Combustion Technology (8-10) Flashcards
Explosion in terms of combustion?
Uncontrolled combustion - explosion
- stoichiometry not controlled
- combustion rates uncontrolled and unconfined.
Carbon Capture
Post-combustion capture of CO2 using sorbents
Amine-CO2 reactions (3)
Carbamate reversion
Bicarbonate formation
Carbamate reversion
Carbamate formation: CO2 + 2RNH2 → RNHCOO + RNH3 (R1)
Bicarbonate formation: CO2 + RNH2 + H2O → HCO3 + RNH3 (R2)
Carbamate reversion: RNHCOO + CO2 + 2H2O → HCO3 + 2RNH3 (R3)
CCS PCC process
- Flue gas from power station is cooled and enters absorption column
2.Rises in column and contacts CO2 with absorbing solvent
3.Remaining low % CO2 flue gas released
4.Solvent is reheated in regeneration column and CO2 captured and delivered offshore.
5.Solvent is recycled back into absorber.
Increase CSS efficiency and 2 problems associated with it
Increasing steam temperature/pressure increases the efficiency of the Rankine cycle reducing CO2 emissions per MWhr of electricity produced.
However, it also increases the demand for the use of higher specification materials.
Fire-side and steam-side corrosion
2 routes of formation of NO from N2 in combustion
Thermal NO (about 5% in coal combustion, > 95% in gas combustion)
Fuel –NO (> 90% in coal combustion)
Thermal NO formation reactions (3)
O + N2 NO + N (R1)
N + O2 NO + O (R2)
N + OH NO + H (R3)
The contribution of reaction (R3) is small for lean mixture, but for rich mixtures it should be considered. Forward reaction of (R1) controls the system, but it is slow at low temperatures (because of high activation energy). Thus it is effective in post-flame zone where temperature is high and the time is available. Concentrations up to 1000 to 4000 ppmv thermal-NO can be observed in uncontrolled combustion systems.
Fuel NO formation reactions (6)
HCN + O NCO + H (R11)
NCO + H NH + CO (R12)
NH + H N+H2 (R13)
CN + O N + CO (R14)
N + OH NO + H (R15)
N + O2 NO + O (R16)
As the fuel is heated and devolatilized, part
of its nitrogen is released and forms small
molecular, gaseous cyano- and cyanide
compounds such as hydrogen cyanide HCN,
and amino compounds such as NH3. This part of fuel-N is termed as volatile-N. For coal the principal species is accepted to be mainly HCN and the following formation mechanism is generally accepted as the main steps in NO formation from fuel-N :
air-fuel equivilance ratio
The air-fuel equivalence ratio, λ (lambda) is defined as the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio.
fuel-air equivalence ratio (phi)
Defined as 1/lambda (air fuel equivilance ratio)
Over fire air (nox control)
Air staging and OFA are commercial techniques. The NO reduction obtained with air staging varies according to the fuel used, but it ranges generally between 10 and 50%.
OFA techniques reduce NOx formation mainly by two mechanisms:
(1) air staging allows deprivation of oxygen, and less mixing of fuel and air in the main combustion zone where fuel nitrogen evolves, thereby reducing fuel-NO;
(2) air staging results in a cooler flame and hence less thermal-NO.
problems with over fire air for nox control
Overfire air staging
decreases NOx.
However, there is an
increase in carbon in ash (CIA)
after 12% overfire air
which would be
unacceptable due to the
increased energy loss.
Selective non-catalytic reduction (SNCR) (nox control) definition and conditions
In this post-combustion control technique, a nitrogen-containing additive (e.g. ammonia (NH3), urea (CO(NH2)2) is injected (normally in upper furnace region) and mixed with flue gases to effect chemical reduction of NO to N2 without the aid of a catalyst.
The SNCR temperature window is about 850-1150 oC for urea, whereas it is about 880-1080 oC for ammonia.
Oxygen required (see reaction)
Selective non-catalytic reduction (SNCR) (nox control) reactions (2)
(NH2)2CO + 2NO + ½ O2 2 H2O + CO2 + 2N2 (R17)
2NH3 + 2NO + ½ O2 2N2 + 3H2O (R18)
Selective catalytic reduction
(SCR) (nox control) definition and conditions. Advantage and disadvantage over non catalysed.
In this technique, a catalyst is used in conjunction with ammonia injection to reduce NO to N2:
depends upon the catalysts used, but is usually within the range of 300 – 400 0C
advantage of SCR over SNCR is that greater NOx reductions (up to 95%) are possible, and the operating window is at lower temperatures. Costs of NOx removal with SCR are generally the highest among all NOx control techniques because of both the initial high capital cost and the high operating costs associated with catalyst replacement.
Selective catalytic reduction reaction
4NO + 4NH3 + O2 4N2 + 6H2O (catalysed)
SCR catalyst criterea and examples
Criteria:
High activity
High porosity
Good stability
Poison resistant
Examples include V2O5 and TIO2
SCR pros and cons
High NO reduction (70 - 95%)
Clean reaction; few by-products
Huge world-wide use
vs
High capital cost
Catalyst expensive
Poisoning
Store & handle NH3
Flow impedance
Low Nox burners Operating principles
Fuel-rich zones generated in flames by
fuel concentrators in primary air/fuel feed annulus
Flame stabiliser or flame-holder controls in-mixing of secondary air
Controlled mixing of tertiary air for good burnout
Release of volatiles and ignition in low excess O2 environments
Control of secondary and tertiary air ratio by movable dampers
Difference between controlled and uncontrolled combustion
Controlled combustion – burner
– local stoichiometry can be controlled
- combustion rates controlled and confined.
Uncontrolled combustion - explosion
- stoichiometry not controlled
- combustion rates uncontrolled and unconfined
KG , explosion constant for a gas
KG is used to characterise the reactivity of a gas explosion
Explosion Prevention Using Inerting
Inert gases such a carbon dioxide, nitrogen, water vapour, argon act as a coolant on the flame.
This reduces the flame temperature and the reactivity of the flame.
If sufficient inerts are added then no flame can propagate and no explosion can occur.
The effectiveness of an inert gas depends on its ability to absorb heat i.e. on its specific heat, Cp.
The displacement of air and reduction of oxygen is a consequence of the use of inerts and a secondary cause of the flame extinguishment.
Measuring effectiveness of inerting
Calculate Rgas using R(gas constant) / Mw
effectiveness of inert gas proportional to Cp/Rgas or cp*mw
limiting oxygen measurement
This is the oxygen in the air plus fuel plus inert gas mixture that occurs when a stoichiometric flame will not propagate an explosion.
Hartmann apparatus
Flammability diagrams created from experiments of mixtures containing
fuel,
Air/O2 and
inert (eg N2, CO2)
using Hartmann apparatus.
