Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis

Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis Volume 5 biomass and biofuel production 5 10 – biomass gasification and pyrolysis

DJ Roddy, Newcastle University, Newcastle upon Tyne, UK

C Manson-Whitton, Progressive Energy Ltd., Stonehouse, UK

© 2012 Elsevier Ltd All rights reserved

5.10.6.2.6 Electrostatic precipitation (wet and dry)

5.10.6.2.7 Specialist tar removal and tar destruction techniques

Comprehensive Renewable Energy, Volume 5 doi:10.1016/B978-0-08-087872-0.00514-X 133

5.10.9.4.2 Hitachi Metals Ltd., Japan 151

However, biomass in its natural state is a very different resource from oil and gas It is a distributed, heterogeneous fuel with a low gravimetric and volumetric calorific value (CV) It is not a direct replacement and is therefore not a fungible resource Biomass can be used in its natural state for the direct provision of heat via combustion, which can in turn be converted to motive power using the Rankine cycle However, this is a limited use of a valuable, renewable hydrocarbon resource, and does not address the need for liquid fuel, chemical feedstock, or even readily deployable heat or power generation at moderate scale with acceptable efficiencies

Converting biomass to a resource that has similar characteristics to the oil and gas it seeks to replace is a valuable strategic intent Techniques such as gasification and pyrolysis can provide gaseous, liquid and solid analogues for natural gas, oil, coal, and derivatives

A number of such ‘substoichiometric technologies’ have been developed for coal in the past few hundred years The worldwide gasification capacity at scale (plants above 100 MWe) now stands at 70 817 MW thermal (MWth) of syngas output at 144 operating plants with a total of 412 gasifiers [1] Equivalent developments for biomass offer the prospect of converting biomass into an energy-dense material that can be moved at low cost to the places where its energy and other attributes can be best used in high-efficiency operations

This chapter begins with a review of the history of gasification and pyrolysis technology development in Section 5.10.2 before moving on to an outline of the basic science and technology that underpins gasification and pyrolysis processes in Section 5.10.3 Section 5.10.4 explores the main types of gasifier design (fixed and fluidized beds, entrained flow (EF), and plasma) Many of the challenges with gasification lie in the provision of suitable feedstocks (covered in Section 5.10.5) and in processing the gas stream to the standards required in various end uses (covered in Section 5.10.6) After recapping on the main gasification system options (Section 5.10.7) and pyrolysis arrangements (Section 5.10.8), a series of case studies is presented in Section 5.10.9 to illustrate how the various elements have come together in practice The chapter concludes with a review of recent and likely future developments and some suggestions for further reading

The material presented in this chapter provides a critical link between other chapters in the Biomass/Biofuels volume of this major reference work Other chapters examine different ways of growing various types of biomass and ensuring their carbon and sustainability credentials, limitations on the ability of biofuels to meet all future demands without a migration from first-generation fuels toward synthetic fuels, technologies for producing synthetic fuels from synthesis gas, and alternative uses for synthesis gas for sustainable production of organic chemicals in addition to fuels At the center of all of that stands the pyrolysis and gasification system required for converting biomass into good quality synthesis gas

5.10.2 Historical Development

While there are tens of thousands of gasifiers operating globally across a range of scales, the majority of the capacity is fueled by coal

Of those that operate on biomass, most have been used for heat and some power applications via steam raising rather than for production of high-quality syngas There is very limited experience of biomass gasifiers providing the quality of syngas necessary for power generation in engines or for conversion to biofuel

Significant advances were made during the oil crisis of the 1970s, although oil price decline halted significant further develop­ments Today’s oil price, in addition to the recognition of the need to address climate change is driving activity in this sector It is only over the past 5 years that biomass gasifier systems have come close to commercial operation, with probably fewer than 50 operating worldwide generating biofuels or power in excess of 1 MWth input rating This relative infancy is due to a number of factors discussed in Section 5.10.10 However, a number of suppliers are currently installing their first ‘commercial’-scale products The production of gas from coal began in 1665 in England [2] In early processes, the coal was converted into coke (the main product) and coal gas (a by-product) by heating it in an airtight furnace (or coke oven) using additional coal as an external fuel This

was effectively a pyrolysis process Larger-scale gasification processes were developed toward the end of the eighteenth century

to provide gas in large quantities, based on converting coke into hydrogen and carbon monoxide [3] Coal gas was first used for lighting purposes in Philadelphia in 1796 The first work on studying gas production from wood was done by P Lebon in

1791 By the 1850s, ‘town gas’ (produced from the gasification of coal) was widely used in London for lighting Winkler started developing the fluidized bed coal gasifier in 1922 The growth of gas works continued until the oil and gas industry started to introduce cheap fuels Over time, the use of industrial gas extended from direct use in lighting and cooking to heating, and then as a chemical feedstock for producing ammonia, methanol, and their many derivatives including various fertilizers More recently, it has been used for electricity generation and ultimately for liquid transport fuel production Parallel developments took place in steam drum and piping technology, leading to gastight equipment that could be operated at pressures above 2 bar – and therefore more compact installations Fully continuous gasification became possible with the commercialization of cryogenic separation of air into oxygen and nitrogen in the 1920s This led to developments like the Lurgi moving-bed pressurized gasification process in 1931 and the Koppers-Totzek EF process in the 1940s [3]

Terminology usage has varied over time and between countries ‘Town gas’ is usually derived from coal, ‘wood gas’ from biomass, and ‘water gas’ from coke Many prefer to reserve the term ‘synthesis gas’ for mixtures of hydrogen and carbon monoxide (only) irrespective of the feedstock Some people use the term ‘producer gas’ to cover all of the above: others reserve it for partial oxidation of coke using humidified air Producer gas was first used to power an internal combustion engine in 1881, with the engine

‘sucking’ the gas from a gasifier – hence the additional term ‘suction gas’

During the Second World War, there was an upsurge in interest in gasification as a source of fuels at a time when fuel supply was problematic Small gasifiers running on charcoal and wood were readily available in the 1940s with more than a million small units

in operation [4] Fuel quality and exhaust emissions are likely to have been highly variable Once liquid fuels became readily available again, interest in gasification fell away However, work continued in developing countries such as China, and then South Africa, Brazil, the Philippines, and Indonesia

The oil crisis in 1973 triggered new interest in coal and biomass gasification, and this was sustained by the 1980 oil crisis In South Africa, Sasol used coal gasification and Fischer–Tropsch synthesis [5] as the basis of their synthetic fuels and petrochemicals industry, making their facility the largest gasification center in the world [6] Commercial facilities with high-value end products have tended to

be more immune to downward swings in oil and gas prices [7] While interest in biomass gasification in developed countries has been intermittent, developing countries have tended to demonstrate an ongoing interest in gasification of agricultural wastes, particularly for energy supply in remote areas Developed countries are now looking more widely at their increasing levels of organic waste production

in the context of resource conservation and climate change abatement, and see gasification as a versatile process for converting organic waste into a range of energy forms, including high-specification transport fuels via the latest gas-to-liquids technologies [8]

5.10.3 Basic Gasification Technology

Gasification is a process in which a solid material containing carbon (e.g., biomass) is converted into a gas by reacting it at high temperature with oxygen which is present at levels insufficient to support complete combustion The aim is to produce a synthesis gas (or syngas) consisting mainly of hydrogen and carbon monoxide Syngas can then be used for chemical or fuel synthesis (hence the name), or as a fuel for direct combustion

The main steps are:

Biomass is heated in a pyrolysis stage to drive off the volatile components that typically make up 70–86% of the dry biomass, leaving a solid char (or biochar) Depending on the details of the gasifier, the heat can come from external sources or from combustion of some of the pyrolysis products The volatile components are mainly hydrogen, carbon monoxide, carbon dioxide, methane, hydrocarbon gases, tar, and water vapor Gas stream composition depends on pyrolysis temperature, pressure, and residence time, as well as the nature of the biomass feedstock Where the heat for the gasification stage comes from combustion of a proportion of the pyrolysis char inside the gasifier, the exothermic reactions are represented by the equations:

C þ O2 → CO2 ΔH ¼ −393:8 kJ mol− 1 ½1

1

C þ O2 → CO ΔH ¼ −123:1 kJ mol− 1 ½2

2 Next comes the gasification stage proper, where higher temperatures crack tars and hydrocarbons in the pyrolysis gas stream and char are partially oxidized Carbon is converted into CO and hydrogen in a reaction called the water gas reaction in which carbon reacts with water vapor derived from the original biomass:

C þ H2O ↔ CO þ H2 ΔH ¼ 118:5 kJ mol− 1 ½3 Another key gasification reaction is the Boudouard reaction:

C þ CO2 ↔ 2CO ΔH ¼ 159:9 kJ mol− 1 ½4

In these reversible, endothermic reactions (3 and 4), higher temperatures favor the production of hydrogen and carbon monoxide Lower pressures also favor the production of carbon monoxide, while higher pressures favor the production of carbon dioxide

The other main reaction is the exothermic water gas shift reaction in which CO reacts with steam to form CO2 and additional hydrogen:

CO þ H2O ↔ H2 þCO2 ΔH ¼ −41 kJ mol− 1 ½5 There is also an important methanation reaction:

C þ 2H2 ↔ CH4 ΔH ¼ −87:5 kJ mol− 1 ½6 The above reactions and others involving feedstock impurities take place simultaneously during the gasification process The relative proportions of gases at the gasifier exit depend on process conditions and the composition of the biomass feedstock The position of the steady-state equilibrium depends in the normal way on temperature and pressure, but at low temperatures the rate of reaction may be so low that equilibrium compositions are never reached in practice For example, below 700 °C, the water gas shift reaction proceeds so slowly that the product composition is said to be ‘frozen’ [2] Gas–solid reactions are slow compared with the gas-phase reactions Another key parameter affecting the outlet gas composition is the amount of oxygen relative to what is required to support complete combustion

5.10.4 Gasifier Designs

A wide range of gasifier configurations have been developed globally, each tailored to different feedstock materials (type and form), different scales, and different required qualities of syngas There are three basic forms of gasification system: fixed bed (updraft and downdraft), fluidized bed, and EF

The main points of difference relate to where biomass is fed into the gasifier (top or side), how it is moved around (under gravity

or via gas flow), the temperature at which it is operated (and in particular whether it is above or below the ash/char melting point), the operating pressure, and the choice of oxidant (oxygen, air or steam)

Some people draw a major distinction between low-temperature gasification ( < 1000 °C) and high-temperature gasification (>1200 °C) With low-temperature gasification, the desired products (hydrogen and CO) typically contain only half of the energy in the gas stream, with the remainder being contained in the methane and higher hydrocarbon tars [3] With high-temperature gasification there is limited methane and/or tar formation, and the gas cleanup and recovery system is therefore simpler Gasification processes typically seek to operate either below the ash softening point (above which it starts to become sticky and prone to agglomeration) or above the slagging temperature (whereupon it becomes fully liquid and therefore removable)

In addition, there are slight variants using techniques such as indirect gasifiers (where heat is applied externally rather than autothermally from partial combustion of the biomass in the gasification stage), plasma arcs, and molten metal baths, either as a core part of the gasification process or as a means of cleanup

5.10.4.1 Fixed Bed

In a fixed bed gasifier, gas flows relatively slowly through a bed of fuel, which therefore must have good permeability This means that ‘lumpy’ feedstock is required rather than crushed or pulverized form The oxidant can be air or oxygen, although commonly for biomass facilities air is used In a fixed bed gasifier, there are four distinct thermal zones In the drying zone, remaining moisture in the fuel is evaporated In the pyrolysis zone, the material is heated to 300–400 °C with no added oxygen, generating a pyrolysis gas laden with liquid hydrocarbon tar and a char In the gasification (reduction) zone, typically in excess of 800 °C, the majority of the char is converted to a syngas In the combustion zone, in excess of 1000 °C, the remaining char is fully combusted, providing the heat required for the reactions in the other zones

There are two main versions of the fixed bed gasifier: ‘updraft’ and ‘downdraft’ In both cases, fuel is added into the top of the gasifier In an updraft gasifier, the oxidant is injected into the base of the vessel and the syngas exits from the top (so the biomass and gases move in opposite directions), and some of the char burns as it falls to provide heat In a downdraft gasifier, the syngas exits through the base of the vessel, while the oxidant is fed in at the top or the side (so the biomass and gases move in the same direction) Some of the biomass is burnt as it falls, and then forms a bed of hot charcoal The different configurations dictate the relative positions of the four zones discussed above

Both types of gasifier are relatively simple, and therefore lend themselves to smaller-scale facilities Updraft gasifiers have high thermal efficiencies (due to high charcoal burnout and good internal heat exchange), can accommodate higher moisture feedstocks (as the countercurrent gas flow dries it from the point of entry), and the relatively low temperature of the raw syngas is suited to the gas cleanup units Additionally, updraft gasifiers can be configured at a wide variety of scales from 10 kWe to > 30 MWe However, in an updraft gasifier, the tar laden gas from the pyrolysis zone passes out in the syngas, whereas in a downdraft gasifier, the tar passes through the combustion zone and therefore can be cracked at temperature Crudely, updraft gasifiers generate of the order of 100 g Nm−3 of tar, whereas downdraft gasifiers produce only 1 g Nm−3 This has a significant impact on the downstream syngas cleanup required This major advantage tends to outweigh the disadvantages of downdraft gasification that include higher particulate carryover, slightly lower gasifier efficiency (due to the relatively high temperature of the exit gases), some scale-size limits, and tighter constraints on feedstock quality in terms of particle size and moisture content Pelletization or briquetting of the biomass is

often necessary The gasifier throat configuration is critical in ensuring tar destruction In general, this constrains the maximum gasifier size to approximately 8–10 MWth (and often smaller), although the units lend themselves to multiple trains to increase capacity

Excessive tar formation can occur during unsteady operation or periods of part-load operation [2] Care needs to be taken before restarting a fixed bed gasifier to ensure that all combustible gases have been vented Fuel blockages and high-temperature corrosion are other common problems They sometimes suffer from product gas nonuniformity as a result of flow maldistribution, but generally they offer high levels of thermal efficiency albeit at relatively low throughputs Slight variations on fixed bed gasifier configurations, particularly with regard to oxidant injection points and gas flow, can offer further optimization of the various attributes Such developments tend to be proprietary

5.10.4.2 Fluidized Bed

This technology was originally developed by Winkler in 1926 for large-scale coal gasification In a biomass fluidized bed gasifier, solid crushed fuel particles are suspended together with a much larger mass of fine inert bed material (e.g., silica sand, dolomite, or even the ash from the fuel itself) in high gas flow New feed particles are mixed with those already undergoing gasification The ash can be discharged dry or agglomerated The low temperatures (< 900 °C) in this gasifier allow the use of reactive feedstock Some fluidized bed gasifiers are designed to be operated under pressure The high gas volumes required for fluidization mean that these gasifiers are often air-blown although oxygen-blown systems are feasible The gas exits the chamber at the top In this type of gasifier, the four stages (drying, devolatilization, gasification, and combustion) are not stratified as in a fixed bed gasifier, but occur simultaneously The tar levels in this type of gasifier are at an intermediate level between up- and downdraft systems at a nominal

∼10 g Nm−3

At start-up, an external means of bringing the sand up to temperature is required During normal operation, a proportion of the injected biomass is combusted in a controlled flow of oxidant in order to maintain the bed temperature Fluidized bed gasifiers are more compact than fixed bed because the intensive mixing in the bed leads to good heat exchange and high reaction rates They can operate at lower reaction temperatures and can thereby tolerate biomass feedstocks with a lower ash melting point or a highly corrosive ash A drawback is that carbon burnout is incomplete because of the range of residence times seen by individual particles There are several types of fluidized bed gasifier

5.10.4.2.1 Bubbling fluidized bed

Biomass is fed in from the side (see Figure 1), with air, oxygen, or steam being blown upward through the bed at a rate that is just high enough to keep the material agitated – typically 1–3 m s−1 Good mixing leads to a faster pyrolysis reaction than in a fixed bed gasifier [10] Oxygen gives a higher quality syngas than air The modest temperatures result in reasonably high levels of methane production Reactors designed to have a larger head space above the bubbling bed tend to produce lower tar levels in the syngas stream [10] Particulate levels can be high as a result of particle attrition in the fluidized bed A cyclone at the syngas exit point catches the ash and char particles Bubbling fluidized bed (BFB) gasifiers have run on many different biomass feedstocks and tend to

be quite tolerant of variation in particle size and moisture content because of the quality of the mixing One of the main risks is bed agglomeration, which can occur when biomass feedstocks with too low an ash melting temperature are employed

Technology providers for BFB gasifiers include Carbona, Foster Wheeler, Enerkem, TRI, EPI, and Iowa State University [9]

5.10.4.2.2 Circulating fluidized bed

Biomass is fed in from the side (see Figure 2) A higher air/oxygen/steam velocity is used (typically 5–10 m s−1) in order to keep the biomass suspended, with the particulates being returned to the fluidized bed via a cyclone and siphon Higher velocities lead to higher levels of particle attrition and therefore higher concentrations of particulates The gasifier needs to be designed against

Biomass

Air/Oxygen Syngas

steam

Figure 1 Bubbling fluidized bed gasifier Reproduced with permission from E4Tech (2009) Review of technologies for gasification of biomass and wastes NNFCC Project 09/008 www.nnfcc.co.uk

Figure 2 Circulating fluidized bed gasifier Reproduced with permission from E4Tech (2009) Review of technologies for gasification of biomass and wastes NNFCC Project 09/008 www.nnfcc.co.uk

erosion by high-velocity particles Circulating Fluidized Bed (CFB) gasifiers tend to switch easily between different feedstocks provided the size is kept below 20 mm The cyclone is designed to separate out both the ash and the bed material and return them to the reactor

Smaller particles tend to be gasified on the first pass and carried over while larger particles remain behind until they have become sufficiently consumed to be carried over into the external recycle loop This makes CFBs particularly suited to the gasification of biomass where particle size and shape can be difficult to control Carbon burnout is considerably better than with a BFB gasifier Pressurized operation is also possible, and is cost-effective if the syngas is required to be pressurized for downstream use [3] A

20 bar system has been developed by Foster Wheeler, using lock hoppers for pressurizing the biomass feed and for primary ash removal

The most prominent technology provider for CFB gasifiers at present is Foster Wheeler, but designs are also being developed by VTT, CUTEC, Fraunhofer, VVBGC, and Uhde/TUB-F [9]

5.10.4.2.3 Dual fluidized bed

Here there are separate, but linked, CFB reactors for gasification and combustion Biomass is fed into the gasifier and converted into syngas and char using steam Suspended char and sand drop into the combustor where the char is burnt in air whose velocity is sufficiently high to keep the heated particles of sand suspended and drive them through a cyclone The cyclone returns the hot particles to the gasifier while releasing syngas The use of steam not only boosts the concentration of hydrogen in the syngas but also increases the methane content The main advantages are the ability to optimize gasification and combustion separately, and the ability to produce a relatively low-nitrogen syngas using air rather than oxygen for the combustion part Cracking catalysts are used

to break down any heavy hydrocarbons in the syngas stream, along with a scrubber for alkali and particulate removal

The technology is at a relatively early stage of development, with REPOTEC, SilvaGas, and Taylor Biomass among the active players [9]

5.10.4.3 Entrained Flow

In an EF gasifier (see Figure 3), pulverized fuel particles and gas flow concurrently and rapidly with inherently short residence times (a few seconds) in the gasifier reactor This type of gasifier can also process atomized liquid feedstock or slurries It is the most common technology for processing coal, and has featured very prominently in successful coal gasifiers since the 1950s [1] All EF

Biomass

Syngas

Air/Oxygen steam

Biomass

Oxygen Steam

Slag Syngas Figure 3 Entrained flow gasifiers Reproduced with permission from E4Tech (2009) Review of technologies for gasification of biomass and wastes NNFCC Project 09/008 www.nnfcc.co.uk

gasifiers are slagging (resultant ash is fused and discharged as molten slag) as the processing temperature is higher This is an important aspect of the design since the formed slag serves as part of the inner vessel wall, providing a heat and corrosion protection layer Liquid slag viscosity must be controlled such as by adding a suitable fluxing agent such as limestone These gasifiers generally use pure oxygen as an oxidant, but many are not suited to waste or biomass streams because these cannot be slurried, and the biomass particles (compared with coal) do not lend themselves to dry-feed Therefore, they are usually only used for biomass in conjunction with pyrolysis preprocessing The majority of EF gasifiers were developed for coal operation and are large-scale, typically at 600 MWth or larger [1]

The high temperatures (typically 1200–1600 °C) of an EF gasifier provide extremely low levels of tar as a result of extensive thermal cracking [11]and very low methane content High temperatures also favor hydrogen and CO production over methane and CO2 A drawback is that the extensive cooling required prior to gas cleanup reduces the thermal efficiency The cost (and energy) penalty associated with satisfying the high oxygen demand of an EF gasifier is significant Operating pressure can be up to 100 bar

EF gasifiers may be able to accept a mixture of feedstocks provided the particle size is adequate and the composition remains reasonably steady over time Given the short residence time involved, it is normal to grind the biomass down to a particle size of less than 1 mm Typically, the feedstock moisture content must be below 15% Unlike other gasifiers, the EF gasifier needs a pilot flame

to provide the initial injection of energy The advantage of a high-quality syngas is offset by the need for a pulverized feedstock EF gasifiers designed for coal operation can sometimes accept 10–15% biomass in a coal blend [2]

Several companies are developing EF gasifier designs for biomass gasification The most prominent are Choren and Range Fuels, with a number of other companies at an earlier stage of development, for example, Pearson Technology, FZK/KIT, and Mitsubishi Heavy Industries [9]

5.10.4.4 Plasma

Plasma is generated by high-voltage discharge between graphite electrodes The torch can reach temperatures of 6000–10 000 °C, which will convert hydrocarbon solids, liquids, and gases to H2 and CO Syngas composition can be regulated by controlling the plasma torches to compensate for variations in feed rate and composition and achieve a steady gasifier temperature Such systems can

be configured such that the plasma is generated in the core gasification vessel, with a body of feedstock not dissimilar to an updraft gasifier configuration, in which case no oxidant is required at all Any inorganic matter is vitrified into an inert slag Alternatively, the plasma can be applied in a separate vessel, downstream of a more conventional gasifier, where the impure syngas, liquids, and residual char are converted to carbon monoxide and hydrogen – high-quality syngas Plasma gasifiers are normally designed to run on waste, with a particular initial interest in medical waste Any heavy metals contained within the waste tend to come out in the vitrified slag The biggest players at present are Westinghouse and Plasco, with InEnTec, Startech, and Solena Group also active in the field [9]

5.10.4.5 Choice of Oxidant

Gasifiers can use oxygen or air as the oxidant Systems that use oxygen tend to be more costly (10–15% increase in capital cost), and also entail the cost or parasitic load of oxygen production (equivalent to 5–7% on operating costs) [7] However, for liquid fuel or Bio-SNG production, oxygen tends to be necessary to avoid nitrogen dilution in the gas stream and to keep the cost of high-spec gas cleanup within acceptable bounds Nitrogen is, however, not a problem where the intended end use is ammonia synthesis Most EF gasifier developers opt for an oxygen-blown design, particularly where fuel synthesis is the aim Fluidized bed developers tend to offer both air and oxygen depending on the application Steam can also serve as an oxidant as well as an indirect heat source, or it can serve as a moderator to reduce the gasification temperature relative to a pure oxygen system

5.10.5 Gasifier Feedstock Supply

It is particularly important to understand the properties of candidate biomass fuels in undertaking process design and specification, especially with respect to fuel preparation and handling and gasifier operations Standards do exist for solid biofuels of all types: the EU has developed via CEN/335 a comprehensive approach to the classification and standardization of solid biofuels and this should be used in transactions between seller and buyer and by process designers in order to assure reliable and certifiable operational conditions The essential first condition that must be satisfied is that feedstock specification and the process design are matched; the gasifier

in particular cannot be omnivorous For gasification, important feedstock attributes are:

• morphology (size, shape) – affecting pressure drop across the bed and consistency of operation

• moisture content – drier feedstocks give a higher quality gas

• energy content – on a dry and ash-free basis most biomass provides about 19 MJ kg−1, but in practice the figures vary considerably

• volatile matter content – important for tar production levels

• elemental composition – important for energy content and for critical contaminants (particularly levels of halogens, sulfur, arsenic, and mercury)

• ash fusion characteristics – both the quantity of ash and its melting characteristics are important

• bulk density – important for energy density, ease of material handling, storage costs and transport costs

Here waste biomass feedstocks are considered first, followed by virgin biomass feedstocks before looking at feedstock handling and reception requirements for a reliable gasification facility

5.10.5.1 Waste Biomass Feedstocks

Over 98% of the potential UK biomass resource is from waste products [12] Municipal, commercial, and industrial waste therefore provide a valuable and ubiquitous source of biomass fuel Waste is increasing in the United Kingdom Annual municipal arisings have been predicted to grow from ∼40 million tonnes to in excess of 50 million tonnes by 2020 [13] Across the spectrum of municipal, commercial, and industrial waste arisings, the key biomass waste fuels are:

▪ Solid recovered fuel (SRF) in its wet and dry forms

▪ Mixed waste wood

▪ Sawmill coproduct and other discarded clean wood

5.10.5.1.1 Solid recovered fuel

SRF is nonhazardous waste that has been processed to provide a consistent, market-orientated fuel with a higher CV, lower moisture and ash content, and controlled chemical content and biomass fraction

In its raw state, municipal waste is typically 50% biomass due to the organics, paper, wood, and textile content Its composition depends very much on local regulations and approach to separation and recycling of household waste Such material is of relatively low CV with uncontrolled form and composition

The production of SRF from nonhazardous wastes creates the opportunity to utilize waste-derived fuels in thermal applications that are more sophisticated than the classical waste disposal route via incineration; in particular SRF is being regarded increasingly

by a number of producers and users as a potential feedstock in gasification

The term SRF arises from work undertaken by the European Commission under CEN/343 to provide a systematic basis for the classification and standardization of fuels derived from nonhazardous wastes This work was undertaken in the anticipation that the energy content of nonhazardous wastes should be exploited in pursuit of increased resource efficiency within the EU CEN/343 therefore set out to define a scientifically informed basis for describing the properties of waste-derived fuels for the purpose of facilitating trade between producer and user, for informing process design, environmental permitting, communication with stakeholders, and for quality management

It will be readily appreciated that it is not feasible to design a piece of sophisticated plant such as a gasifier without tailoring the design to the known properties of the fuel This is true for a conventional coal gasifier and it is equally the case for a gasifier intended for operation on biomass or a waste-derived fuel Given the variable provenance and properties of waste materials, it becomes an indispensable condition that some method must be applied by which the physical and chemical properties of a waste-derived fuel can be specified and assured, if they are to be used as a gasifier feedstock The CEN/343 approach provides a rigorous method to do this

The production of SRF is usually carried out by mechanical biological treatment (MBT) This process is a combination of mechanical sorting and biological stabilization of the residue Alternatively, it can be provided by an autoclave process In this case, the biogenic content of the primary fuel output tends to be higher but wetter, and requires significantly higher input energy for production Importantly, manufacturers of SRF are starting to provide fuel to a European quality specification Example processes are EcoDeco and Herhof MBT material has a CV of 15–20 GJ te−1 (EcoDeco indicate 17 GJ te−1 [14]) and a biomass content of at least 60%

5.10.5.1.2 Mixed waste wood

Waste wood arises from various sectors including municipal arisings, retail, the wood and wood products sector, furniture, transport, and packaging The majority of this residual waste wood is mixed, containing contaminants in the form of glues and resins in chipboard and medium-density fiberboard (MDF), paint, plastic coatings, and so on These contaminants inhibit recycling and widespread energy recovery from the arisings in the United Kingdom, since Waste Incineration Directive compliant plant is required

Waste wood is an important source of biomass feedstock, although there is debate over available resource In the Carbon Balances Report [15], ERM estimates that there are ∼7.5 million tonnes of waste wood produced annually in the United Kingdom, of which only 1.2 million tonnes is recycled or reused and 0.3 million is incinerated with energy recovery This leaves ∼6 million tonnes or 80% that

is currently disposed of to landfill In line with other waste sources, waste wood arisings are not envisaged to decrease However, a significant number of proposed renewable energy plants are predicated on waste wood for all or part of the feedstock stream Waste wood has a CV of ∼15 GJ te−1 depending on the exact moisture and storage condition, and a biomass content of ∼90% or more, depending on exact sourcing of the material Waste contractors commonly chip the material for ease of transport, and seek small gate fees or zero cost disposal routes for this chipped, prepared material This often includes local delivery within a 25 mile radius Waste wood does not receive the same level of gate fees as most types of SRF, and may even command a price However, the typically higher biogenic content and reduced ash content can offer enhanced outturn product revenues and lower costs This can partially offset the loss of revenues on the weighbridge

5.10.5.2 Virgin Biomass Feedstocks

This subject is covered in great detail in Perlack’s chapter, so all that appears here is a brief UK perspective

5.10.5.2.1 Virgin woodchip

In the United Kingdom, half of the commercial forestry is operated by the forestry commission, with the balance under private management Approximately 9 million green tonnes are extracted per annum for timber production Green timber has 50–55% of moisture as harvested, although with seasoning it can be reduced to 30% naturally over time, without additional heat This material can be utilized as woodchip, although its use is in direct competition with sawlog Small roundwood is less valuable than sawlog, so woodchip can be sourced from this material

5.10.5.2.2 Forestry and arboricultural arisings

Other than saw-wood, there is a variety of lower grade timber available from forestry and the urban environment In managing forestry, brash (removal of ancillary stems), thinning (trees which are too small for extraction), and poor quality final crops can be extracted The arboricultural arisings in England, Scotland, and Wales by forest district is estimated to be ∼ 670 000 [16] oven-dried tonnes per annum This is equivalent to 1 300 000 green tonnes

Similarly, in the urban environment and on road and rail-sides, tree management produces arboricultural arisings These are usually chipped, and often landfilled, but are increasingly being viewed as another energy biomass source

5.10.5.2.3 Sawmill coproduct

Sawmill coproduct is an alternative and valuable source of woody biomass Sawmills recover ∼50% of the input material as sawn product, with the balance being coproduct in the form of bark, sawdust, and woodchip With the latest equipment and sawing efficiency improvements, sawmill recoveries are improving slightly, but this still represents a significant source of biomass material Current outlets from sawmills have been historically to the boardmill industries for the production of chipboard and MDF Increases in levels of using recycled material have put pressure on this market, which is dominated by a few large players in the United Kingdom Increased board production in other parts of Europe with lower manufacturing costs continues to apply down­ward pressure on the coproduct price Therefore, sawmills are considering other outlets, such as onsite combustion for energy generation and pellet manufacture

Sawmill coproduct has a high moisture content, up to 55% since much of the sawn softwood produced in the United Kingdom is unseasoned Therefore, the CV is relatively low at ∼7.5 GJ te−1 It also means that transport costs per gigajoules for this material can

be relatively high

In 2004, UK sawmills consumed 5.1 million tonnes of softwood to produce ∼2.5 million tonnes of sawn log [17] Therefore,

∼2.5 million tonnes of coproduct was generated, of which ∼1.7 million tonnes was utilized by the UK panel board industries This coproduct was generated by 235 sawmills nationally, of which 18 sawmills individually produced over 40 000 tonnes of coproduct per annum This material commands a price, and an increasingly higher one, as the number of biomass projects seeking pure materials increases

in the dry state is assumed to be 18 GJ te−1, although it is usually harvested at ∼ 50% moisture, and dried to ∼25% to provide a feedstock of 12 GJ te−1

5.10.5.3 Typical Fuel Characteristics and Key Contaminants

Table 1 collates and compares UK data on key biomass feedstock characteristics for both waste and virgin materials

Although many of the macroscopic properties of biomass are remarkably similar across a number of species, it is important to note that minor constituents can vary with the species and undoubtedly with the environment and soils in which they are grown (Scientific literature is prolific on the subject of mineral take-up from the environment, with some plant species being especially effective in accumulating, lead, zinc, mercury, etc.)

This is particularly important when considering the properties of biomass ashes, which in themselves are notably dissimilar to coal ashes, both in the amount and also their chemical composition Biomass ash is generally quite different to coal ash, and tends

to contain large quantities of salts Typical components are potassium, calcium, phosphorus, sodium, magnesium, iron, and silicon This has implications for chosen gasifier operating conditions, especially with respect to ash fusion temperatures and the volatile

Table 1 Summary of key biomass feedstock characteristics

Chlorine 0.6% (up to 1%) 0.03% (up to 0.4%) 0.01% (up to 0.04%)

Sulfur 0.15% (up to 1%) 0.03% (up to 0.2%) 0.01% (up to 0.1%)

Heavy metals Hg 0.5 mg kg−1 (up to 10 mg kg−1) Hg 0.05 mg kg−1 (up to 0.2 mg kg−1) Hg 0.05 mg kg−1 (up to 0.2 mg kg−1)

As 1.0 mg kg−1 (up to 100 mg kg−1) As 1.0 mg kg−1 (up to 10 mg kg−1) As 0.10 mg kg−1 (up to 2 mg kg−1)

behavior of certain alkali metal oxides at elevated temperatures Furthermore, gas processing operations may be sensitive to small levels of both alkali metals and heavy metals in the deactivation of catalysts

5.10.5.4 Feedstock Reception and Handling

Feedstock must be safely admitted into the facility, and stored in a way which satisfies both the technical requirements of the processing plant and also any regulations regarding odor, leaching, and wind disturbance issues

An important factor in any biomass system is feedstock processing This is dictated by the feedstock type, delivered form, and gasifier form requirements Fixed bed gasifiers require lumped fuel (large chips, pellets, or briquettes), fluidized bed gasifiers require

a finer particle or floc, and EF gasifiers require very fine particle size capable of being injected in dense phase flow or as a liquid Therefore, the feedstock must be supplied in, or processed into the appropriate form This can be achieved by shredding, hammer-milling, pelletizing, or briquetting The separate use of heat is often necessary since moisture content is critical, ideally using waste heat from the process to control humidity of the fuel It is normal to aim for a moisture content in the range 10–20% Above that range the thermal efficiency of the gasifier begins to drop away: below that range the energy required for drying grows rapidly Thermo-mechanical options that combine both aspects of fuel conditioning include pyrolysis and torrefaction

Each of these processes has an energy penalty that can add significantly to the operational cost An EF gasifier that may offer superior syngas characteristics will require either very finely milled solid material (possibly torrefied) or a pyrolysis oil These processes in particular decrease the overall process efficiency significantly and increase costs A gasification system that can handle a relatively simple shredded or chipped material may offer efficiency and cost advantages upstream, although if this comes with a significant gas quality penalty, the cost of subsequent downstream gas processing may be higher

The storage and handling of lump fuel is relatively straightforward Woodchip or pellets can be tipped directly onto flat floors, bunkers, moving floor systems, or even blown into silos Floc is difficult to handle: it is extremely low density that makes it volumetrically inefficient to handle; it must be carefully handled to prevent wind disturbance and escape; and it is difficult to move out of containers and through process plant Storage facilities are typically designed for at least 10 days feedstock supply Other important elements of the feedstock handling system can include controlling biomass supply rate into the gasifier, controlling biomass distribution across the inlet, maintaining gasifier pressure and temperature conditions during feedstock injection, conveying of biomass over extended vertical and horizontal distances, removal of foreign objects, and automating the whole process to reduce labor costs and possible health risks Common problems are material bridging, plugging, tar accumulation

on entry valves, and physical damage to conveying screws

5.10.6 Gas Processing

The gas processing chain is also critical, and often overlooked In addition to the primary components in the syngas (CO, H2, CH4, N2, CO2), there are a variety of impurities which are a function of feedstock and gasifier configuration These have an impact on the operability and longevity of downstream equipment, particularly the power generator or catalyst, the final emissions to air and water from the facility, and quality of output fuel The critical impurities are tars, particulates, sulfur, and chlorine compounds, nitrogen compounds such as HCN and NH3, heavy metals, alkali metals, and polyaromatic hydrocarbons including dioxins Additionally, it may be necessary to adjust the ratios of CO and H2 in the gas stream, and even reduce or remove components such as CO2

5.10.6.1 Contaminants and Their Impacts

Table 2 lists the main contaminants in a raw gasifier product stream and summarizes their impacts when left untreated