Until recent times, the history of fuels was essentially the history of biofuels. Although there is evidence of coal-burning as early as 3000 years ago, its contribution remained relatively small until about 200 years ago. Indeed, bio-energy was still dominant in many areas of life well into the industrial revolution, with wood for heat, tallow candles made from animal fats fro light, and grasses as "fuel" for the only means of transport - the horse.

The move from bio-fuel to fossil fuel was a key feature of the industrial revolution. For many countries, the high temperatures needed for iron smelting could be achieved only in furnaces using charcoal. The impurities and variable nature of coal made it unsuitable for smelting, and attempts to reduce it to a type of charcoal had little success initially. But in the early 1700s, an effective "coal charcoal" was produced, and within a few decades, this new fuel, now called coke, was replacing charcoal throughout the growing industrial sector.

By the end of the nineteenth century, coal was dominant in the world's industrialized countries. The twentieth century saw the rise of oil and natural gas, but it is worth mentioning that coal consumption also increased five -fold between 1900 and 2000.

Will the twenty-first century see the reverse process namely from coal back to wood? Could Bio-fuels completely replace fossil fuels? Theoretically it is possible!

Energy Crops

The term energy crop is used in the widest sense, to include any plants that are grown specifically for use as fuel or for conversion into other bio-fuels. Includes, for instance, wood for burning, plants for fermenting to ethanol and crops whose seeds are particularly rich in oils (but not of course those that are grown primarily for food).

Energy Crops have attracted increasing attention in recent years, for several different reasons namely:

• The need for alternatives to fossil fuels, to reduce net CO₂ emissions
• The search for indigenous alternatives to imported oil
• The problem of surplus agricultural land

Energy Crops can be divided up in to two headings Woody crops and Agricultural crops

WOODY CROPS
In modified conventional forestry, whilst energy is still not the main product, it is part of the plan. Coniferous trees are planted at higher than usual density and vigorously thinned after a few years, using integrated harvesting techniques to produce chipped wood. The remaining trees grow to maturity in the normal way. There have been trials of this system, but the main developments in recent years have been with woody crops planted and harvested entirely for energy production.

AGRICULTURAL CROPS
Globally the most grown crops for bio-energy purposes are sugar, cane and maize. Both are C4 crops, with the high yields needed for a favourable energy balance. The main interest lies in their seeds, which can be converted to a diesel substitute, known as bio-diesel.

Wastes

There are two main types of wastes. The wastes arising direct from uses of biomass for example forestry, agriculture and animal husbandry. Then there’s the household, or more generally, municipal wastes including the specific wastes associated with industrial processes.

WOOD RESIDUES
Operations such as plantations and trimming felled trees generate large volume forestry residues. They are often left to rot on site. Solid biomass, mainly forestry residues, fuels some 6% of Austria’s generating capacity which two thirds of it in a CHP plant. The investment required for the harvesting and processing machinery means that the resource is unlikely to become economically viable until a market for wood is fully established.

TEMPERATE CROP WASTES
World-wide residues from wheat and maize, the two main temperate cereal crops, amount to more than a billion tonnes per year, with an estimated energy content of 15-20 EJ. They have uses as bedding, feed, etc. but in major cereal-growing regions more than half may remain unused. In East Anglia in the UK about 80% of the nearly 2 million tonnes of straw produced annually is surplus to agricultural requirements. The UK market for straw-burning systems for space heating has been relatively slow to develop.

TROPICAL CROP WASTES
The total energy content of the annual residues of the world’s two main tropical food crops, sugar and rice, is estimated as about 18 EJ - similar to the total for temperate crops. In this case, however, significant quantities are already being used as fuels. Bagasse, the fibrous residue of sugar cane, is used in sugar factories as a fuel for raising steam, and to produce electricity for use in the plant. Rice husks are among the most common agricultural residues in the world, accounting for about one-fifth of unmilled rice dry weight.

ANIMAL WASTES
Animal manure can be a major source of greenhouse gases. It is estimated, for instance, to account for 10% of methane emissions in the USA. Sewage sludge can be treated by anaerobic digestion. By the mid-1994, 26 projects with a total generating capacity of 33MW were supplying electricity under the NFFO scheme, but recent years have seen no new projects [Source: Renewable Energy, “Power for a sustainable Future”, Second Edition, Oxford]. Poultry litter, a mixture of chicken droppings and material such as straw, wood shavings etc. has an energy content in the range of 9-15GJ/t depending on its moisture.

MUNICIPLE SOLID WASTE
The average household in the world’s industrialized countries generates rather more than a tonne of solid waste per year, with an energy content of about 9 GJ per tonne. It appears therefore that the average UK household could in principle supply one tenth of its total annual energy consumption of about 90GJ from its own wastes. In practice there is also three main ways in which Municipal Solid Waste (MSW) is treated at present:

• Disposal in landfills
• Combustion
• Disposal in anaerobic digesters

LANDFILL GAS

A large proportion of municipal solid waste is biological material and its disposal in deep landfills furnishes suitable conditions for anaerobic digestion. It was known for decades that landfill sites produced methane, and systems were fitted to burn it off safely, but the idea of using this Landfill Gas (LFG) developed only in the 1970’s. The digestion in a landfill takes place over years, rather than weeks of “wet” systems. In developing a site, each area is covered with a layer of impervious material after it is filled, and the gas is collected by an array of interconnected perforated pipes at depths of up to 20m in the refuse. In a large well established landfill there can be several kilometres of pipes, with as much as1000 m³ per hour of gas being pumped out. In theory, the lifetime yield per tonne of wastes in a good site should lie in the range 150-300 m³ of gas, with between 50% and 60% by volume of methane, which suggests a total energy of 5-6 GJ per tonne of refuse. In practice, at the average UK gas extraction rate, the heat energy output per tonne of wastes (as collected) is rather less than 2 GJ, but yields are extremely variable and often unpredictable - a factor that tends to deter potential investors.

The gas may be used directly to fire kiln, furnaces or boilers, but there are rarely enough large users close to a landfill site, and the output is increasingly used to generate electricity for local use or for sale. The generators are driven either by large internal combustion engines or by gas turbines. Assuming a gas-to-electricity energy efficiency of perhaps 35%, this brings the overall energy efficiency of the system below 10%. A site containing a million tonnes of MSW might support an electrical capacity of perhaps 2 MW over a 15-20 year generating lifetime. Despite the low energy conversion efficiency, LFG plants have been amongst the most financially attractive of the systems receiving contracts under the NFFO scheme, with an annual electrical output in 2002 of some 2700GHh from an installed capacity of about 400 MW.

An example of the cost calculation that REH follows is:
[Source ETSU 1999]. For a site large enough to support 2 MW of generating capacity over an operating life of 15 years with an average load factor of 88%. The following calculation assumes a discount rate of 8 %.

 

Financial Data:
Plant cost: £750 per kW
O&M costs: 1 p kW/h
Discount rate: 8%
There are 8760 hours in a year, so with 88% load factor, the 2 MW plant will generate:
2000 x 8760 x 0.88 = 15.42 million kWh per year. Say for example the repayment on £1000 at 8 % over 15 years is £117. The total plant cost is £ 1 500 000, so the total annual repayment is: 1500 x £ 117 = £ 175 000 = 17.55 million pence per year. Expressed as a cost per kWh of output, this is 17.55 / 15.42 = 1.14 p kW/h. Adding the O&M costs gives a final electricity unit cost of 2.14 p per kWh.

COMBUSTION OF WOOD AND CROP RESIDUES
Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient: For example [Source: Renewable Energy, Power for a sustainable Future, Second Edition] How much wood is needed to bring one litre of water to the boil?

Data
Specific heat capacity of water = 4200 J Kg-1 K-1
Mass of 1 litre of water = 1 kg
Heat value of wood = 15 MJ kg-1
Density of Wood = 600 kg m-3
1 cubic centimetre (1 cm3 = 10-6 m3)

Calculation
Heat energy needed to heat 1 litre of water from 20 °C to 100 °C = 80 x 4200 J = 336 kJ
Heat energy released in burning 1 cm3 of wood = 15 x 600 x 10-6 MJ = 9.0 kJ
Therefore: Volume of wood required = 336 / 9.0 = 37 cm3

Conclusion
Experience suggests that on an open fire much more than two thin 20 cm sticks would be
needed. But a well-designed stove using small pieces of wood could boil the water with
as little as four times this “input” – an efficiency of 25%

Wood burning produces a wide range of pollutants, and the past few decades have seen
many programmes in developing countries for the design and dissemination of improved
stoves.

CHARCOAL
Charcoal is traditionally produced in the forests where the wood is cut. The “kiln”, consisting of stacked wood covered with an earth layer, is allowed to smoulder for a few days in the near absence of air, typically at 300 – 500 °C, a process called pyrolysis. The volatile matter is driven off, leaving the charcoal, the “char” component. Charcoal is almost pure carbon, with about twice the energy density of the original wood and burning at a much higher temperature, so it is much easier to design a simple and efficient stove for use with this high quality fuel.

[Source of picture: Reference from Duiker Combustion Engineers]

COMBUSTION OF MUNICIPAL SOLID WASTES
In many countries in continental Europe and elsewhere, refuse incineration with heat recovery, or Energy-from-Waste (EfW) is an important means of waste disposal. The heat may be used directly for district heating, or for power production (often in CHP plant). The inert ash can be used as hardcore. Countries with successful recycling and composting programs have often seen parallel growth in EfW, which accounts for 30-60% of MSW disposal in most West European countries. World installed capacity is over 3 GW, about half of it in Europe. EfW is becoming financially attractive in large cities in Britain because of the shortage of suitable landfill sites and high cost of transporting wastes to distant sites. Encouraged by support under the NFFO scheme, UK capacity

tripled during the 1990s, and all MSW combustion plants in the UK now make use of their heat output. A state of the art incinerator has been built on the Isle of Man. REH had no involvement with this project and wasn’t established yet. It is also known as the 'Energy-from-Waste Plant'. It reduces the amount of waste burried at landfill sites in the North of the Island, namely Point of Ayre, whilst producing electricity for the island. The plant can deal with all the burnable waste produced by the island everyday.

ANAEROBIC DIGESTION
Biogas and landfill gas is forms of anaerobic digestion of wastes. In landfill’s case the digester is the landfill itself and the operator has only limited control of the processes. The digestion of “wet wastes” is quite different. The feedstock, dung or sewage, is converted to a slurry with up to 95% water, and fed into a purpose-built digester whose temperature can be controlled. The process of anaerobic digestion is complex, but it appears that bacteria break down the organic material into sugars and then into various acids which are decomposed to produce the final gas, leaving a residue whose composition depends on the systems and the original feedstock. The biogas produced by a digester can be used to produce heat or electric power – or in many cases both. A Danish government programme investigating the economics of anaerobic digestion concluded after a six-year trial that a large-scale biogas plant could be profitable in three realistic conditions were met:

• the plant is operated in combined heat and power mode
• the gas is sold at a price comparable to that of natural gas
• credit is given for the disposal of other wastes

ANAEROBIC DIGESTERS FOR MSW
As an alternative to recovery of biogas from landfills, MSW can be subjected to more carefully controlled processing in the type of large digester described above. Under these conditions gas yields are much higher and digestion is complete within matter of weeks rather than years. An advantage over landfill is that these digesters can be that these digesters can be closer to urban areas, reducing transport costs. They also require less land, but do need higher capital investment and processing costs which is a disincentive.

GASIFICATION
Gasifiers are used for gasification. Gasification is not a new process. “Coal gas” the product of coal gasification was widely used in the UK for many decades and “wood gas” was used for heating lighting and even vehicle fuel. Both were superseded by natural gas although wood gasification reappeared during the coal shortages of World War 2. In contrast with anaerobic digestion of waste, gasification is used for chemical processes by which gaseous fuel is produced from a solid fuel (Wood chips, agricultural residues, saw dust or waste). This process is done by adding hot steam and oxygen with the solid fuel. This results into a mixture of combustible components (mainly carbon monoxide and hydrogen, with some methane, higher hydrocarbons and condensable tars) together with carbon dioxide and water. The gasification reaction does not occur easily in which the operating temperature and atmospheric pressures of the gasifier vary in a wide range. It is therefore essential to have the correct gasifier. If the output from a biomass gasifier can be cleaned and used to run the gas turbine, the possibility arises of a self-contained biomass integrated gasification combined cycle (BIGCC) system, for local power generation.

Basic Process:
There are many different designs of modern gasifiers, but essentially one basic process which is hot steam and oxygen interacting with the solid fuel

Gasification Reaction:
Operating temperature vary from 100 ºC – 1000 ºC
Pressure: 1atmosphere – 30 x atmosphere
If air is used rather then pure oxygen, nitrogen will be present and the energy content of the resulting gas would only be 3-5 MJ/m3, about a tenth of natural gas. The additional cost of the oxygen plant makes this more economic viable with large systems.

Conversion Efficiency:
Conversion efficiency from the energy of the solid fuel to that of the resulting gas varies widely, from as little as 40% in relatively simple systems to 70% or more in the most sophisticated plants.

The Biomass CHP system, with emphasis on Downdraft Gasifiers:
System’s like these converts prepared fuel into producer gas. Unlike other gasifiers it does not require additional gasses/diesel, the combustion engine starts & runs on producer gas.

Combined Heat and Power (CHP) system

Process

• Uses downdraft gasifiers to turn fuel into a combustible gas
• The gas is then filtered to remove tars and dust.
• The gas is then cooled and fed to a spark an ignition engine
• Then used for heat and power.

Fuels normally accepted
Softwoods: [spruce, pine, willow]
Hardwoods: [beech, oak, poplar]
Industrial wastes: [palletwood pieces and furniture off cuts]

Size of typical plant
To generate 250 kW > Area:10mx15m ; Height: 4m

Maintenance
Gasification systems are normally unmanned and work automatically. One person is normally required to deal with handling of the biomass, to move materials from a reception area to a biomass dryer. A dumper truck or digger and shovel could be used. Dried materials would then be stored to provide a “buffer” of material of 2-3 days [12-18t] to feed the gasifier. The gasification system is automatic and can be unmanned.

Performance
For each 250 kg of prepared biomass, this will give approximately 250 kWe, generating an electricity income of £15/h, assuming a price of 6p/kWh or £0.06/kWh. (250 kWe x £0.06/kWh = £15/h)

Biomass input i250 kg/h
Char/ash output 7-10 kg/h
Condensate (water) 30 kg/h
Engine exhaust (within emissions limits) -
Electrical output (gross) 300 kWe
Electrical output (net) 250 – 275 kWe
Recoverable heat 450 – 600 kWth
Electricity production cost £30/MWh
ROO incentive £30/MWh

PYROLYSIS TO PRODUCE BIO-OIL
The method involves heating the bio-material with a carefully controlled air supply. It must not burn, of course, and as the aim is a liquid product, gasification must be minimised. The resulting reactions are complex and hard to predict, giving a range of oils, acids, water, solid char and uncondensed gasses, depending on the feedstock and operating conditions. The product, bio-oil, usually has about half the energy content of crude oil, and contains acid contaminants that must be removed. Variations on the basic process include solvolysis, the use of organic solvents at 200 - 300 °C to dissolve the solids into an oil-like product, and fast pyrolysis, requiring temperatures of 500 – 1300 °C and high pressures (50 – 150 atmospheres).

SYNTHESIZING LIQUID FUELS
The first stage in the synthesis is a shift reaction, to adjust the proportions of H2 and CO to the ratio required in the desired product. Methanol, for instance, is CH3OH and therefore needs two H2 molecules for each CO, a hydrogen-to carbon monoxide ratio of 2:1. In the Fischer-Tropsch process the two components are passed over a suitable catalyst at high temperature and pressure, and the product, initially formed as a gas, is condensed.

FERMENTATION TO PRODUCE ETHANOL
Fermentation is an anaerobic biological process in which sugars (C6H12O6) are converted to alcohol by the action of micro-organisms, usually yeast. The required product, ethanol (C2H5OH), is separated from other components by distillation. Unlike methanol, ethanol cannot simply substitute for petrol, but it can be used as a gasoline extender in gasohol, which is petrol containing 26% ethanol. The liquid resulting from fermentation contains about 10% ethanol, which is distilled off. The complete process requires a considerable heat input, usually supplied by crop residues. Brazil’s Pro-Alcool programme, producing ethanol from sugar residues, is the world’s largest commercial biomass system.

VEGETABLE OILS TO BIO-DIESEL
Vegetable oils are quite different from liquid fuels. They occur naturally in the seeds of many plants, and are extracted by crushing. Their energy content of 37 – 39 GJ/t is only a little less than that of diesel (about 42 GJ/t). They can be burned directly in diesel engines, either pure or blended with diesel fuel, but incomplete combustion is a problem, leading to carbon build-up in the cylinders, and conversion of the vegetable oils to bio-diesel is preferred. There are reports that in 2002 UK drivers were mixing vegetable oil from their local supermarkets with their diesel fuel. This is illegal – unless they take care to pay the tax – and may harm the engine. [Source: Renewable Energy, Second edition]