Renewable pt 2

Life cycle analysis

a compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product throughout its life cycle

What are LCAs used for?

to guide material selection, design, resource optimisation and inform policy and regulations

Types of LCA

cradle to grave

cradle to gate

gate to gate

gate to grave

LCA Steps

Define goal and scope

Compile inventory of inputs/outputs

Evaluate environmental impats

Interpret results

What are some simple impact questions?

does the process use signifiacant energy?

does it use significant transportation?

how is the energy needed generated?

how efficient is the energy generation?

Global warming potential

the contribution of a greenhouse gas emission to climate change, based on its radiative forcing and atmospheric residence

Examples of other impact categories

Acidification

Ecotoxicity

Eutrophication

Human health criteria

Ozone depletion

Photochemical smog

Abiotic depletion potential

annual production/reserves² (resource depletion)

What’s the most important thing for a LCA?

Data quality

What issues are there with LCA?

Comparing significance of emmission factors

How can impacts be normalised

impact/output (mass, energy or financial)

Carbon footprint calculation

input mass*activity (energy/transport)*emission factor

General composition of fuel

Moisture

Volatile material

Carbon

Inert materials

Calorific value test

a small sample is compacted and placed in a bomb calorimeter where it is combusted in the presence of oxygen and the net temperature rise is recorded.

Gross calorific value

total CV minus non-compbustibles

Net calorific value

Total CV including non-combustibles

CHNS elemental analyser

Measures Carbon, Hydrogen, Nitrogen and Sulphur in a material by combusting a sample (at ~1000^oC) to produce CO₂, H₂O, NO_x and SO_x, which is then quantified using Gas Chromatograph detector

What is fixed carbon?

The carbon that remains after volatile matter is burnt off

Proximate analysis

a small sample is heated to 110^oC and held. The difference in weight is measured. (MOISTURE)

The sample is then heated to 950^oC (in nitrogen) and the weight is measured (VOLATILES)

The sample is then cooled to 820^oC and the gas flow is changed to air to burnout residual carbon material (FIXED CARBON)

The final weight is measured (ASH)

Fuel ratio

ratio of fixed carbon to volatiles on a dry ash free basis

What is volatile content useful for?

Ensuring flame stability

Volatile release/slow pyrolysis

Similar to proximate analysis but slower heating rate (5degC/min). A derivative plot is used. can be used to find when specific volatiles are released

Why is early weight loss a problem?

can be an issue for if the fuel is pre heated prior to combustion - could be an issue for storage

R factor

weight loss in devolatilisation/volatile content

What does R factor show?

When heated at a realistic rate more volatiles are released than found in the fuel in proximate analysis when the sample is heated slowly.

Intrinsic reactivity

Similar to proximate analysis but uses air flow rather than beingdine under nitrogen. Can be done isothermally or not

What does intrinsic reactivity show?

how a material changes (in terms of reactivity) during the combustion process. In general material becomes less reactive during combustion because it is thermally annealing over time and therefore the combustion process can start to slow down.

The rate of combustion at lower temperatures is dictated by this reactivity level

X-ray fluorescence (XRF)

Electrons are displaced from their characteristic atomic orbital positions.

This releases a burst of energy that is characteristic of a particular element.

This release of energy is then registered by the detector in the XRF instrument, which in turn identifies and quantifies the energies by element. 

What is XRF used for

It flags up the presence of slagging and fouling elements like Si, K and Na as well as chlorine and other elements that create pollution problems or boiler issues like corrosion

Why is mineral content important?

some minerals have a lower melt temperature which makes them more likely to slag the boiler. Ash composition can effect methods of removal from emissions

Ash fustion testing

determines the softening and melting behaviour of solid fuel ash at high temperatures, using a high-temperature furnace to heat a prepared sample of ash in the shape of a cone or pyramid.

Test the fusibility and slagging potential of the mineral component in the fuel

Sintering strength

the improved mechanical properties, such as tensile strength and hardness, that a material gains through the sintering process, where particles are bonded together by heat and pressure.

Sintering strength test

A fuel is turned to ash at a low temperatures and pressed into a pellet

The pellet is then sintered at a fixed temperature to simulate what happens as the ash is deposited on boiler walls.

What does sintering strength show?

what sort of problems might occur with build up/removal/ash bridging etc. significant levels of volatile elements  can increase sinter strength which makes boiler maintenance more difficult

Self heating stages

exothermic adsorption/condensation of water vapour

respiration of plant cells/microorganisms

reaches temperature for spontaneous ignition

Characteristic temperatures in ash fustion test

initial deformation,

sphere

hemisphere

flow

Tap Density

This is a measure of bulk density after 1-1000 taps/shakes whereupon the fuel pellets become more settled and the density increases

What can density influence?

CV

handling limits

grindability

burnout rate

Why is particle size reduced?

Large particles burn slower as they take longer to heat up and form thicker chars

Loss of Ignition (LOI)

carbon materials left in ash

determined by heating sample of ash residue to 900 degC for 24 hrs and measuring weight loss