 350 MWe PFBC power plant is planned in Japan and another is on order in USA (to be operated at SPORN). UK has gathered a large amount of data on a 80 MWe PFBC plant in Grimethrope during its operation from 1980-1992 and is now offering commercial PFBC plants and developing second generation PFBC. ABB-Sweden is the leading international manufacturer which has supplied the first three demonstration plants in the world and is now offering 300 MWe units plants. In India, BHEL-Hyderabad has been operating a 400 mm PFBC for the last eight years and has collected useful research data. IIT Madras has a 300 mm diameter research facility built with NSF (USA) grant. A proposal by BHEL for a 60 MWe PFBC plant is under consideration with the Government of India.

Integrated Gassification Combined Cycle (IGCC)

The integrated gassification combined cycle is a process in which the fuel is gasified in an oxygen or air-blown gasifier operating at high pressure. The raw gas thus produced is cleaned of most pollutants (almost 99 percent of its sulphur and 90 percent of nitrogen pollutants). It is then burned in the combustion chamber of the gas turbine generator for power generation. The heat from the raw gas and hot exhaust gas from the turbine is used to generate steam which is fed into the steam turbine for power generation.

Often, IGCC is referred to as "Cool Water" technology, a name drawn from the ranch in California's Mojave Desert that once occupied the site where it was developed. Coal all shorts burns so well with the Cool Water technology -upto 99 percent of sulphur contamination is eliminated.

The main subsystems of a power plant with integrated gasification are:

• Gasification plant
• Raw gas heat recovery systems
• Gas purification with sulphur recovery
• Air separation plant (only for oxygen blown gasification)
• Gas turbine with heat recovery steam generator
• Steam turbine generator

The feedstock which is fed into the gasifier is more or less completely gasified to synthesis gas (syngas) with the addition of steam and enriched oxygen or air. The gasifier can be fixed bed, entrained or fluidised bed. The selection of the gasifier to achieve best cost efficiency and emission levels depends upon the type of fuel.

In the gas purification system, initial dust is removed from the cooled raw gas. Chemical pollutants such as hydrogen sulphide, hydrogen chloride and others are also removed. Downstream of the gas purification system, the purified gas is reheated, saturated with water if necessary (for reduction of the oxides of nitrogen) and supplied to the gas turbine combustion chamber.

The IGCC technology scores over others as it is not sensitive with regard to fuel quality. Depending on the type of gasifier, liquid residues, slurries or a mixture of petcoke and coal can be used. In fact, the IGCC technology was developed to take advantage of combined cycle efficiency of such low-grade fuels (Fig. 9)

IGCC technology is also environment friendly. In IGCC, pollutants like sulphur dioxide and oxides of nitrogen are reduced to very low levels by primary measures alone, without down-stream plant components and additives like limestone.

The low NOx values are achieved by dilution of the purified syngas with nitrogen from air separation unit and by saturation with water. The direct removal of sulphur compounds from the syngas results in the effective recovery of elemental sulphur, yielding a saleable raw chemical product. Gasification and gas cleaning are an extremely effective filter for contaminants harmful to both gas turbines as well as environment. The IGCC technology is not only environment friendly, but also efficient in power generation (upto 50 percent).

However, IGCC is an expensive option. Some companies claim that they have found an answer to the cost issue with a new technology for producing methanol. They believe that fitting this system, which produces methanol at twice the rate of conventional methods, on the back end of the gasifier units on an IGCC plant can cut the capital cost by 25 percent. The technology achieves this saving by reducing the number of gasifiers the IGCC plant needs - provided the full capacity of the power station is not required for base load running. This enables the operator to make full use of the gasifers, which account for 50-60 percent of the cost of an IGCC and become prohibitively expensive under part-time operation. When power is not required, they can be switched to methanol production. This provides the additional fuel to meet full power output at time of peak demand.

The additional benefits will not make an IGCC unit competitive with a combined cycle gas turbine (CCGT) plant where there is adequate supply of natural gas. However, a 500 MW unit could compete with traditional coal-fired technology. The biggest difficulty may arise in securing a long-term purchase contract for methanol that will allow the plant operator to keep the gasifiers in continuous operation.

The use of gasification for power generation is perceived by many as a complex and expensive technology. However, recent experience in both developed and developing countries reinforces its relevance to power generation. In India, in particular, the IGCC technology is of great relevance as we do not have huge reserves of hydrocarbons. Since coal is available, more project developers can go in for coal-based IGCC plants.

The merits of advanced clean coal combustion technologies over pulverised fired conventional combustion are enlisted in Table 8.

Table 8 : Merits of Advanced coal combustion systems