Biomass

Biomass is a term that refers to living matter. When applied to Renewable Energy Sources it refers to those organic materials which are viable as sources of energy or may be converted to Biofuels which may be used as energy sources. The majority of Biomass resources fit into the broad categories of:

  • energy crops (i.e. crops which are specifically grown for their energy potential)
  • residues (by products of existing crops)
  • waste products produced directly or indirectly from the solar conversion process photosynthesis.

Thus biomass is a solar technology in which the first conversion process is photosynthesis. These may be processes which simply convert the form to one more suitable for use (e.g. chipping, reducing water content) or transform the biomass material into a number of other compounds with more versatile or appropriate properties. When biomass is used directly in an energy application without chemical processing then it is combusted. Conversion may be effected by thermochemical, biological or chemical processes. These may be categorised as follows

  • Combustion
  • Gasification
  • Pyrolysis
  • Liquefaction
  • Anaerobic Digestion
  • Fermentation
  • Acid Hydrolysis
  • Enzyme Hydrolysis
  • Esterification

The advantage of biomass and biofuels is that unlike other renewable technologies they have the potential to be used in all applications currently using fossil fuels. Coal is used for direct heating and power generation for which biomass can be used directly. Applications that specifically make use of the Natural Gas and the Petroleum products Petrol and Diesel require the conversion of the biomass fuel into an appropriate gas or liquid form. Biomass material that is used in a conversion technology is referred to as the feedstock to the conversion process.

Conversion Technologies

Combustion is the direct burning of biomass to produce heat. This heat may then be used directly (for example for heating or crop drying) or used to produce electricity (by using the heat of combustion to generate steam). Biomass feedstock may be added with a fossil fuel feedstock such as coal to produce electricity. Such technology is referred to as co-generation. Only reasonably dry feedstocks are appropriate for efficient conversion. These may be dried by waste heat from the generation process. Biomass projects do not necessarily have to be" high-technology". Examples range from simple wood stoves used domestically for cooking in the developing world to CHP and similar technologies to those representing the state of the art in electricity generation from fossil fuels. The more efficient combustion processes utilise fluidised beds that enable the burning of all the solid material. This is also aided by increasing the surface area of the fuel by processes such as chipping which reduce the particle size. However, heating biomass materials produces a high proportion of volatile matter that typically represents 75% of the fuel's energy. Thus it is important that these gases are also combusted and not simply exhausted with the flue gases. Firing biomass powder in ceramic gas turbines is thought to be one of the most promising technologies and is being actively explored. Commercial exploitation should be possible within ten years [14].

Gasification is the incomplete combustion of solid biomass forming a mixture of gases referred to as Producer Gas. Carbon Monoxide is the predominant component of useful heat value. Other gaseous products depend on whether the conversion is achieved with air or pure oxygen, whether water is introduced into the feedstock and the temperatures reached in the process. Hydrogen will be produced if water is present and temperatures in excess of 900oC are reached. Excess water will permit the creation of Carbon Dioxide which will weaken the heating value of the mix. Air facilitates a low temperature conversion, however a lower heat value gas will be created as Nitrogen will be present in the final mix. Producer Gas has a relatively low heat value (4MJ/m3) and is therefore best used in situ for heating or power generation. Amongst the modern technologies appropriate are BIG-STIG (Biomass Integrated Gasifier-Steam Injected Gas Turbine) and IGCC (Integrated Gasification Combined Cycle) [14].

Pyrolysis is the heating of biomass in the absence of air. Pyrolysis has been conducted for many centuries in the production of charcoal. However, there may be many final products (solid, gas and liquid) of which traditional methods concentrate on only one i.e. the charcoal. This represents at best one-third the energy content of the material. Efficient gasification techniques collect a form of producer gas rich in hydrogen and carbon monoxide, pyrolysis oil (also referred to as bio-oil) as well as charcoal. Different techniques permit conversion into different proportions of each. State of the art technologies are fast pyrolysis that converts 60% into gas and flash pyrolysis that produces a similar proportion in the form of bio-oil.

Liquefaction is a process where a reducing gas such as hydrogen is added to a slurried feed under favourable temperatures and pressures an oxygenated liquid is produced with a high heating value. The high pressures required give rise to a high equipment cost, in addition there are technical problems with feeding slurries. These disadvantages help explain why there is relatively little interest in this process.

Anaerobic Digestion is the process in which biomass is broken down by living organisms in the absence of air (anaerobic). The energy product of this transformation is referred to as Biogas. The technology is well established in the developing world, particularly in Asia. In China alone an estimated 4 million plants are in operation [15]. The primary and desired constituent of Biogas is Methane. Other gases, mainly Carbon Dioxide and Hydrogen Sulphide are also produced. The breakdown of constituents is dependent on a number of factors. In particular the Carbon/Nitrogen (C/N) ratio of the biomass source, the pH, temperature and concentration of undesirable elements such as heavy metals and antibiotics as well as plant design and rate of loading of feedstock.

The best biogas feedstocks are animal (including human) wastes, whilst other constituents may be added, for example crop residues, these will usually affect the C/N balance of the mix to its detriment. Operating conditions produce a gas of over 50% Methane and heat value over 20MJ/m3 (Natural Gas: 37MJ/m3). The resulting gas may be used for heating applications or electricity generation. The Carbon Dioxide and Hydrogen Sulphide is best removed if the gas is to be used for power engines. This can be achieved relatively easily and will improve the heat value of the gas. Small-scale biogas plants as typified by rural India and China offer a 'total solution' to waste treatment. The production of methane is only one of the desirable products. The effluent slurry that is the other main product has almost the same amounts of nutrients as the feedstock and is without the pathogens of the source material. This makes an efficient fertiliser that causes lower leaching than manufactured nitrate fertilisers. The slurry is also a good and inexpensive cattle feed.

There is room for improvement and expansion as the capital expenditure can be reduced by locating larger plants fed from a number of local intensive farm operations. Whereas other biomass materials largely provide a seasonal variability corresponding with crop harvesting schedules, anaerobic digestion of animal slurries offers either a constant supply of material where fully intensive systems are involved, or, where summer grazing is permitted, a seasonal variability which may complement other sources.

Fermentation, Acid Hydrolysis, Enzyme Hydrolysis are used in the fermentation of simple sugars. These sugars may be present in the biomass feedstock or produced by breaking down the source material.

Three broad types of feedstock are appropriate to ethanol production:

  • Sacchariferous (sugars) for example sugar cane/beet, sweet sorghum
  • Amylaceous (starches) for example cereals, cassava, and potatoes
  • Cellulosic (fibrous) for example agricultural and timber residues, paper pulp.

The starches and cellulosic materials must initially be reduced to monosaccharides. For the starches this is promoted by enzyme action, the cellulosic by acid hydrolysis or enzymatic splitting. Sugars are then fermented using yeast. The resulting alcohol is too dilute for final use and must be distilled to increase the concentration. A concentration of 95% alcohol may be achieved by simple distillation, however anhydrous (100%) is required when the final product is to be mixed with gasoline. Anhydrous ethanol has an energy value of approximately 2/3 that of gasoline and may be used to replace gasoline in engines with relatively little modification. However its low volatility at temperatures below 10oC create problems for engine starting at such temperatures.

These alcohols are an unsuitable replacement for diesel oil in engines, such demands however can be met through the use of vegetable oils produced from agricultural crops grown for such purpose. Appropriate feedstocks include safflower, cottonseed, soya, rape, castor, coconut, peanut and palm. The main process simply involves extracting the oil from the seeds or beans, though the feedstock may require cleaning beforehand. However, vegetable oils have a much higher viscosity than diesel oil that can give rise to pressurisation problems within the engine.

Esterification, where the oil is combined with an alcohol such as methanol or ethanol in the presence of a catalyst, removing the glycerine that gives rise to these problems. Rape Methyl Ester (RME) is one such ester that has been widely investigated. It may be combined with diesel in any proportion to produce a much healthier fuel with no sulphur. Typical energy content of 35-45MJ/kg may be achieved. 3000kg of rape seed produces 1000kg of RME. The main by-product (oil meal) may be used as an animal feed, though it may require the addition of the amino acid lysine which may be prohibitively expensive.

Local and Global potential

Biomass is currently the most significant working renewable technology and accounts for 14% of world primary fuel use [14]. This proportion is equivalent to less than 2% of the rate at which energy is stored within 'incidental' biomass throughout the world. However, there are sharp divisions in the level of exploitation in Developed and Developing Nations. In 1991 biomass contributed 2% to EU and 35% to developing countries primary energy use. It is estimated that the EU has the potential to increase its proportion threefold by 2005 [14].

All forms of biomass are relatively low-density energy sources. Their efficiencies are highly dependent on the operational details of production, harvesting, transportation and processing. Storage may also give rise to degradation, changing the balance of ingredients in the feedstocks, usually detrimentally.

Energy Value

For location

In Year

Contribution (%)

Type

Notes

200

World

2050

50

all

a

128

World

2050

 

Energy Crops

a

25

World

2050

 

Dung

a

14

World

2050

 

Forestry Residues

a

13

World

2050

 

Cereal Residues

a

12

World

2050

 

Bagasse

a

10

World

2050

 

Existing Forests

a

3

World

2050

 

Urban Refuse

a

270

World

existing

 

Energy Crops

b

47

Latin America

existing

 

Energy Crops

b

31

Africa

existing

 

Energy Crops

b

270

World

existing

 

Energy Crops

c

0.04

UK

2005

4

Controlled waste

d

0.02

UK

2005

2

Landfill Gas

d

0.32

UK

2005

30

Energy Crops

d

0.64

UK

2005

59

Energy Crops

e

0.08

UK

2005

8

Energy Crops

f

7

USA

existing

 

Residues

g

Notes Based On

a 400Mha land for energy crops, 25-75% residues recovered, best available technology
total world primary energy use 400EJ/year

b Productivity 300GJ/ha/year, energy crops grown on degraded land which was once forested and is no longer settled on or land which has been affected by salt but may sustain salt tolerant biomass crops

c 10% of existing forests, crop and pastureland

d Maximum Practical Resource, total UK electricity production 1EJ

e 3Mha land (slightly more than all present woodlands), total UK electricity production 1EJ

f Maximum Projected Contribution, total UK electricity production 1EJ

g Total Recoverable Resource

Where a year is given this refers to an economic and technically based prediction
‘existing’ refers to converting currently degraded land or current recoverable resource completely to biomass production/conversion

Table 3: Estimates and Predictions of Biomass Energy Contributions and Potential, EJ (1018J)

The UK has concentrated its efforts in the commercial exploitation of biomass in the form of landfill gas and the burning of waste for power generation. This will never represent a very significant energy supply and represents only one solution to the problem of dealing with domestic and commercial wastes. Recovery of the energy in waste materials may be better achieved by recycling the material and thus reducing the energy required in manufacturing.

Operating efficiencies of Biogas plants effectively require large-scale operations linked for example to intensive animal farming. However the predominance of antibiotics used within this industry has a significant impact on the production, as these are toxic to the methane-producing bacteria.

Biomass production is most relevant in countries with agricultural surpluses but with an energy deficit [15]. Whilst the UK is not self sufficient in food, the Common Agricultural Policy of the EU promotes the alternative use of agricultural land through the set-aside scheme. Through the efficient production of Biomass crops, the UK has the potential to produce nearly 60% of its electricity needs.

The developing world is the largest user of biomass fuels but these are frequently used inefficiently. Where forestry crops are utilised, then management of the resource is not always carried out to maximise yields. The developing world does have the potential to produce a very significant amount of biofuels, giving countries which have few fossil fuel reserves the opportunity to export fuels.

Table 3 represents an accumulation of data on the potential in various regions of the world.


The biomass industry

Biomass industry is represented by those Biomass fuel sources and equipment that is able to process the raw fuels. Organisations are involved in all stages of Biomass to Energy production. These stages may be specified as:

  • Production
  • Harvesting and pre-processing
  • Conversion
  • Distribution

Production. Products may be derived from what may be constituted as waste sources (e.g. animal manure, crop residues, municipal waste, forest thinning) or crops which are specifically grown to produce a biomass product. Efficiency of sourcing these products, particularly with regard to the waste products is achieved through good management and altering processes to make use of these products. The production of Biomass fuels should be seen as part of a process that achieves other objectives. Biomass crops should have the following characteristics:

  • high yield
  • low production inputs
  • high energy value crops
  • low moisture content (when used for combustion applications)
  • suited to the local climate,
  • tolerant to specific conditions, e.g. when they are intended for use on denuded land/where high salt levels present

Table 4 lists the crops which have been recognised as having potential within the European Union.

Wastes which are considered to have good potential as Biomass feedstocks include

  • Bagasse (from Sugar Cane)
  • Rice Hulls
  • Rice Straw
  • Peanut Hulls
  • Oat Hulls
  • Switchgrass
  • Wheat Straw
  • Wood

Harvesting and pre-processing. Biomass fuels need to be prepared in order to meet the requirements of the conversion facility. The form (particle size and shape) is one of three main characteristics which need to be considered, the others being moisture content and chemical composition. As biomass is a relatively low bulk density fuel, optimisation of loading transportation vehicles is important and thus the form is also important in this respect. For short rotation coppice crops, it is usual to harvest and process on site (or close to site where processing machinery requirements cannot be met on site).

Crop European Region
   
Sweet Sorghum South/Central
Miscanthus South
Jerusalem Artichoke South/Central
Artichoke (tubes) South/Central
Rapeseed -
Sugar Beet -
Wheat -
Eucalyptus South
Poplar North/West/Central
Willow North/West
Robinia South
Conifers North/West/Central

Table 4: Energy Crops [11,14]

The two main techniques are cut and chip and cut and bundle. The majority of equipment of this type has been developed for the pulp industry that is based around a more mature stock than short rotation coppicing which is generally accepted as most appropriate for energy crops. A number of companies have developed equipment for the special needs of biomass crops.

For example, the Swedish Frbbesta tractor towed stick harvester and the British Coppice Willow Harvester. Both cut the wood, gather the bundles, tie them and then eject them. Other harvesters have been developed for other biomass sources. For example the Nicholson-Koch mobile harvester designed to recover residues and the Austoft sugar cane harvester. Decisions on whether to chip or bundle are also determined by the moisture content of the wood and what is appropriate for the equipment which will process it. For chips in fixed grate combustors, consistent particle size is important to avoid compaction of the feedstock and incomplete combustion. Chips also degrade quickly and are not a suitable form for storage.

Forms of converted feedstock include refuse-derived fuel, derived from municipal waste and used for combustion. Particle size reduction of agricultural residues may be achieved by grinding. A number of commercial organisations are involved in this sector. In particular, marketing plant for the different processes outlined in the introduction.

Distribution. Final products of conversion are one or more of heat, electricity, biofuels and by-products. Where the product is heat or electricity, then this product is no different to that produced from any another fuel. The biofuels are a different product to their fossil fuel rivals. A number of companies have been established marketing their own form of these biofuels. One example is the U.S company NOPEC who produce a methyl ester from esterification of soya and other vegetable oils [16]. It is claimed that this product may be used in any diesel engine without modification and that it causes less wear than comparable low-sulphur petroleum based diesel fuels.

Main International Projects

The EU has recently adopted a White Paper [17] which sets out a 'Strategy and Action Plan' to double renewable contribution to primary energy needs by the year 2010. It assumes a 1995 base figure of 6% that is to rise to 12%. The target for biomass contribution is to treble the 1995 figure of 45Mtoe to 135Mtoe. In particular, the following are stated as requiring active promotion:

  • Co-firing or fossil fuel substitution in coal power plants and in existing district heating networks
  • New district heating or cooling networks as an outlet for co-generation with biomass Greater access to upgraded fuels such as chips and pellets and a more intensive exploitation of appropriate forest, wood and paper industry residues
  • New scaled up IGCC (Integrated Gasification in Combined Cycle) systems in the capacity range of 25-50 MWe based on a mixture of biomass and waste derived fuels
  • Clean energy generation from municipal waste either by thermal treatment, landfill gas recovery or anaerobic digestion as long as energy generation from waste complements and does not replace waste prevention and recycling

ALTENER II, the programme which sets out to promote renewables by tackling non-technical barriers will be the main vehicle used to meet these objectives.

The U.S. Department of Energy set up the National Biomass Power Program [18] in 1991. Its aim is to help establish a sustainable option for helping to meet the expected new electric generating capacity needed world-wide. Core activities of the Biomass Power Program include

Working with the power industry on co-generation and to replace fossil fuels with biomass in existing boilers

  • Evaluating and developing advanced technologies such as gasification and pyrolysis
  • Assessing the characteristics of biogas produced from various gasification technologies
  • Developing clean-up technology for high temperature biogas
  • Supporting small system demonstrations
  • Analysing biomass power systems
  • Sponsoring cost-shared feasibility studies with industry

Barriers to biomass technologies

As a source of energy, biomass is doubtless considered as the lowest technology renewable energy solution and this runs contrary to western culture which largely looks to technology to define changes in society [19].

Biomass, with its historical position in Western society and prominent position in developing economies is considered by a significant proportion of the population as not being a solution for Western economies. There is also a perception that the development of bio-mass as an energy source will give rise to more intensive use of land and all that entails with regard to ecological impact.

Whilst many biomass utilisation technologies have been in existence for many years, their efficiencies often fall far short of those required for Biomass to become a serious, marketable rival to fossil fuels. The following will be of concern for the majority of biomass materials:

Economic Viability of Production. Biomass and biofuel production, both historically in the UK and currently in the developing world has relied on the benefits of the products produced. Where wastes are concerned, its relevance in regard to waste treatment. These factors are less significant in the developed world where biofuels need to offer a direct replacement for fossil fuels and the products of their conversion (e.g. electricity). Whilst subsidies are relevant in enabling the development of biomass industries they will need to compete with fossil fuels in the long term. Costs for conventional and biomass fuels are provided in Table 5. Although assumptions are made for each costing, all (apart from the separate predictions for RME and bioethanol) are current. Reduction of costs depends on the widespread adoption of the techniques and finding markets for by-products,

Fuel ECU/Litre ECU/KWh
     
Conventional Fuels    
Gasoline/Diesel 0.07  
Electricity generation   0.02 - 0.04
     
Biofuels    
bioethanol 0.50a, <0.30b  
RME 0.90a, <0.30b  
CHP   0.075
Direct Combustion, Solid biomass   0.11
Ceramic Gas Turbine   0.05
IGCC   0.057

Notes
a: current cost
b: realistic cost for 2005

Table 5: Production Costs for Biofuels and Conventional fuels, ECU [11,14]

Optimisation of Energy Ratio. For agricultural and forestry crops the energy required to produce the final product has been secondary in the developed world, where fossil fuels have been relatively inexpensive. Where the final product is to be energy then it would be futile to produce a product which has an energy ratio (energy value of final product/total energy required to produce it) below one and questionable for an energy ratio below two. Energy costs for crop production include production, transport and use of fertilisers and pesticides, machinery and irrigation. Added to this will be energy costs for transportation and any pre-processing of the biomass before processing into fuel. Some fuels will incur energy costs because other ingredients are necessary to create the final biofuel.

Whilst residues are a by-product, there will still be an energy ratio which not only reflects the energy required to process the materials (together with any overheads such as transportation) but also the efficiency of the system which has produced the residue itself.

Table 6 gives examples for production of ethanol from three different biomass sources. Whilst none represents a particularly efficient process, their is a striking difference between them.

Feedstock Energy Ratio
Sugar Cane 2.75
Sugar Beet 0.56
Wheat 1.50

Table 6: Ratios of fuel energy output to total energy inputs to produce bioethanol [15]

Establishing a good energy ratio is thus an essential factor to consider, particularly when the subsidy of biofuels is considered. Under favourable purchase prices of biomass from the producer and subsidies for production a negative fuel balance could ensue.

Maximising Energy Content. Biomass fuels can theoretically be used in all applications for which fossil fuels are currently used, However, the biofuel frequently has a significantly lower energy value than its fossil fuel competitor. This produces two problems. First, the fuel becomes relatively expensive to transport. Second, where the fuel is intended to directly replace an existing fuel, performance will be reduced. This latter point is particularly relevant to gasoline and diesel replacements. The first point means that the economics of biofuel production are greatly affected by the distances between production of the biomass, plant producing the biofuel and consumer of the fuel. This often limits the useful application of biomass fuels to the approximate location that they are sourced. However, as biomass tends to be sourced away from centres of population, biofuel plants are generally established to provide fuel for farms that produce the crop.

Gases MJ/m3 MJ/kg MG/L
       
Natural Gas 37    
Producer Gas 4-11    
Biogas 22    
Hydrogen   118 8
       
Liquids      
Gasoline   44 30-33
Diesel   43 35
LPG   51 26
Ethanol   24 20
methanol   20 16
Pyrolysis Oil   20-25  
Vegetable Oils   37-40 34-37
Oxygenated Slurry 35-40    
       
Solids      
Coal   28  
Dry Wood   12-20  

Table 7: Energy Content of Conventional and Biofuels


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