By Ragnar Lundqvist, Foster Wheeler Energia Oy, Helsinki, Finland
Biofuels present many challenges for power plant operators, but technology developments and the vast experience of the Nordic countries means that they are growing in popularity all over the world.
The Kyoto protocol, although not ratified to the extent intended, the EU White Paper proclaiming an increase from six per cent to 12 per cent in energy production from renewables, and the common desire to use sustainable fuel sources will all help to increase the use of biofuels for generating electricity and heat.
Equipment manufacturers have been providing energy production equipment for biomass fuels since the 1920s. The first units were stoker or grate type boilers burning wood, wood waste or agricultural waste such as bagasse. They were used typically in industrial plants for generating process steam. The steam parameters were low and not much emphasis was put on efficiency.
Due to the energy crisis in the early 1970s there was an enhanced commercial demand for technologies that could use low grade fuels. At this time, the bubbling bed boiler (BFB) was introduced for burning biofuels such as wood, bark, wood waste and sludge. In the middle of the 1970s circulating fluidized bed technology was developed and then commercialized in the 1980s. This was another technology step to further utilize biofuels as a source for energy production in an efficient way.
Foster Wheeler has been delivering biofuel technology since the 1920s
Today all three of these technologies exist in parallel. However, during the late 80s and 90s, the fluidized bed boiler won terrain over the grate fired technology due to its higher efficiency and lower gaseous emissions.
Grate-fired boilers have special merits for certain fuels. Applications such as burning chicken litter or dried olive waste are good examples. The chart shown (see Figure 1) is divided into three different technologies, namely grate-fired, BFB and CFB boilers. The references shown are boilers burning biofuels only and boilers where co-combustion takes place.
With co-combustion, biofuels are used as one fuel source and are combusted with another fuel, usually coal or oil. This is a very typical scenario and results from the fact that the availability of biofuels may be limited due to seasonal variations, and the overall availability of the fuel may be low compared to the amount of energy that needs to be produced. In such cases a boiler designed with multi-fuel capability ensures energy year-round.
Biofuels are also typically low in energy and low in bulk density so transporting large quantities is not economically feasible. This is another reason for co-combustion and is why plants based on only biofuels are usually relatively small in size.
Over the years of technology development, the steam parameters in biomass-fueled plants have also increased both in terms of temperature and pressure. This means an increase in the efficiency of a plant. In the early years, saturated steam was sufficient for the industrial processes. But with increasing demand for in-house electricity from industrial plants, the use of backpressure turbines was soon introduced and today most applications include a cogeneration scheme.
The development of fresh steam temperatures over the years has seen a corresponding increase in plant efficiency
Simultaneously the efficiency of the industrial application in power generation has become more and more important, and now high efficiencies are demanded by power plant owners.
Figure 2 shows the increase in steam temperature over the years. Today the temperature typically varies from 440°C to 540°C depending on application and the cogeneration scheme. Plant efficiencies have increased from being less than 20 per cent to 36 per cent (LHV basis) in the most modern power plants.
Figure 2 also shows that there is a large variation in the steam temperature of biomass power plants. The reason for this is that biofuels restrict, or can actually determine the steam conditions, especially the temperature. The fuels are organic materials and use nutrients while growing, so biofuels such as wood contain alkali salts and chlorides. If these compounds exist in sufficient quantities, they will produce corrosive substances in the combustion process. This may in turn lead to fouling of heat surfaces or even corrosion of the metal surfaces resulting in boiler tube leaks.
The attack on the metal surfaces will depend on the temperature. The higher the temperature, the more severe a corrosive attack can be. It is of utmost importance to know the fuel quality and then in the design select such steam parameters and layout so that corrosion or fouling of the heat surfaces will not take place.
The Nordic countries have been forerunners in implementing technologies that use biofuel and have also been the first to use the latest and most advanced designs in order to improve efficiency and plant reliability. In the Nordic region over the last ten years, most boilers installed in the 5 MWe and above size range have been boilers based on BFB or CFB technology. Typically they have been installed in cogeneration plants where in addition to electricity, heat or steam is generated for district heating or industrial use. There are several good examples of these plants.
One example is the CFB boiler at the Mälarenergi AB combined heat and power (CHP) plant in Västeras, Sweden. This is one of the largest and oldest CHP plants in the country. It consists of four existing units using oil and pulverized coal as fuel. The total production capacity is about 500 MW electricity and 900 MW district heat. Coal has been the main fuel, covering approximately 65 per cent of heat production. All existing boilers have been furnished with efficient desulphurization and DeNOxequipment.
The fourth unit was converted from oil firing to pulverized coal firing in 1983 with downrating of the unit capacity to 155 MWe/250 MWdh. This was due to a reduction in main steam flow from 750 t/h to 550 t/h. The boiler for Unit 4 is a once-through design with a reheat cycle having main steam pressure of 171 bar and steam temperature of 540°C.
In 1998 Mälarenergi AB in Västeras started constructing a new boiler (Unit 5) mainly for biomass firing. The steam cycle in this boiler is connected to the existing turbine, steam, condensate and feedwater systems of Unit 4. Unit 5 is now producing 200 t/h (55.5 kg/s) of main steam and is bringing the turbine of Unit 4 back to full capacity. With Boiler 5, the Västeras CHP plant consumes about 1.1 million m3 of biomass fuel to replace 120 000 t of coal per year corresponding to one third of today’s usage. Hence emissions of carbon dioxide have been reduced by 340 000 tonnes per year.
Foster Wheeler was selected to construct the new boiler as a turnkey supplier. The delivery includes a Circulating Fluidized Bed (CFB) boiler with auxiliaries, structural steel, boiler house, electrification, instrumentation and control system, flue gas condensing plant as well as erection and commissioning. The project began in December 1998 and start-up was in October 2000, two months ahead of the original schedule. The boiler is now in normal commercial operation.
Cross-sectional drawing of the CFB boiler at Malarenergi AB in Vasteras, Sweden
Figure 3 shows the boiler outline and also lists the biofuels used. Unit 5 is typically operated at full load most of the time in order to maximize the use of biomass. The load of Unit 4 will follow the requirements of the district heating network.
The steam parameters for the new CFB are dictated by the existing steam turbine. Steam pressure and temperature are the same as for the once through boiler as shown in Table 1.
The feedwater temperature at full load is 250°C for both boilers.
The Swedish emission taxation for NOxand SOx has made it necessary for power plant owners and operators to strive for low emission levels. Hence the new M
As the main fuels used at Mälarenergi are different low sulphur biomass fuels, limestone feeding for SOx reduction is not required. However limestone-feeding equipment is provided as a provision for when high sulphur fuels such as peat and coal are used. Dust emissions are controlled with a fabric filter. The emission limits are shown in Table 2.
A unique feature of this project is the connection of a once-through boiler and a natural circulation boiler, both with reheat, to a common turbine. To address this challenge, the parallel operation of the two boilers was analyzed in a special work group formed by the owner and the supplier at an early stage of the project.
Furthermore a training simulator was designed particularly for this combined mode of operation. With this and with thorough analyses done, the commissioning and the commercial operation of the new unit has been very smooth.
In the Västeras boiler, both the high temperature superheater and reheater are Intrex heat exchangers. This way the risk for chlorine-induced corrosion and fouling is greatly reduced. Also, the ability to control the heat transfer rate in the final reheater enables of the amount of spray water needed for reheat steam temperature control to be minimized. This results in an improvement in the plant’s net efficiency.
The interest in sustainable energy and biomass-fired power plants is also growing in other parts of the world. For example, Germany has now passed a law, the so-called biomass law, which gives plant owners using biofuels a financial incentive; the price they receive for power generated, up to a capacity of 20 MWe, is DEM170/MWh (¢8/kWh) for 20 years provided that the plant efficiency is higher than 29 per cent (LHV basis). This has created a tremendous interest in energy conversion technology for biomass fuels. These plants will be mostly condensing power stations generating only electricity.
There have been a few biofuel based cogeneration plants built in Germany before this new law. In Germany, two boilers have been built burning industrial wood waste and demolition wood in Germany. The first one at Hornitex Werke Beeskow Kunststoffe und Holzwerkstoffe GmbH in Beeskow has been in operation since 1996, while the second at Hornitex Energie GmbH in Horn Bad Meinberg has been operating since 2000. These have a steam production capacity of 30.5 kg/s and 33.4 kg/s respectively at 480°C and 89 bar. This corresponds to an electrical output of 25 MWe and 31 MWe respectively.
The steam parameters for temperature and pressure in these two plants are lower than in the Swedish example since the fuel contains some compounds that could cause high temperature corrosion. This is prevented by using lower steam temperatures and the Intrex heat exchanger design. The Beeskow unit is one of the first using this heat exchanger type and has now been in operation for five years without corrosion.
The use of biofuels will continuously increase as there are both political and social driving forces promoting the use of sustainable energy. The incentives in electricity prices implemented in certain countries in the European Union will further increase the use of biofuels.
Power stations based on biofuels only will continue to be relatively small in size due to the low energy content and low bulk density of the fuel, which prevents, from an economic point of view, transportation of the fuel long distances. This fact will promote localized energy production, which could solve some energy demand problems in developing countries, for example.
Biofuels are not homogeneous materials. The quality and form varies. The fuels typically also contain alkalis such as potassium and chlorines. These can cause fouling and corrosion. Special emphasis must be paid on the fuel quality and characteristics in the design of a boiler.
With bubbling and circulating fluidized bed technologies the biofuels can be burnt efficiently and with low gaseous emissions meeting the most stringent regulations. With special boiler designs, power plant efficiencies can be very high compared to the efficiencies in the past. Grate-fired units will also be used especially when fuels such as chicken litter or olive waste are burnt.