Renewable pt1

Why do we seek alternative ways of producing energy?

Demand and consumption

depletion of non-renewable sources

price spikes

climate change

sustainability

Sustainability

development that meets the needs of the present generation without compromising the ability of future generations to meet their needs.

Clean up Technology (CUT)

removal of a pollutant stream or emission using a new or existing technology

Cleaner Technology (CT)

improvement through redesign of product or service delivery

Renewable Energy

a way of producing energy that does not exhaust resources because it uses resources that can be renewed continually

Waste

•a material, substance, or by-product eliminated or discarded when the cost of recycling or re-use exceeds the cost of discarding it

Calorific value

the energy contained in a fuel that is released as heat when the fuel undergoes complete combustion with O₂ under standard conditions.

primary energy intensity

Advantages of biomass

•Renewable

• Improves energy independence and security

• Helps reduce waste

• Helps reduce GHG emissions:

i)Dependence on fossil-fuel based power plants,

ii)Prevents CH₄ emissions from decaying biomass

• Readily available and reliable

Disadvantages of biomass

• High cost

• Can result to deforestation

• Large space requirements

• Fuel vs food debate

• Adverse environmental impacts due to air pollutants

Physio-chemical conversion

used to produce oil from various biomass crops, such as oilseed rape, linseed

Example of physio-chemcial conversion

Oil converted to methyl esters (biodiesel) and glycerine via trans-esterification (reacting with methanol in the presence of a catalyst).

Biodiesel

a fuel comprised of mono-alkyl esters of long chain fatty acids that derived from vegetable oils or animal fats

Biodiesel advantages

•Produced from renewable resources/wastes

•Use in existing diesel engines

•Lowers dependence on fossil fuels

•Grown, produced and distributed locally

•Produces less toxic pollutants than petroleum products

•Vehicles that run with biodiesel achieve a 30% fuel economy

•Positive economic impact on the supply chain

•Reduced CO₂ emissions (up to 70%)

•Improves engine operation of the vehicle

•Accidental spills cause less damage and is safer than petroleum

Biodeisel disadvantages

•Variation in quality

•Not suitable for use in low temperatures

•Could harm the rubber hoses of some engines

•More expensive than petroleum

•Food shortage

•Increased use of fertilizers

•Clogging in engine

•Regional availability and suitability

•Pressure on local water resources

•Use of petroleum diesel in the supply chain for biodiesel production

•10% higher NOx emissions than other petroleum products

Bio-chemical conversion

the conversion of biomass into corresponding products using biocatalysts

Anaerobic Digestion

converts organic material to biogas in the absence of oxygen by using microorganisms.

Digestate can be used as fertiliser

Pre-treatment is essential - size reduction, sanitation, pre-heating

Advantages of anaerobic digestion

•Offers valuable by-products

•Reduces the amount of odour

•Improved water quality as phosphorous and other metals are removed

•GHG reduction from the farms by 66%

•Certain businesses paying a carbon levy for their CO₂ emissions can offset their levy costs against the carbon savings from running their own biogas plant.

•Reduces disease-causing pathogens in manure

Disadvantages of anaerobic digestion

•High installation cost

•High system operation and maintenance requirements

•Economically feasible for larger farms

•High land use

•Requires constant flow of raw materials to run efficiently

•Does not convert as large a proportion of the carbon to biogas, as i.e. when using gasification

Advantages of fermentation (bioethanol production)

•More cost-effective compared to other biofuels

•Ethanol-blended fuels reduce GHG emissions (3.9% for E10; 37.1% for E85)

•Exhaust gases of ethanol are much cleaner (more complete combustion)

•Decreases the need on oil imports

•Contributes to jobs creation at a local/national level

Disadvantages of fermentation (bioethanol production)

•High land use requirements – Biodiversity

•Distillation mostly depending on fossil fuels

•Food versus biofuel, spike in food prices

•Ethanol has high affinity for water. Can be transported only by railroad or auto

•Ethanol is hard to vaporize; starting a car in cold weather may be difficult

•Bioethanol is not as efficient as petroleum. Burning 1L of ethanol gives 34% less energy compared to petrol.

Thermo-chemical conversion processes

Combustion

Gasification

Pyrolysis

Liquefaction

Why is thermochemical conversion better than biochemical conversion?

higher efficiencies 

quicker reaction times

Why are feedstocks pelletised?

•Improves process controllability

•Minimizes feedstock losses

•Requires drying, grinding and pressing of the material

•Feedstock more robust

•Lower losses along the supply chain

Why do feedstocks undergo torrefaction

•Reduces degradation of stored material

•Reduces energy costs for grinding

•Potentially reduces flue gas emissions

•Many volatiles may be removed

•Increases energy density

•Increases bulk density

Torrefaction

low T treatment (200-350 degC) under low O₂

Liquefaction

heating of aqueous slurries or organic wastes (250 – 375 ^oC) at elevated pressure (180 bar) for 20-60 minutes. Suitable for feedstocks with high moisture and ash content

Pyrolysis

Converts feed into bio-oil, char and biogas using temperatures above 500 degC in the absence of oxygen

Advantages of pyrolysis

•Can replace coal and naturas gas, causing a reduction in climate change

•Easier to control contamination of air emissions

•Products with multiple applications

•Pyrolysis plants are flexible and easy to operate

Disadvantages of pyrolysis

•Generates potentially toxic residues

•May produce potential toxic air emissions (acid gases, dioxins, furans etc.)

•Limited scale-up

•Pyrolysis of plastic wastes not sufficient due to low capacities,

•Running cost for pyrolysis of plastic wastes much higher compared to landfilling

Gasification

converts organic matter into its gaseous components. Produces synthetic gas (syngas), consisting of H₂ and CO, at higher T of 1200^oC

At lower T produces CO, H₂, CH₄, CO₂ and tar

5 Stages of Gasification

Drying

Pyrolysis - evaporates of volatiles

Combustion - exothermic

Cracking - breaksdown tar, ensures complete combustion

Reduction - removes O2 from products

Advantages of gasification

•Use of various types of biomass and waste,

•Less oxygen use results to less emissions such as NOx and SOx,

•Significant reduction of weight/volume of organic matter (more efficient than pyrolysis and combustion)

•Allows the use of efficient power generating technologies

•Produced synthetic gas has many uses in the chemical industry

Disadvantages of gasification

•High-cost installation,

•Complex process that consists of multiple phases,

•Syngas has to be purified before use,

•Some limitation in feedstocks due to the need of high calorific values,

•Requires energy from fossil fuels to initiate reactions.

Combustion

the simplest thermo-chemical conversion process that takes place in the presence of air,

Produces heat, power or combined heat and power (CHP),

Combustion Advantages

•Mature technology, simple and readily available,

•Low operating cost,

•Power generation at large scale

Combustion Disadvantages

•Low efficiency in small scale,

•High installation cost,

•Lower energy load from biomass compared to fossil fuels,

•High ash volumes that need appropriate management

Stoker Boiler

Furnace capable of burning a range of solid fuels,

Can burn larger particle sizes of fuel compared to pulverised fuel (PF) boilers,

Different types exist, each with varying grate designs

Types of stokers

Overfeed

Underfeed

Spreader

zones of overfed and underfed stokers

1.Fresh Coal Zone

2.Drying Zone

3.Distillation Zone (VM+CO+CO₂+N₂+H₂)

4.incandescent Coke (CO₂+N₂+H₂O) (combustion)

5.Ash Zone

Spreader Stokers

Raw materials fed by a rotating feeder,

Speed of feeder directly proportional to the steam output,

Can burn various types of biomass

Achieves a fast burnout

Fuel evenly distributed across the grate,

Can satisfy large shifts in steam demand

Issues with Stoker boilers

•Low burn temperatures make it less attractive for electricity generation,

•Slower burnout than PF boilers, limits the furnaces ability to adapt, further reducing its appeal for electricity generation,

•Large number of process variables make process control difficult

•Compared to PF boilers, stokers have lower fuel burnout leading to high ‘carbon in ash’. This is a direct loss of useable fuel. 

What can make ash unusable as by-product?

High carbon content