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.
