Volume 5 biomass and biofuel production 5 04 – biomass power generation

Volume 5 biomass and biofuel production 5 04 – biomass power generation Volume 5 biomass and biofuel production 5 04 – biomass power generation Volume 5 biomass and biofuel production 5 04 – biomass power generation Volume 5 biomass and biofuel production 5 04 – biomass power generation Volume 5 biomass and biofuel production 5 04 – biomass power generation Volume 5 biomass and biofuel production 5 04 – biomass power generation

A Malmgren, BioC Ltd, Cirencester, UK

G Riley, RWE npower, Swindon, UK

© 2012 Elsevier Ltd All rights reserved

5.04.1 Why Is There a Trend to Build Stand-Alone Biomass Power Plants?

5.04.7.2 Liquid Biofuels for Power Generation/Combined Heat and Power

5.04.11.1.1 Oxides of sulfur (SOx)

5.04.11.1.2 Oxides of nitrogen (NOx)

5.04.11.1.4 Volatile organic compounds 51

5.04.1 Why Is There a Trend to Build Stand-Alone Biomass Power Plants?

The burning of biomass can make a significant contribution to international objectives of CO2 reduction It will provide a dispatchable source of renewable energy at a time when the power grid is becoming increasingly reliant on intermittent wind energy Biomass is seen as a renewable and carbon-neutral energy source as new plants or trees grow in the place of the ones that are harvested, absorbing the same amount of CO2 as is released when the harvested plants are burned The cycle time for this is a few years as opposed to fossil fuels, which take many millions of years to form

There will be some fossil fuel consumed in connection with planting, producing and applying fertilizer, harvesting, transport, etc., but on the other hand, if the plant was left to decompose in nature, it would be likely to produce methane, which is a more powerful greenhouse gas (GHG) than CO2 This results in a negative methane emission of 41 g kWh−1 in a direct-fired biomass power plant burning biomass residue [1]

The fossil fuel used in transport can be replaced with biodiesel and the amount of transport can be limited by using locally sourced biofuels as far as possible, thus reducing the carbon footprint of production and transport Solid biomass fuels are generally

of significantly lower bulk density and have lower energy content per kilogram than fossil fuels, which makes transport more costly

So the preference will be for locally sourced fuels when they are available

So in short, biomass fuels are renewable, sustainable, and environmentally friendly if they are produced and used in a sensible and responsible way, but can also cause irreversible damage to the environment if produced or used in other ways They can benefit local communities and in some cases can even be beneficial to biodiversity They can be used to compensate for one of the major weaknesses of wind power, its intermittent and unpredictable availability, as biomass can be stored and dispatched when needed There are many technical and logistical challenges to fit biomass into the current power infrastructure, but this is likely to change when the generation mix changes as older fossil-fueled power stations are decommissioned

5.04.2 Is Biomass Power Generation Sustainable?

The ability to generate electricity in a sustainable way without long-term detrimental impact on the environment has become a very topical issue over recent years This debate concerns aspects like climate change, biodiversity, deforestation, impact on indigenous populations and wildlife, groundwater levels, use of farmland to grow fuels instead of food, and many more

The increase in the use of biomass from agriculture and forestry for power production as well as for transport fuels has added to pressure on farmland and forest Large-scale production of biofuels will have consequences for biodiversity and water resources It is important that these questions are handled in a sensible and responsible way so that no irreversible detrimental impact is caused The sustainability of energy crops has been extensively researched The results of this work in the United Kingdom are summarized in good practice guidelines for the production of energy crops and extraction of forestry residues [2, 3] UK grants for the production of energy crops are conditional on implementing the recommendations in the guidelines, including recom­mendations on transport distances to the end user

Some early studies into the effect of energy crop plantations on biodiversity indicate that there can even be some positive effects

[4] compared to traditional cereal production

The potential for production of biofuels is large enough (see Figure 1), as biofuel production can support even ambitious renewable energy targets and still adhere to strict environmental standards The European potential for environmentally compatible primary biomass production from agriculture and forestry, for example, has been predicted to increase from around 3.8 EJ in 2010

to around 8.3 EJ in 2030 [5] This does not include residual biomass materials It is estimated that a further 4.2 EJ could be available from sources like agricultural residues, wet manures, wood processing residues, the biodegradable fraction of municipal solid waste, and black liquor from the pulp and paper industry To put this in perspective, the total electricity consumption in the European Union was around 10 EJ in 2007

Creating a sustainable supply chain for biomass supporting biodiversity and adhering to high environmental and ethical standards

is a substantial challenge A separate chapter in this volume addresses biomass sustainability in detail The large scale required to fit into the infrastructure of existing power generation plants and the existing cost structure created by the current electricity prices and support mechanisms for renewable energy will require new logistical solutions Biomass-based power generation lends itself well to

Figure 1 Environmentally compatible bioenergy potential from primary agriculture and forestry in Europe Adapted from [5]

the combined heat and power (CHP) concept where smaller distributed plants are providing heat for district heating to their local communities as well as electricity and are burning locally produced biomass fuels This type of installation can deliver overall conversion efficiencies twice that of a dedicated electricity generation plant although the conversion efficiency for electricity generation

is lower than for a dedicated generation plant This type of installation is obviously easier to implement in colder climates where the need for district heating is higher It can be difficult to install district heating in existing buildings

5.04.3 Life-Cycle Analysis

While power generation from biomass has been promoted as a mechanism for reducing the net emissions of CO2 and other GHGs, there have been concerns over the fossil fuel used for planting, harvesting and transporting the material as well as the manufacture, transport, and application of fertilizers and pesticides Life-cycle analysis (LCA) is a method used to provide information on the cumulative environmental impacts over the life cycle of a process and can be used to assess the overall impact of different alternative fuels The carbon balance for biomass compared to other fuels used in power generation is shown in Figure 2 and a selection of biomass fuels are compared in Figure 3

A fuller treatment of the various ways of computing the LCA of a biomass fuel is the subject of a separate chapter in this volume

5.04.4 How Does Biomass Power Generation Pay?

The decision to invest in a biomass combustion plant will normally be based on commercial considerations This decision will be governed by current and expected future power price, legislation, expected investment and price levels for the fuel, and also the expectations for future government support for biomass combustion Biomass is typically not available at a cost comparable to coal

Figure 2 Comparison of life-cycle CO equivalent emissions from different power generation technologies Adapted from [6]

0 10 20 30 40 50 60 70 80 90 100

Emissions of CO2 equivalents (kg CO2 MWh–1)

Figure 3 Emissions of greenhouse gas from production and delivery of different biomass fuels to power stations in the United Kingdom, expressed as

CO2 equivalents Adapted from [7] PKE, Palm Kernel Expeller cake; SRC, short rotation coppice

or natural gas, so some additional incentive is required to make biomass-firing happen Currently, this incentive is, in most countries, in the form of feed-in tariffs or some sort of obligation/quotas

Many biofuels are internationally traded commodities with highly variable prices over seasons and years Figure 4 shows the level of variation that was seen in the prices of sunflower meal, citrus pulp, wheat feed pellets, rape meal, sunflower husk pellets, and palm kernel expeller cake (PKE) over the period from 2000 to 2007 A high level of covariation between the different commodities is obvious Examples of factors influencing the prices are weather, crop success, freight costs, supply and demand, political stability in the region of origin, relative prices of alternative products and market dynamics of its core market such as paper and pulp, animal feed, board manufacturers, and road transport fuels

The 2003 peak was caused by a combination of factors: very hot weather in southern Europe and Ukraine, reduced crop yield, high freight demand to China, and reduced vessel availability due to port bottlenecks The year 2006/07 saw an even higher price increase driven by poor weather conditions in key areas, high freight costs, low stocks from 2006, increasing proportion of corn going into fuel, changes in attitude to animal feed in Asia, etc It does not help that the market is characterized by a lack of price

Biomass prices the last 7 years

Figure 4 Historical price development for a number of biomass materials Price doesn’t include transport and handling [8] PKE, Palm Kernel Expeller cake

transparency, high volatility, and poor credit rating of some players This is clearly a high-risk environment to make long-term capital investments and the traditional strategy for generators is to avoid high-risk projects

The two most fundamental factors in the commercial evaluation of a potential fuel for a power station are the available volume and price A power generating unit producing 100 MW of electricity at a thermal efficiency of 35% will require in the order of

500 kilotonnes of high-quality biomass fuel per year if it is operating around the clock This is the equivalent of 6 lorries per hour if deliveries take place 8 h a day and the required store to provide a buffer for a long weekend of 4 days would have to hold

5500 tonnes or 8000 m3 if the fuel is wood pellets or PKE but 30 000 m3 if it is dry sawdust This is obviously a situation that requires a high level of logistic control

The traditional commercial model used by many power generators is based on a few large contracts with a few suppliers and large traded units This model is not suitable for domestic biomass fuels as many production units are relatively small farms Exceptions to this are fuels like PKE and olive residue where the fuel is the residual product of a large-scale manufacturing operation The significant extra administration that is required to manage a large number of contracts with smaller suppliers will add to the cost and risk of the use of biomass fuels or create a business opportunity for organizations that are already operating in this type of market, for example, the cereal and grain market

5.04.5 Legislation and Regulation

In 1997, many governments signed up to the Kyoto Protocol and made commitments to reduce their CO2 emissions and help tackle climate change The methods used in different countries to promote this development vary widely By early 2010, at least

83 countries had some mechanism or policy for the promotion of renewable generation Most common is a feed-in tariff, which

is used in at least 50 countries Renewable obligations or quotas are used in 10 countries [9] The legislation promoting renewable technologies is different in each country and is therefore a complex issue and difficult to discuss in general terms in

a way that covers the situation everywhere Below are a few comments on EU legislation from a British perspective

In the European Union, there are a number of directives directly regulating the power industry The EU Integrated Pollution Prevention and Control (IPPC) Directive specifies that best available techniques (BATs) for minimizing the environmental impact of a process should be applied Environmental emissions from power plants are regulated by either the Large Combustion Plant Directive (LCPD) or the Waste Incineration Directive (WID) via the IPPC process, depending

on the fuel The LCPD limits emissions of nitrogen oxides (NOx), SO2, and particulate material from power plants with a thermal input at least 50 MW The WID comes into play when the plant incinerates or coincinerates wastes WID imposes stricter limits on emissions into the air, soil, surface water, and groundwater than LCPD Member states are obliged to report national emissions of listed pollutants to the European Pollution Emission Register (EPER), operating under the umbrella of the IPPC Directive

The LCPD is a European directive and is therefore applicable to all large combustion plants in the European Union It introduces stringent emission limit values (ELVs) for all combustion plants over 50 MWth By 1 January 2008, all ‘new’ combustion plants (those in operation after 1987) had to comply with LCPD or opt out and operate no more than 20 000 h before closing by 2015 at the latest Most plants have been forced to fit flue gas desulfurization (FGD) equipment and make combustion modifications to reduce NOx to meet the LCPD requirements

The Industrial Emissions Directive (IED) was approved by the European Parliament in July 2010 The intention of this directive

is to combine a number of pieces of EU legislation into one single directive and also tighten the emission limits further from those

in the LCPD (see Section 5.04.5.1) The IED is planned to come into force in 2016 and plants that are opted out will be allowed to operate under their current emission limits for 17 500 h between 2016 and 2023 [10]

5.04.5.1 Emission Limits

In the United Kingdom, the EU IPPC Directive has been transposed into the pollution prevention and control (PPC) regime Under PPC, power stations are regulated by the Environment Agency (EA) Permits issued under PPC must be based on the BATs, taking into account the local environmental conditions, geographical location, and technical characteristics of the specific installation This emphasis on the application of BAT has replaced the best available technology not entailing excessive cost (BATNEEC) to reduce the environmental impact of the process BAT does still include an economic assessment but this is of less weight than previously (Table 1)

The WID is an EU directive with the purpose to limit, as far as practicable, negative effects on the environment, in particular pollution by emissions into the air, soil, surface water, and groundwater, and minimize the risks to the environment and human health from the incineration and coincineration of waste The Directive defines stringent operational and technical conditions and emission limits for plants incinerating and coincinerating waste to safeguard a high level of environmental and health protection Despite being an EU-wide regulation, its interpretation has varied between countries One example is tallow, which can be cofired in non-WID-compliant plants in some European countries, while it has been classified as a WID substance in other countries and therefore it is legal to burn it in only WID-compliant plants

Table 1 Emission limits for a large combustion plant under the current LCPD and suggested IED [11]

(existing plant) (existing plant)

SO2, coal plant > 500 MWth (mg Nm−3)

NOx, coal plant > 300 MWth (mg Nm−3) Particulates, coal plant > 300 MWth (mg Nm−3)

400

500 50

200

200 20IED, Industrial Emissions Directive; LCPD, Large Combustion Plant Directive

5.04.5.1.1 Renewable Obligation

The Renewable Obligation (RO) is the UK Government’s primary mechanism to support the production of renewable electricity

It was introduced in April 2002 and obliges electricity suppliers to source an increasing percentage of electricity from renewable sources The obligation rises each year, starting at 3% in 2002/03 in England and Wales and rising to 15.4% by 2015/16 Electricity generators using renewable sources are awarded Renewable Obligation Certificates (ROCs) in proportion to their renewable generation Suppliers demonstrate compliance by redeeming these certificates that they have acquired from the generators via a market mechanism The alternatives are to pay a buyout penalty for each ROC certificate they cannot provide or

to purchase ROCs from a supplier who has a surplus The buyout payments are recycled to those suppliers that redeem ROCs (often referred to as the ‘green smear’ or ‘recycle’) A banded structure was introduced in 2009 where 1 MWh of electricity generated from renewable sources earns a number of ROC certificates ranging from 0.25 to 2 depending on the type of renewable generation used Cofiring of regular biomass earns 0.5 ROC while stand-alone biomass generation earns 1.5 ROC for the same fuel and if it is using energy crops or is a CHP plant, it earns 2 ROC

A further support mechanism for renewable generation in the United Kingdom is the ‘Levy Exemption Certificate’ (LEC), which

is awarded to generators for generation of electricity from nonfossil sources and relieves them from paying the climate change levy

[12], which is an environmental tax levied on electricity, natural gas, coal, petroleum, and hydrocarbon gas One LEC is awarded for each MWh of electricity that is generated from renewable sources

5.04.6 What Technology Choices Are Available?

5.04.6.1 Technology Development

Boilers have been a major part of industrial applications since the industrial revolution in the 1700s and still are Power and heat can nowadays be distributed more efficiently, and larger and more efficient units can be constructed feeding many end users through distribution networks for both electricity and heat

Fire-tube boilers were developed early on, with the hot combustion gases passing through tubes submerged in water that is brought to the boil, producing steam The heat losses in such a system are low as both the fire and the flue gas are kept within the shell containing the water, and thus most heat losses are absorbed by the water The size and steam pressure are, however, limited by the containment capacity of the shell These units are still in use in many places but not for modern power generation

The next development was the water-tube boiler developed in the second half of the 1800s Here, the steam production takes place in tubes with the water flowing through them This has the advantage that the production capacity can be increased by simply adding more tubes and the smaller diameter tubes can contain a much higher pressure than the larger shell On the other hand, the combustion chamber has to be insulated much more heavily as the heat loss through walls is not recovered by the water as in the shell boiler

The insulation problem was later solved by making the furnace walls out of the water tubes, and thus allowing wall losses to be recovered by the water again This is the prevailing technology used in all large- and most medium-sized boilers today

The combustion in a boiler is controlled by four factors:

The temperature has to be high enough for the combustion reactions to take place at a rate that allows complete combustion in the particular plant Combustion time is the final factor and will together with the temperature define the size of the fuel particles that can be used Smaller particles burn faster but need more investment in milling plant as well as more energy for the milling Unburned carbon in the ash is a loss of efficiency, leading to higher fuel costs as well as increasing the problem with deposition of the ash and can make it impossible to use the ash in cement manufacture or construction projects

Increasingly sophisticated methods to control and reduce the emissions of harmful substances in the flue gases have been introduced over the years Textile filters for dust collection have been developed and are no longer only particle collectors Today, they use limestone, sodium carbonate, and active carbon to capture sulfur oxides, heavy metals, and hydrochloric acid Injection of ammonia or urea (SNCR – selective noncatalytic reduction) can be used to reduce the emission of NOx, and if even higher NOx

reduction is required, a catalytic converter (SCR – selective catalytic reduction) can be used

CHP is the simultaneous production of heat and power (i.e., electricity) At a large scale, a CHP unit is part of a power station This is an effective way to improve the efficiency of fuel utilization from below 50% in a conventional power-only generation plant

to 80–90% or even higher in a CHP plant This is done by using the power station waste heat as a heat source for district heating or for some industrial process that does not require high-quality steam The conversion efficiency from fuel to electricity is usually somewhat lower than for a dedicated electricity generator, but the overall efficiency (from fuel to electricity plus usable heat) is much higher If the heat customer does not require constant heat, then the cost of generating electricity during periods of low or no heat demand will be higher This concept is most efficient in countries with a climate that requires buildings to be heated to some degree all year-round or in the vicinity of an industry with a constant need for low-quality heat The CHP concept fits nicely with smaller biomass-fired power stations positioned close to a local fuel source and a district heating network or an industrial heat customer A separate chapter in this volume looks at biomass CHP in more detail

5.04.6.2 Fixed and Moving Grates

The simplest combustion configuration is to build a bonfire on the ground It is a small step to put the fuel on a grate allowing air to pass up through the fuel This is the grate-fired configuration and is the oldest solution used in boilers This configuration can be improved by using a fan to force more air through the grate or by using a more sophisticated grate design like a moving grate, vibrating grate, or a chain grate for better control of the combustion process and higher capacity per square meter of grate The fuel will go through drying, pyrolysis, and char burnout while on the grate, and after complete burnout, the ash will fall off the edge of the grate into the ash pit

The grate does not permit accurate control of the combustion conditions for individual fuel particles as the airflow through the grate varies with the thickness of the fuel bed A thinner area of the bed will allow more air through and this will result in more intense combustion, which will make the bed even thinner The segregated flow of oxygen-rich and lean gas leaving the bed tends to be difficult to mix well Secondary air jets and a contracting cross section in the furnace exit are used to improve mixing The fixed grate consists of a perforated grate that is stationary It is often water cooled and can be sloping, thus allowing the fuel

to slide down the grate when new fuel is pushed onto the feed end This is a mechanically robust construction with lower cost than grates with moving parts

The chain grate consists of a moving belt that the fuel can rest on while it is burning This gives good control of the residence time

of the fuel It looks similar to the traction belt used on tanks and takes the fuel on a journey traveling from one end of the combustion chamber to the other, where it falls over the edge and ends up in the ash pit The belt is made out of metal to resist the combustion heat and is perforated to allow combustion air to pass through it The combustion process is controlled by the airflow through the grate and the speed of the grate

The vibrating grate is based on the same concept as the traveling grate but instead of moving the grate, which is sloping, it is shaken at regular intervals The shaking makes the fuel bed resting on the grate move toward the ash pit This is a much simpler construction than the traveling grate and most moving parts can be kept outside the hot section, but the vibrations cause strain on the mechanical parts of the boiler Burmeister & Wain has built a number of biomass boilers and converted existing boilers to biomass boilers based on a vibrating grate technology They use a water-cooled grate with a low degree of slope and a vibrator that shakes the bed at regular intervals, typically something like 20 s of shaking every 5 min It has, according to the manufacturer, “very high availability, low maintenance and low consumption of spare parts” [13]

The spreader stoker system is a hybrid between suspension firing and grate firing A spreader throws the fuel onto the grate It is often used together with traveling grates or vibrating grates Smaller fuel particles will ignite and burn while still suspended in the air and the larger particles are given sufficient time to burn out after landing on the grate This means that the output from the boiler can

be increased without increasing the load on the grate

The underfeed stoker uses a screw feeder to push fuel up through an opening in the center of the grate This creates a pile of unburned fuel above the screw The fuel will then travel toward the edge of the grate while it is burning This technique is often used

in smaller biomass installations for heating applications but not in power generation

5.04.6.3 Suspension Firing

Suspension firing takes place when small fuel particles are burning while suspended in the combustion chamber This is common in large utility boilers It requires that the fuel particles are small enough to burn before falling to the floor or are carried out of the combustion chamber by the combustion gases

The suspension firing concept allows the load of the boiler to increase in proportion to the volume of the boiler rather than to the area of the bottom surface as is the case for grate combustion The size of a suspension-fired boiler grows much more slowly than

a grate boiler when the output increases Other advantages with this concept are quick response to load changes, low excess air levels, high efficiency, wide fuel diet, a system that is straightforward for automatic control, and a potential for significant upscaling

A modern suspension-fired power station boiler usually allows about 2 s for complete burnout of the fuel particle, which is why coal has to be milled to such a fine powder (< 75 μm) before it is burned It is commonly held that biomass particles that are less dense, more porous, and have a much higher volatile content can be up to 1–2 mm and still burn satisfactorily in such a boiler if the temperature and the oxygen concentration are high enough

Compared with a grate-based system, this means that a costly and energy-demanding milling plant will be required unless the fuel is supplied as a powder of sufficient fineness The suspension-fired boiler will also need burners that introduce the fuel and air into the combustion chamber in a way that creates favorable conditions for mixing and ignition of the fuel and air to create stable combustion A more sophisticated control system is required than for simpler systems like grate firing Another disadvantage with this concept is that peak temperatures can be high, leading to thermal NOx formation

The principal variations to the introduction of fuel and air are wall-fired, corner-fired, and downshot boilers (see Figure 5) The wall-fired boiler has a number of burners, each capable of producing a stable flame, mounted on one or two opposing walls The corner-fired or tangentially fired concept is that the burners are placed in the corners of the furnace and send air and fuel into a fireball in the center of the combustion chamber This means that rather than having individual discrete flames from each burner as

in the wall-fired concept, there is only one flame with lower peak temperatures and longer residence times for the fuel particles The downshot concept is, finally, a variation on the wall-fired theme where the flames are directed downward giving the fuel particles longer residence time, as they move down and turn to leave the combustion chamber through an opening in the top (see Figure 5, left figure) This configuration is mainly used for low-volatile and slow-burning coals such as anthracite

A number of fossil fuel suspension-fired plants have been converted to burn biomass, wood pellets in particular One of the earliest conversions was Hässelbyverket power plant just north of Stockholm in Sweden The plant converted their three boilers of

110 MWth each from coal and oil type to wood pellet firing The pellets are milled in the original Babcock vertical spindle mills and burned in the original burners, both with small modifications Other examples of large plants converted from fossil fuel plants to wood pellet plants are Helsingborg in Sweden and Les Awirs in Belgium There are many examples of smaller oil-fired boilers in the size range from 20 MWth and upward that have been converted to suspension-fire milled wood pellets

5.04.6.4 Fluidized Beds

The fluidized bed boiler is gaining in popularity and has overtaken the grate-fired boiler in biofuel power generation applications The principle is very simple: air is blown through a bed of sand and fuel particles at a velocity that is sufficient to suspend the particles on the airstream but not able to lift them permanently out of the bed, that is, 1–3 m s−1 at 800–900 °C This makes the particle bed behave very much like a bubbling fluid and it is called a bubbling fluidized bed (BFB) This bed of constantly moving sand and fuel particles gives very good contact between fuel particles and the air and also very homogeneous conditions, which make it possible to keep peak temperatures low, resulting in low emissions of NOx The residence time in the bed is long compared

to the conditions in a suspension-fired system The sand particles give the bed well-defined fluidization properties and maintain the function of the bed even during fluctuations of disturbances in fuel feed The good contact and long residence time allow combustion with good burnout of relatively large fuel particles This makes fuel preparation cheaper and less energy demanding This boiler is also more flexible than a conventional boiler with a wider turndown ratio and very good environmental performance The capital cost, finally, is low

A BFB boiler (see Figure 6) has a dense bed where the biomass fuel is dried and pyrolyzed Around 30–40% of the combustion air is introduced through the nozzles at the bottom of the bed (see Figure 7) and the rest in the freeboard above the bed where gases and fine particles burn This type of boiler can handle fuel with a wide range of particle sizes and fuel blends The best performance

is achieved if the majority of the fuel particles are in the size range 5–50 mm Finer particles tend to blow out of the bed and burn in

Downshot Corner/tangentially fired Wall fired

Figure 5 Principal configuration of suspension-fired boilers (courtesy of RWE npower)

Foster wheeler BFB boiler

157 MWth, 37 MWe, 60.2 kg s–1, 105 bar, 535 °C

Äänevoima Oy, Äänekoshi, Finland

Figure 6 Modern bubbling fluidized bed (BFB) boiler (courtesy of Foster Wheeler with permission)

Figure 7 Primary air nozzles in fluidized bed boiler (courtesy of Foster Wheeler with permission)

the freeboard causing hot zones, which increases NOx production as well as slagging tendencies Too large particles will not fluidize properly and can cause the bed to collapse

Common sizes for BFB boilers are 10–300 MWth Currently, the largest BFB power boiler for biomass fuels in the United Kingdom is the 44 MWe boiler at Stevens Croft in Scotland

The circulating fluidized bed (CFB) boiler (see Figure 8) takes the principle of the bubbling bed one step further and with increased fluidization velocity the particles are lifted out of the bed and follow the gas out of the combustion chamber They are then separated from the gas in a cyclone or beam separator and returned to the bed This usually takes place at velocities of 5–10 m s−1

and allows higher turbulence levels and a higher combustion density resulting in more compact boilers The size of a CFB increases more slowly than a BFB when the steam capacity is scaled up (see Figure 9) A modern biomass CFB boiler can be operated with

NOx emissions of less than 150 mg Nm−3, less than 200 mg SOx and CO Nm−3, and less than 20 mg dust Nm−3

A weak point in the CFB boiler design is the particle separator, which is large and has traditionally been lined with thick refractory that is exposed to heavy erosion from the fuel particles Recent advances in design have made it possible to reduce the amount of refractory by using steam-cooled cyclones and thinner refractory

Another important development in CFB design has been to move the final superheater to the return leg of the particle separator

It is covered by the hot recirculating particles, which are gently fluidized to control the recirculation rate This creates an

Figure 9 Historical sizes of installed circulating fluidized bed boilers (courtesy of Foster Wheeler with permission)

environment with high heat transfer, low concentrations of corrosive gases, lower peak temperature, and a constant cleaning of the superheater tubes by the fluidized particles This design can operate at higher steam temperatures and with more corrosive fuels than the traditional design, and use cheaper steel qualities in the superheater

The most common cause of corrosion in a CFB boiler is the chlorine-induced corrosion This occurs mostly in the convective heat exchangers It tends to appear together with high fouling rates due to the presence of alkali and chlorine in fuels that are high in calcium and potassium but low in silica The mechanisms of this corrosion are currently not well understood but it can be aggravated by the presence of Zn, Pb, and metallic aluminum This is particularly important if the fuel contains recycled biomass materials that often contain significant amounts of chlorine (from PVC and wood preservatives) and lead and zinc (from paints and wood preservatives)

Alkali chloride corrosion takes place at metal temperatures above 450 °C in carbon steels, and lead/zinc/tin chlorides are corrosive at metal temperatures between 360 and 440 °C It is therefore important to choose the steam and gas temperatures in a plant intended to burn recycled material so that these temperature windows are avoided as far as possible

The bed sand has to be chosen carefully to reduce the agglomeration risks with the chosen fuel No sand works well with all biomass fuels, so a matching procedure is required Wood ash can react with SiO2 from the bed sand at temperatures as low as

700–900 °C, forming a layer of calcium or potassium silicate on the bed particles and causing agglomeration Herbaceous and agricultural fuels like straw, cereals, and grass do not have the same problem with quartz sand as wood does

One way to reduce the problem of bed agglomeration is to increase the removal rate of bed material and the addition rate of makeup material This reduces the amount of alkali material available in the bed for the formation of a sticky coat on the bed particles A better way is to change the bed material to a less troublesome one if such a material can be identified

The main trend these days seems to be that fluid bed technology is used in most biomass boilers, with the simpler BFB for smaller installations and CFB for anything over 50 MWe Leading suppliers [14] are offering commercial solutions for 300 MWe

plant fired on 100% biomass and supercritical plant up to 800 MWe for cofiring biomass with coal The main reason for not offering supercritical biomass plant is lack of steels that can resist the corrosion at the metal temperatures reached in such a plant

5.04.6.5 Gasification

Power generation by gasification takes place in two steps The first is the actual gasification, which is a pyrolysis process where the solid fuel is heated with insufficient oxygen for combustion The result is a gas that can be burned, which is the second step The combustion takes place as a separate process physically removed from the gasification process The combustion can take place in a gas turbine, a combustion engine, or a boiler This technology is still in the development phase and has been fraught with technical difficulties Essent is now operating a gasifier commercially at its Amercentrale power station in Holland This gasifier uses waste wood and feeds the resulting gas to the 600 MWe CHP boiler, Amer 9 Gasification is covered in detail in a separate chapter in this volume (see Chapter 5.10)

5.04.7 Potential Biofuels

5.04.7.1 Solid Biofuels

Biofuels are completely different from fossil fuels in almost every possible way As a generalization, solid biomass fuels as a group are soft and fibrous products from plants that have been harvested recently as opposed to hard and brittle coal, which has been geologically deposited deep underground for over 50 million years Biomass tend to be produced on a much smaller scale than the large mining operations used to extract coal and both the bulk density and the heat content per weight of biomass are considerably lower than for coal while the moisture content of fresh biomass is higher All this means that transport and handling costs are a much larger part of the total cost of the fuel

The fact that the properties are so different means that equipment developed for the agricultural sector can sometimes be more suitable than power station equipment that is designed for coal Although the biomass tends to generate a dust that stays airborne longer than coal, it is not experienced as dirty in the same way as coal dust Most dry biomass needs to be stored under cover to avoid self-heating, decomposition, and growth of spores and fungi, although product development is changing this situation Thermally treated wood pellets that are much more hydrophobic are starting to enter the market While they are more expensive to produce, they can be stored in the open, saving the investment in undercover stores

Both the lower bulk density (200–800 kg m−3) and the lower calorific value (typically 15–20 MJ kg−1 on a dry basis) compared to coal means that the volume to store as well as required transport capacity for biomass is significantly larger per gigajoule One method to increase the bulk density is pelleting or briquetting, which also improves the bulk handling properties of the fuel, and the drier pellet/briquette will have a higher calorific value, thus reducing the costs for transport, but it is also more expensive to make (Figure 10)

The ash content of many biomasses is lower than that of coal, that is, less than 5%, with the ash content of clean stem wood often well below 0.5% There are also biomass types with very high ash content like rice husks that can have more than 15% ash The composition of the ash in most biomass is more troublesome than coal ash from a slagging/fouling/bed agglomeration point of view with a high content of alkali like calcium (Ca) and potassium (K) Calcium- and potassium-containing ash deposits very easily

on surfaces forming fouling in the form of CaO, CaSO4, and K2SO4 These deposits harden on superheaters if not removed frequently by soot blowing Some biomass fuels can have high concentrations of chlorine, which increases the risk of fouling and particularly high-temperature corrosion in the superheaters, but most of them contain significantly less sulfur than the average coal

Figure 10 Wood pellets (courtesy of RWE npower)

The format of delivery for biomass fuels is different than for coal as well and requires different systems for reception and handling The fuel handling system on a power station has to be designed for particular types of fuel as the handling characteristics vary widely Take straw and coconut shell as two extreme examples: it is easy to see that the requirements and volumetric capacity for

a plant delivering the same power output would be completely different for these two fuels

The chemical characteristics of the fuel in question have a strong impact on its combustion (carbon burnout, slagging, fouling) and environmental (NOx, SOx, dust) performance as well as fly ash salability, load capability, and overall plant efficiency Some of the key factors here are calorific value, moisture, ash content, ash composition, trace element content, and ash fusion temperature The most important domestic biomass fuel in the United Kingdom is currently wood It comes in many forms like virgin wood from the forest, recycled wood from packaging and other sources, demolition wood, forestry residue, and energy crops like willow and poplar Wood is usually delivered as sawdust, wood chips, or pellets Dry wood has a calorific value in the order of

19–20 MJ kg−1 and typically well below 1% ash The moisture content varies, with wood pellets typically containing less than 8% and recently felled wood up to 60%

Energy crop is anything that is grown specifically for the purpose of using it as a fuel Some of the most popular energy crops are Miscanthus giganteus (also known as elephant grass), Salix (also known as SRC willow), Populus (also known as SRC poplar), switchgrass, and reed canary grass (Figures 11 and 12)

5.04.7.1.1 Globally sourced biomass

Olive residues, PKE (see Figure 13), shea meal, and other residues such as sunflower husks and citrus pulp pellets are currently produced in the Mediterranean, South East Asia, Indonesia, South America, and Africa and exported to Western countries for use as fuel in power stations Other examples of biomass fuels on the market are peanut husks, cocoa meal, coconut fiber, and soy residue They are all by-products from other primary products like olive oil and palm oil Many are traditionally used for animal feed but have in recent years found an alternative market as fuels for power generation

Figure 11 Miscanthus (furthest away), switchgrass, and reed canary grass (nearest) (courtesy of Rothamsted Research, UK with permission)

Figure 12 A field of Miscanthus crop in southern England (courtesy of RWE npower)

Figure 13 Palm oil fruit (∼40 mm long) The central kernel can be seen in the left picture Palm kernel expeller cake is the residue from extraction of the oil content of the kernel (courtesy of RWE npower)

5.04.7.1.1(i) Delivery format

Biomass fuels are available in a number of different formats, varying from a fine dust and sawdust to chips, pellets, briquettes, and bales and as liquids

Chips and dust are the formats requiring least postharvest processing and are often the cheapest fuel if local production is available Chipping can be done directly in the forest using a mobile plant The chips are typically between 10 and 50 mm They can

be milled to form wood dust (sawdust) They have the advantage that they can be stored in the open as long as they are carefully monitored for self-heating and spontaneous ignition But their bulk density is substantially lower than that of pellets, so transport will be more expensive per unit of energy

Pellets and briquettes are generally more cost effective to transport due to their higher bulk density of typically 600–700 kg m−3

and are less prone to ‘hang-up’ in the bunkers and conveyors They are, though, considerably more expensive to produce Pellets are biofuel compressed into small cylinders with a typical diameter of 5–15 mm and a length of 10–50 mm (Figure 10) They have a higher and more standard bulk density than the raw materials and being clean and dry (< 10% moisture) they are easier to transport and handle They can be stored much longer than other wood sources but they can be very dusty and have to be stored under cover and dry

Biomass can also be delivered to the power station in bales This format is mostly used for straw and requires special equipment

to remove strings and break up the bales or a plant designed specially to burn bales Bales are relatively easy to transport and have a good bulk density They can also be stored in the open for shorter periods of time A modern large bale can weigh 300–500 kg Forestry residues are any part of the tree remaining when the primary product (logs) has been removed These residues are collected and either chipped in the forest or compressed into bales and transported from the forest by lorry for chipping by the end user, which again requires specialized equipment, that is, a chipping plant

5.04.7.2 Liquid Biofuels for Power Generation/Combined Heat and Power

Diesel and heavy fuel oil can be replaced with liquid biomass fuels like rapeseed oil, palm oil, tallow, and tall oil Palm oil has become very sensitive from a public relations point of view due to all the media coverage around palm plantations and their