Firing Fuels with Low Heating Values

Table 1.6 shows a few gaseous fuels with low heating values. When fuels with low heating values are fired in a boiler, one has to be concerned about the following:

1. Low combustion temperature resulting in a low gas temperature at the furnace exit.

Sometimes a supplementary fuel such as oil or natural gas is fired to a small extent to maintain a stable flame. If a superheater is located directly at the furnace exit, the steam temperature may have to be checked as the LMTD will be significantly lower.

2. Large amount of flue gases will be generated for the same steam generation while firing the low Btu fuel. This increases the convective heat transfer and also the gas velocities in various heating surfaces. If a boiler is operated on natural gas as well as a low Btu fuel, then the turndown will be lower due to low gas velocities and flow distribution concerns while firing natural gas at low loads.

3. The boiler exit gas temperature will be higher due to the higher flue gas flow, and

this decreases the boiler efficiency.

4. The back pressure will also be higher due to the higher gas pressure drop with larger flue gas flow. Fans may have to be checked for suitability on low-Btu fuel firing. Tables 1.12 through 1.14 show the performance of a boiler with two different fuels.

Example 1.13

A boiler generating 100 t/h of saturated steam at 28.7 kg/cm 2 g is fired with two fuels

as shown in Table 1.12. Column 1 shows the low heating value fuel, and the other is the typical natural gas with a high heating value and the fuel analysis.

Table 1.13 shows the flue gas analysis and the various losses and boiler efficiency. It may be seen that the % volume water vapor is much lower with the low BTU fuel. The convective heat transfer coefficient will be high, while the nonluminous coefficient will

be much smaller. The back pressure is nearly double with the low BTU fuel. Fan power consumption will be very high due to the high back pressure. The exit gas temperature is about 40°C higher due to the higher mass flow of flue gas with low-BTU fuel. If a superheater is present, depending on its location (convective or radiant), the steam temperature could also be significantly different.

TABLE 1.12

Fuels Fired in the Boiler

Gas, % Volume

Low Heating Value Fuel

Nat Gas

Methane 0.1 97 Ethane

0 1 Hydrogen

2.4 0 Carbon monoxide

23.3 0 Carbon dioxide

14.4 0.5 Nitrogen

56.4 1.5 Water vapor

3.4 0 LHV, kcal/kg

11,480 HHV, kcal/kg

28 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 1.13

Losses and Efficiency

Low Heating Value Fuel Nat Gas

Dry gas loss 8.88 3.59 % Air moisture

0.10 0.10 % Fuel moisture

1.84 10.63 % Casing loss

0.30 0.30 % Unacc/margin

0.50 0.50 % Efficiency, lhv

89.79 94.17 % Efficiency, hhv

88.39 84.89 % Furnace back pr

162.00 mm wc % vol CO 2 23.60 8.25 % H 2 O

Fuel flow

5187 kg/h

TABLE 1.14

Boiler Performance on Natural Gas and Low-Btu Fuel

Boiler Load, %

Boiler duty 55.82 55.81 MM kcal/h Ambient temp.

26.7 26.67 °C

Relative hum.

Excess air

FGR

Fuel input (hhv) 66.05 63.43 MM kcal/h Heat rel. rate (HHV)

kcal/m 3 h Heat rel. rate (HHV)

kcal/m 2 h Steam flow

kg/h Steam pressure

38.7 38.7 kg/cm 2 g Steam temp.

± 5°C Feed water temp.

± 5°C Water temp. lvg eco

± 5°C Blowdown

Boiler exit gas temp.

± 5°C Eco exit gas temp.

± 5°C Air flow

kg/h Flue gas to stack

kg/h Fuel

Nat. gas

Low BTU

Max gas velocity—boiler 34 44 m/s Economizer

9 14 m/s

Combustion Calculations

Firing Fuels with Low Heating Values