Palm Kernel Shells: An Attractive Biomass Fuel for Europe

palm-kernel-shellsEurope is targeting an ambitious renewable energy program aimed at 20% renewable energy in the energy mix by 2020 with biomass energy being key renewable energy resource across the continent. However, the lack of locally-available biomass resources has hampered the progress of biomass energy industry in Europe as compared with solar and wind energy industries. The European biomass industry is largely dependent on wood pellets and crop residues.

Europe is the largest producer of wood pellets, which is currently estimated at 13.5 million tons per year while its consumption is 18.8 million tons per year. The biggest wood pellet producing countries in Europe are Germany and Sweden. Europe relies on America and Canada to meet its wood pellet requirements and there is an urgent need to explore alternative biomass resources. In recent years, palm kernel shells (popularly known as PKS) from Southeast Asia has emerged has an attractive biomass resources which can replace wood pellets in biomass power plants across Europe.

What are Palm Kernel Shells

Palm kernel shells are the shell fractions left after the nut has been removed after crushing in the Palm Oil mill. Kernel shells are a fibrous material and can be easily handled in bulk directly from the product line to the end use. Large and small shell fractions are mixed with dust-like fractions and small fibres.

Moisture content in kernel shells is low compared to other biomass residues with different sources suggesting values between 11% and 13%. Palm kernel shells contain residues of Palm Oil, which accounts for its slightly higher heating value than average lignocellulosic biomass. Compared to other residues from the industry, it is a good quality biomass fuel with uniform size distribution, easy handling, easy crushing, and limited biological activity due to low moisture content.

Press fibre and shell generated by the palm oil mills are traditionally used as solid fuels for steam boilers. The steam generated is used to run turbines for electricity production. These two solid fuels alone are able to generate more than enough energy to meet the energy demands of a palm oil mill.

Advantages of Palm Kernel Shells

PKS has almost the same combustion characteristics as wood pellets, abundantly available are and are cheap. Indonesia and Malaysia are the two main producers of PKS. Indonesian oil palm plantations cover 12 million hectares in Indonesia and 5 million hectares in Malaysia, the number of PKS produced from both countries has exceeded 15 million tons per year. Infact, the quantity of PKS generated in both countries exceeds the production of wood pellets from the United States and Canada, or the two largest producers of wood pellets today.

Interestingly, United States and Canada cannot produce PKS, because they do not have oil palm plantations, but Indonesia and Malaysia can also produce wood pellets because they have large forests. The production of wood pellets in Indonesia and Malaysia is still small today, which is less than 1 million tons per year, but the production of PKS is much higher which can power biomass power plants across Europe and protect forests which are being cut down to produce wood pellets in North America and other parts of the world.

PKS as a Boiler Fuel

Although most power plants currently use pulverized coal boiler technology which reaches around 50% of the world’s electricity generation, the use of grate combustion boiler technology and fluidized bed boilers is also increasing. Pulverized coal boiler is mainly used for very large capacity plants (> 100 MW), while for ordinary medium capacity uses fluidized bed technology (between 20-100 MW) and for smaller capacity with combustor grate (<20 MW). The advantage of boiler combustion and fluidized bed technology is fuel flexibility including tolerance to particle size.

When the pulverized coal boiler requires a small particle size (1-2 cm) like sawdust so that it can be atomized on the pulverizer nozzle, the combustor grate and fluidized bed the particle size of gravel (max. 8 cm) can be accepted. Based on these conditions, palm kernel shells has a great opportunity to be used as a boiler fuel in large-scale power plants.

Use of PKS in pulverized coal boiler

There are several things that need to be considered for the use of PKS in pulverized coal boilers. The first thing that can be done is to reduce PKS particle size to a maximum of 2 cm so that it can be atomized in a pulverized system. The second thing to note is the percentage of PKS in coal, or the term cofiring. Unlike a grate and a fluidized bed combustion that can be flexible with various types of fuel, pulverized coal boilers use coal only. There are specific things that distinguish biomass and coal fuels, namely ash content and ash chemistry, both of which greatly influence the combustion characteristics in the pulverized system.

PKS-biomass

PKS has emerged as an attractive biomass commodity in Japan

Coal ash content is generally greater than biomass, and coal ash chemistry is very different from biomass ash chemistry. Biomass ash has lower inorganic content than coal, but the alkali content in biomass can change the properties of coal ash, especially aluminosilicate ash.

Biomass cofiring with coal in small portions for example 3-5% does not require modification of the pulverized coal power plant. For example, Shinci in Japan with a capacity of 2 x 1,000 MW of supercritical pulverized fuel with 3% cofiring requires 16,000 tons per year of biomass and no modification. Similarly, Korea Southeast Power (KOSEP) 5,000 MW with 5% cofiring requires 600,000 tons per year of biomass without modification.

PKS cofiring in coal-based power plants

Pulverized coal-based power plants are the predominant method of large-scale electricity production worldwide including Europe. If pulverised fuel power plants make a switch to co-firing of biomass fuels, it will make a huge impact on reducing coal usage, reducing carbon emissions and making a transition to renewable energy. Additionally, the cheapest and most effective way for big coal-based power plants to enter renewable energy sector is biomass cofiring. Palm kernel shells can be pyrolyzed to produce charcoal while coal will produce coke if it is pyrolyzed. Charcoal can be used for fuel, briquette production and activated charcoal.

Share of Renewables in Energy Supply of UK

The Earth is facing a climate crisis, as the burning of fossil fuels to generate electricity and power our cars overloads the atmosphere with carbon dioxide, causing a dangerous atmospheric imbalance that’s raising global temperatures.

A report from the UN’s Intergovernmental Panel on Climate Change (IPCC) released earlier this month cautioned that the planet has just 12 years to dramatically curb greenhouse gas emissions, by overhauling our energy systems and economies and likely, our societies and political systems. Even a half degree rise beyond that would cause catastrophic sea level rises, droughts, heat, hunger, and poverty, spelling disaster for our species.

UK’s Commitment to Climate Change Mitigation

The UK government has committed to reducing carbon emissions by 80% of 1990 levels by 2050, a process that will involve overhauling our energy supply, which is responsible for 25% of greenhouse emissions in the country, just behind transport (26% of all emissions). But it may be too little too late. The government has already said it is reviewing these targets in light of the IPCC report and in the spring began consulting on a net-zero carbon emissions target for 2050.

But despite these dire prognoses and the enormity of the task facing us as a species, there’s reason to be optimistic. The UK has already managed to cut greenhouse gas emissions by 43% on 1990 levels, with much of the reduction coming from a 57% decline in emissions from energy generation. This is in part thanks to several providers offering you the chance to have a 100% renewable domestic energy supply.

Reduction in Coal Usage

The use of coal has plunged nearly overnight in the UK. In 2012, 42% of the UK’s electricity demand was met by coal. Just six years later, in the second quarter of 2018, that figure had fallen to just 1.6%. Emissions from coal-fired power stations fell from 129 million tonnes of CO2 to just 19 million tonnes over the same period.

A coal-free Britain is already on the horizon. In April 2017, the UK logged its first coal-free day since the Industrial Revolution; this past April we extended the run to 76 consecutive hours. In fact, in the second quarter of 2018, all the UK’s coal power stations were offline for a total of 812 hours, or 37% of the time. That’s more coal free hours than were recorded in 2016 and 2017 combined and in just three months.

When the UK does rely on coal power, it’s primarily to balance supplies and to meet demand overnight and during cold snaps, such as during the Beast from the East storm in March. The UK is so certain that coal is a technology of the past, that the government has plans to mothball all seven remaining coal-fired power stations by 2025.

Share of Renewables in Energy Supply

The decline in coal has been matched by an explosion in renewable energy, particularly in wind power. In the second quarter of 2018, renewables generated 31.7% of the UK’s electricity, up from under 9% in 2011. Of those, wind power produced 13.3% of all electricity (7.1% from onshore turbines farms and 6.2% from offshore wind farms), biomass energy contributed another 11% of the UK’s electricity, solar generated 6% and hydro power made up the rest of renewables’ pie share.

The UK’s total installed renewables capacity has exploded, hitting 42.2GW in the second quarter of 2018, up from under 10GW in 2010. That includes 13.7GW of onshore wind capacity and 7.8GW of offshore wind capacity—a figure which will get a boost with the opening in September of the world’s largest wind farm, the Walney Extension, off the coast of Cumbria, itself with a capacity of nearly 0.7GW. Solar panels contributed another 13GW of renewable capacity, and installed plant biomass infrastructure reaching 3.3GW.

However, while renewables are transforming electricity generation in the UK, our energy system consists of more than simply electricity. We also have to account for natural gas and the use of fuel in transport, and renewables have made fewer in roads in those sectors.

The UK is meeting just 9.3% of its total energy needs from renewable sources, short of the 15% it has earmarked for 2020 and far behind its peers in the EU, where Sweden is already running on 53.8% renewable energy.

Conclusion

Emissions are dropping overall in the UK, largely due to an ongoing revolution in electricity generation and a decisive move away from coal. But these reductions have concealed stagnant and even increasing levels of greenhouse gas emissions from other sectors, including transport and agriculture.

Our transition to a sustainable economy has begun but will require more than wind farms and the shuttering of coal-fired power stations. It must encompass electric vehicles, transformed industries, and ultimately changing attitudes toward energy and the environment and our responsibility toward it.

Waste Management in Sweden: Perspectives

Sweden is considered as a global leader in sustainable waste management and in the reduction of per capita carbon footprint. The country consistently works to lower its greenhouse gas emissions, improve energy efficiency and increase public awareness. Over the past 10 years, Sweden developed methods of repurposing waste, so less than one percent of the total waste generated in the country makes it to landfills. To accomplish this, the country changed their perspective of garbage.

Increase Recycling

Recycling is a part of Swedish culture. Residents regularly sort recyclable materials and food scraps from other waste in their homes before disposal. This streamlines the recycling process and reduces the effort required to sort large volumes of waste at larger recycling centers. As another way to promote recycling, the Swedish government created legislation stating recycling centers must be within 1,000 feet of residential areas. Conveniently located facilities encourage citizens to properly dispose of their waste.

Repurpose Materials

Citizens are also encouraged to reuse or repurpose materials before recycling or disposing of them. Repurposing and reusing products requires less energy when compared to the recycling or waste disposal process. As Swedes use more repurposed products, they reduce the volume of new products they consume which are created from fresh materials. In turn, the country preserves more of its resources.

Invest in Waste to Energy

Over 50 percent of the waste generated in Sweden is burned in waste-to-energy facilities. The energy produced by these facilities heats homes across the country during the long winter months. Localized heating — known as district heating — has improved air quality throughout the nation. It’s easier and more economical to control the emissions from several locations as opposed to multiple, smaller non-point sources.

Another benefit of waste-to-energy facilities is that ash and other byproducts of the burning process can be used for road construction materials. As a whole, Sweden doesn’t create enough waste to fuel its waste to energy plants — the country imports waste from its neighbors to keep its facilities going.

In the early 1990’s, the Swedish government shifted the responsibility for waste management from cities to the industries producing materials which would eventually turn to waste. To promote burning waste for energy, the government provides tax incentives to companies which make more economically attractive.

Impact of Waste-to-Energy

Although Sweden has eliminated the volume of trash entering landfills, they have increased their environmental impacts in other ways. Waste-to-energy facilities are relatively clean in that most harmful byproducts are filtered out before entering the environment, though they still release carbon-dioxide and water as their primary outputs. On average, waste-to-energy plants generate nearly 20 percent more carbon-dioxide when compared to coal plants.

 

waste-management-sweden

Coal plants burn and release carbon which is otherwise sequestered in the ground and unable to react with the earth’s atmosphere. Waste-to-energy facilities consume and release carbon from products made of organic materials, which naturally release their carbon over time. The downside to this process is that it frees the carbon from these materials at a much faster rate than it would be naturally.

The reliance on the waste-to-energy process to generate heat and the tax incentives may lower Swedish motivation to recycle and reuse materials. The country already needs to import trash to keep their waste-to-energy plants running regularly. Another disadvantage of this process is the removal and destruction of finite materials from the environment.

Even though Sweden continues to make strides in lowering their environmental impact as a whole, they should reevaluate their reliance on waste to energy facilities.

A Blackout, Big Oil, and Wind Energy

During the first quarter of 2017, workers installed a wind turbine somewhere in the US every 2.4 hours. Wind provided 5.6% of all the electricity produced in the US in 2016. That’s more than double the amount of wind power in 2010. The whole world is seeing similar growth.  The wind industry isn’t without controversy. Critics blame it for the scope of a blackout in Australia. On the other hand, international oil companies have begun to build off-shore wind farms.

Critics’ case against wind energy

According to its critics, wind power is unreliable. The wind doesn’t blow all the time. It doesn’t blow on any predictable pattern. Wind turbines require some minimum wind speed for them to work at all. And if the wind is too strong, they can’t operate safely and must shut down.

Wind can cross one or the other of these thresholds multiple times a day. They operate at full capacity for only a few hours a year. So the theoretical capacity of a wind farm greatly exceeds its actual output.

The times turbines can generate electricity do not coincide with rising and falling demand for electricity. This variability creates problems for stabilizing the grid. Critics further claim that the wind industry can’t operate without massive government subsidies.

Wind power and South Australia blackout of 2016?

South Australia depends on wind energy for about 40% of its electricity. It suffered seven tornados on September 28, 2016. Two of them, with winds almost as fast as Hurricane Katrina, destroyed twenty towers that held three different transmission lines. Nine wind farms shut down.  Within minutes, the entire state suffered a massive blackout.

What contributed the most to the blackout? South Australia’s high dependence on wind power? The weather? Or something else?

Renewable energy skeptics quickly claimed the blackout justified their position. The wind farms simply failed to provide enough electricity in the emergency. Wind and solar energy, they say, are inherently unreliable. South Australia’s heavy reliance demonstrates an irresponsible policy based on ideology more than technological reality.

Certainly, the weather would have caused a disturbance in electrical service no matter what source of electricity. People near the downed transmission lines could not have avoided loss of power. But prompt action by grid operators makes it possible to bypass problem areas and limit the extent of the outage.

On closer examination, however, the correct answer to the multiple-choice question above is C: something else.

Wind turbines have “low voltage ride through” settings to keep operating for brief periods when voltage dips below the threshold at which they can operate correctly. If low-voltage conditions occur too frequently, the turbines have a protection mechanism that turns them off.

  • Ten wind farms experienced between three and six low-voltage events within two minutes. But the turbines were operating on factory settings. No one performed any testing to determine good settings under local conditions.
  • The agency that regulates the Australian electricity market knew nothing about the protection feature. It blamed the wind farms, but surely someone on staff should have been familiar with the default operation of the turbines. After all, the agency approved purchase and installation of the turbines. It had all the documentation.
  • Two gas generating plants that should have supplied backup power failed to come online.

The weather caused a problem that became a crisis not because of technical limitations of renewable energy, but because of too many different organizations’ incompetence.

If the wind is too strong, wind turbines can’t operate safely and must shut down.

One homeowner in South Australia didn’t suffer from the outage. He didn’t even know about the blackout till he saw it on the news. He had to test the accuracy of the news reports by opening his oven and noting that the light didn’t come on.

It turns out he had installed solar panels just a few weeks earlier. And since power outages in his part of South Australia occur almost every month, he decided to install a Tesla Powerwall as well.

He can’t use it to power his entire house, but it takes care of the lights and the television. It stores enough electricity for 10 hours of off-grid power.

Big oil and wind power

International oil companies have not joined the chorus of wind-industry skeptics. Several of them, including Royal Dutch Shell, have begun to invest heavily in off-shore wind farms. Especially in the North Sea. Oil production there has steadily declined for about 15 years.

Exploring for new oil fields has become too risky and expensive. These oil companies have decided that investing in wind energy helps their cash flow and makes it more predictable.

Oil companies have more expertise in working on offshore platforms than do companies that specialize in wind energy. Instead of building a foundation for turbines on the ocean floor, at least one oil company has begun to explore how to mount them on floating platforms.

Traditional wind energy firms have been operating turbines in the North Sea for years, but the oil companies have begun to outbid them. Their off-shore expertise has helped them drive down their costs.

So far, American oil companies have shown less interest in wind farms. If they decide they’re in the oil business, they will eventually lose market share to renewable energy companies. If they decide they’re in the energy business, they’ll have to start investing in renewable energy. And if any decide to invest heavily in solar power besides or instead of wind, they will still be following the lead of Total, a French oil company.

For that matter, the coal business is dying. Perhaps some of them will have enough sense to invest in renewables to improve their cash flow.

Renewable Energy Production in Australia: The Plan to Reduce Coal Use

Recently there has been a lot of talk in how a country can improve their ecological footstep. One way of doing so is definitely changing the way the respective country produces its energy. Australia has recently been headlining the news in regard to the renewable energy situation. Australia’s energy production is looking towards a new future with a specific aim on solar and wind power.

If Australia plans on keeping its water resource at a steady level, it has got to go from its use of coal to renewable sources. Thanks to its abundance in both solar and wind energy, Australia has quite the advantage when it comes to green energy production possibilities.

Unfortunately though due to their geographic position, the water supply is limited for the country. So much so, that the coal industry was taking a toll on the water supply due to the large quantities of water needed when producing energy from coal. As a result, moving over to wind and solar energy fueled productions is a viable option seeing how both respective energy productions do not require water.

The news that Australia was listed as a “water-stressed company” was released by the World Resource Institute; a non-profit organization based in Washington D.C. Moreover, on this past May 13th The Sydney Morning Herald also wrote that 73% of Australia’s electricity needs were met by the use of coal. In respect to these findings and Australia’s continuous growth, it is imperative that new resources are used for energy production.

Australia has been making headlines in renewable energy sector.

Fortunately, Australia’s geography is a big resource as well when it comes to studying the possibilities of implementing the new energy productions. It was in fact calculated that the dimensions of the solar power farm needed to meet the country’s demands would result in occupying only 0.1% of Australia’s total land mass; I think we can all agree on the fact that that land could be spared for a solar farm.

And on that note, the government is taking the matter seriously, and has called upon everybody to try and better the situation. The incentives call upon small businesses and households as well by reminding them that there are the possibilities of installing their own solar panels, heat pumps, solar water heaters, and more.

Thanks to the various incentives, the Green Energy Council has stated that there is a lot of activity in the sector, including at least 58 different projects focused on implementing the renewable energy sources. As a consequence of these projects, the council has also stated that there would be an income of $10 billion in investments, 6,141 new jobs, and 5,482 megawatts of renewable energy capacity. Definitely great numbers to look forward to!

Tackling China’s Smog Problem with Renewable Energy

smog-chinaChina is currently facing serious environmental problems, with potentially few solutions. Currently, this is mostly taking the form of serious smog issues plaguing North China, with more than 24 cities on red alert. However, with airports being shut down due to lacking visibility and the economy of China being heavily disrupted, action needs to be taken to solve this serious smog problem.

While limited action has been taken, perhaps renewable energy is the key to cutting down China’s smog.

How Bad Is the Problem?

The smog problem in China has become increasing worse from 2015 to 2017, with more than 90 micrograms of pollution per meter squared. These levels of air pollution are similar to the levels recorded previous to 2014, when the Chinese premier declared a war on pollution due to the health dangers posed by rising air pollution levels.

However, since 2015, levels of air pollution have risen once again. This pollution has had hard hitting effects on urban areas, especially the Chinese capital Beijing, and has caused widespread disruption to the lives of Chinese citizens and economy of the country.

The air pollution leads to the cities becoming hotter than ever. Urban Heat Island effect, which refers to buildings absorbing the sun’s heat well, is exacerbated by the smog. In fact, a car in the heat can reach temperatures of 114 degrees Fahrenheit after just 20 minutes, making travelling on hot days nearly unbearable for any living creature. In order to decrease the heated condition of China, it is essential to decrease the amount of smog covering the cities.

What Has the Chinese Government Done?

The Chinese government has taken limited action in an attempt to minimize the air pollution being created in the country. This includes the Atmospheric Pollution Prevention Plan, which acknowledged the danger posed by air pollution levels and aimed to reduce coal usage in urban areas like Beijing.

However, this is not representative of the main action the government has taken. Primarily, the Chinese government has focused on individual areas and attempting to reduce local pollution levels through efficient coal burning and banning the burning of waste materials, especially on farms. These solutions, while effective on a short-term basis, are not all that is needed, though.

Investment in renewables can reduce China's dependence on coal for power generation

Investment in renewables can reduce China’s dependence on coal for power generation

China needs to reduce its overall usage of coal produced energy, which currently stands at 64 percent of total energy consumption. While this has already been happening in China, the further introduction of renewable energy could be of great help to China’s pollution levels.

How Could Renewable Energy Help?

Many people believe renewable energy to be a small affair, something undertaken by the Western world in a vain attempt to reduce our collective guilt concerning climate change and wastage levels. This is simply not the case. Renewable energy is a $120 billion industry that receives investment and application across the world. This includes solar energy, waste-to-energy technology, wind energy, hydroelectric energy and many more attempts to reduce overall energy usage.

Through investment in renewable energy, China could reduce its dependence on coal and increase the efficiency of its energy production and economy. Smog is directly created by China’s use of coal for its energy production, and by investing in other renewable means, China can simultaneously improve its health situation.

In fact, the obviously positive nature of investment in renewable energy can be clearly seen through the Chinese government’s already existing plans to further incorporate it into the economy. In the five-year plan announced in 2016, the Chinese government explicitly stated it would decrease air pollution levels through investment in wind, solar and biomass energy production technologies.

While the plans additionally included investment in making the coal industry more efficient and reducing emissions on an industrial and commercial level, clearly renewable energy is believed to be a valid alternative energy source.

Overall, it is clear that renewable energy can certainly help with China’s serious smog problem. Whether this should be in tangent with further investment in the coal industry or necessitate the end of widespread coal usage in China is still a question for debate.

About the Author

Emily Folk is freelance writer and blogger on topics of renewable energy and conservation. To get her latest posts, check out her blog Conservation Folks, or follow her on Twitter.

Use of Palm Kernel Shells in Circulating Fluidized Bed Power Plants

Palm kernel shells are widely used in fluidized bed combustion-based power plants in Japan and South Korea. The key advantages of fluidized bed combustion (FBC) technology are higher fuel flexibility, high efficiency and relatively low combustion temperature. FBC technology, which can either be bubbling fluidized bed (BFB) or circulating fluidized bed (CFB), is suitable for plant capacities above 20 MW. Palm kernel shells (PKS) is more suitable for CFB-based power plant because its size is less than 4 cm.

With relatively low operating temperature of around 650 – 900 oC, the ash problem can be minimized. Certain biomass fuels have high ash levels and ash-forming materials that can potentially damage these generating units. In addition, the fuel cleanliness factor is also important as certain impurities, such as metals, can block the air pores on the perforated plate of FBC unit. It is to be noted that air, especially oxygen, is essential for the biomass combustion process and for keeping the fuel bed in fluidized condition.

The requirements for clean fuel must be met by the provider or seller of the biomass fuel. Usually the purchasers require an acceptable amount of impurities (contaminants) of less than 1%. Cleaning of PKS is done by sifting (screening) which may either be manual or mechanical.

In addition to PKS, biomass pellets from agricultural wastes or agro-industrial wastes, such as EFB pellets which have a high ash content and low melting point, can also be used in CFB-based power plants. More specifically, CFBs are more efficient and emit less flue gas than BFBs.

The disadvantages of CFB power plant is the high concentration of the flue gas which demands high degree of efficiency of the dust precipitator and the boiler cleaning system. In addition, the bed material is lost alongwith ash and has to be replenished regularly.

A large-scale biomass power plant in Japan

The commonly used bed materials are silica sand and dolomite. To reduce operating costs, bed material is usually reused after separation of ash. The technique is that the ash mixture is separated from a large size material with fine particles and silica sand in a water classifier. Next the fine material is returned to the bed.

Currently power plants in Japan that have an efficiency of more than 41% are only based on ultra supercritical pulverized coal. Modification of power plants can also be done to improve the efficiency, which require more investments. The existing CFB power plants are driving up the need to use more and more PKS in Japan for biomass power generation without significant plant modifications.

Cofiring of Biomass

Cofiring of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Cofiring (or co-combustion) involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels.

Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Cofiring of biomass has the major advantage of avoiding the construction of new, dedicated, biomass power plant. An existing power station is modified to accept the biomass resource and utilize it to produce a minor proportion of its electricity.

Cofiring of biomass may be implemented using different types and percentages of biomass in a range of combustion and gasification technologies. Most forms of biomass are suitable for co-firing. These include dedicated energy crops, urban wood waste and agricultural residues such as rice straw and rice husk.

The fuel preparation requirements, issues associated with combustion such as corrosion and fouling of boiler tubes, and characteristics of residual ash dictate the co-firing configuration appropriate for a particular plant and biomass resource. These configurations may be categorized into direct, indirect and parallel firing.

Direct Co-Firing

This is the most common form of biomass co-firing involving direct co-firing of the biomass fuel and the primary fuel (generally coal) in the combustion chamber of the boiler. The cheapest and simplest form of direct co-firing for a pulverized coal power plant is through mixing prepared biomass and coal in the coal yard or on the coal conveyor belt, before the combined fuel is fed into the power station boiler.

Indirect Co-firing

If the biomass fuel has different attributes to the normal fossil fuel, then it may be prudent to partially segregate the biomass fuel rather than risk damage to the complete station.

For indirect co-firing, the ash of the biomass resource and the main fuel are kept separate from one another as the thermal conversion is partially carried out in separate processing plants. As indirect co-firing requires a separate biomass energy conversion plant, it has a relatively high investment cost compared with direct co-firing.

Parallel Firing

For parallel firing, totally separate combustion plants and boilers are used for the biomass resource and the coal- fired power plants. The steam produced is fed into the main power plant where it is upgraded to higher temperatures and pressures, to give resulting higher energy conversion efficiencies. This allows the use of problematic fuels with high alkali and chlorine contents (such as wheat straw) and the separation of the ashes.

Biomass Gasification Power Systems

Biomass gasification power systems have followed two divergent pathways, which are a function of the scale of operations. At sizes much less than 1MW, the preferred technology combination today is a moving bed gasifier and ICE combination, while at scales much larger than 10 MW, the combination is of a fluidized bed gasifier and a gas turbine.

Larger scale units than 25 MW would justify the use of a combined cycle, as is the practice with natural gas fired gas turbine stations. In the future it is anticipated that extremely efficient gasification based power systems would be based on a combined cycle that incorporates a fuel cell, gas turbine  and possibly a Rankine bottoming cycle.

Integrated Gasification Combined Cycle

The most attractive means of utilising a biomass gasifier for power generation is to integrate the gasification process into a gas turbine combined cycle power plant. This will normally require a gasifier capable of producing a gas with heat content close to 19 MJ/Nm3. A close integration of the two parts of the plant can lead to significant efficiency gains.

The gas from the gasifier must first be cleaned to remove impurities such as alkali metals that might damage the gas turbine. The clean gas is fed into the combustor of the gas turbine where it is burned, generating a flow of hot gas which drives the turbine, generating electricity.

Hot exhaust gases from the turbine are then utilised to generate steam in a heat recovery steam generator. The steam drives a steam turbine, producing more power. Low grade waste heat from the steam generator exhaust can be used within the plant, to dry the biomass fuel before it is fed into the gasifier or to preheat the fuel before entry into the gasifier reactor vessel.

Schematic of integrated biomass gasification combined cycle

The gas-fired combined cycle power plant has become one of the most popular configurations for power generation in regions of the world where natural gas is available. The integration of a combined cycle power plant with a coal gasifier is now considered a potentially attractive means of burning coal cleanly in the future.

Biomass Fuel Cell Power Plant

Another potential use for the combustible gas from a biomass gasification plant is as fuel for a fuel cell power plant. Modern high temperature fuel cells are capable of operating with hydrogen, methane and carbon monoxide. Thus product gas from a biomass gasifier could become a suitable fuel.

As with the integrated biomass gasification combined cycle plant, a fuel cell plant would offer high efficiency. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.