Chapter 10

Combustion

Solid Fuel

Ariziel Ruth D. Marquez

SOLID FUEL

• Coal – originated from the arrested decay of the remains of trees, bushes, ferns, mosses, etc.

• Contents of solid fuel

▫ Free moisture

▫ Combined moisture

▫ Carbon, C

▫ Net hydrogen, H

▫ Nitrogen, N and sulfur, S

▫ Ash

SOLID FUEL • Free moisture – amount of water the solid fuel

adsorbed in its surface. It is obtained experimentally by heating the sample for one hour at 104oC to 110oC. The loss in weight is called moisture content.

• Combined moisture – water that maybe formed theoretically from the oxygen and hydrogen present in the fuel.

• Total moisture – equivalent to oxygen in the ultimate analysis.

SOLID FUEL • Net hydrogen – hydrogen other than that in the

moisture and combined moisture. It is the burnable hydrogen.

• N and S – usually present in small amount. Often neglected though present in amounts ranging from 1-3%.

• Ash – the mineral contents of coal. The inorganic constituents which are not burned.

Two types of analysis 1. Proximate Analysis – laboratory method of analysis

based on the volatilization characteristics of solid fuel ▫ Moisture – the loss in weight after heating at 104oC for one hour

▫ Volatile Combustible Matter (VCM) – the loss in weight of the sample upon ignition in a covered crucible for 7 minutes at 250oC. Mostly combined water, C and H.

▫ Fixed carbon (FC) – combustible matter left after ignition. It is pure carbon.

▫ Ash – residue left from complete combustion of the sample in open air at 725oC.

Two types of analysis 2. Ultimate Analysis (Air-dried/as fired basis) – reports the

percentage by weight of ash and each element in coal ▫ Elemental composition of the solid fuel expressed as %C, %H, %S,

%N, %Ash and the balance %O.

Example: Proximate and Ultimate Analysis of Pennsylvania Coal PROXIMATE ULTIMATE Moisture 3.2% Carbon 79.9% VCM 21.0% Hydrogen 4.85% FC 69.3% Sulfur 0.69% Ash 6.5% Nitrogen 1.30% Ash 6.50% (Oxygen 6.76%)

Relation of Proximate and Ultimate Analysis

• C in VCM

▫ Total C = C in VCM + Fixed C

• Net H

▫ Total H = Net H + H equiv to O (H2O)

• Combined H20

▫ Total H2O = Combined H2O + Free H2O

Estimation of Coal • Dulong Formula

HHV = 14,544 C + 62,028 (H – O/8) + 4050 S

where : HHV - higher heating value of the coal [btu/lb] C & S - weight fractions of carbon & sulfur, respectively (H – O/8) - weight fraction of the net hydrogen • Calderwood Equation C = 5.88 + 0.00512 (B – 40.5 S) +/- 0.0053 [80 – 100(VCM/FC)] 1.55

where : C, S, VCM & FC – in weight percent B - higher heating value (HHV) in BTU/lb

Combustion of Coal

• Complete combustion

COAL FLUE GAS CO2, H2O SO2,O2, N2

AIR

REFUSE Ash

Combustion of Coal

• Incomplete combustion

COAL FLUE GAS CO2/CO, H2O SO2,O2, N2

AIR

REFUSE

Unburned combustibles Ash

Unburned combustibles

1. Same composition as the original coal

▫ Refuse – unburned coal

Ash

refusecoal VCM

FC

VCM

FC

COAL FLUE GAS

AIR

REFUSE % FC % VCM % Ash

Unburned combustibles

2. Essentially carbon

▫ Refuse – Coked coal Unburned coal Ash

▫ Refuse – Coked coal Ash

refusecoal VCM

FC

VCM

FC

COAL FLUE GAS

AIR

REFUSE % FC % FC % VCM % Ash % Ash

Combustion of Coal

Ex. Fuel: 37.0 % FC Refuse: 7.40%FC 40.5 % VCM 8.25% VCM 8.9 % Ash 84.35% Ash HV = 14350 BTU/lb Calculate for the percent of carbon lost in the refuse and the percent of HV lost in the refuse. Ex. Fuel: 4.0% H2O Refuse:62.00% H2O 23.0 % FC 11.00%FC 64.0 % VCM 3.00% VCM 9.0 % Ash 24.00% Ash HV = 14000 BTU/lb Calculate for the percent of carbon lost in the refuse and the percent of HV lost in the refuse.