Chapter 9, Problem 9.70P

Interpretation Introduction

(a)

Interpretation:

The rate (SCF/h) at which the biogas produced in the digester should be calculated along with the total heating value (Btu/h) of the gas.

Concept introduction:

The total solid obtained is the product of amount of water that is converted to solid under water treatment process and the solid of the feed digester can be calculated as follows:

${\text{Total solid=Solid}}_{\text{(water)}}{\text{L/day×Solid}}_{\text{(digester)}}\text{kg/L}$

The total heating value is obtained as:

Total heating value for gas = Fuel flow rate × LHV

Where, LHV is lower heating value.

Interpretation Introduction

(b)

Interpretation:

The rate of heat transfer (Btu/h) between the hot water and the sludge should be calculated along with the volumetric flow rate (ft³/h) of the water passing through the heat exchanger.

Concept introduction:

The heat transfer between hot medium and the sludge is calculated as: $Q=h\times A\times \left(T_{\text{medium}}-T_{\text{sludge}}\right)$

Here, h is the heat transfer coefficient and A is the sludge surface area while T_medium and T_sludge are the temperature of hot water medium and the sludge respectively.

Interpretation Introduction

(c)

Interpretation:

The fraction of the digester gas that must be burned to heat the water from 160? to 180? should be obtained and it should be commented that what will happens to the other 20% of the heating value.

Concept introduction:

The biogas is burned in 80% efficient boiler, there is some amount of moisture content present in gas, so that some amount of heat is used to remove these moisture content.

The mass flow rate will be calculated from this formula:

$m=\frac{Q}{C_p\times dT}$

Where m is the mass flow rate, Q is heat flow rate a, C_p is heat capacity and dT is change in temperature.

Interpretation Introduction

(d)

Interpretation:

If there is excess digester gas available after meeting the process-water heating demand, then the potential uses should be explained.

Concept introduction:

The boiler furnaces have the heavy application of digester gases which are consumed to heat the furnace. But, when the water-heating demand is filled, there remains the gases and energy consumed in heating the boiler and these sources of energy can be consumed in various other purposes.