Renewable Energy Technology 3(2+1)

Lesson 8. Performance evaluation of gasifiers

1. Start up of the gasifier

Small amount of ignited charcoal will be placed in the bottom of the gasifier and further feed material will be added.  The blower connected with the gasifier will be was turned on and air is supplied into the reactor.  Air supply was regulated by regulating valves and measured using rotameters before entering into the reactor.  This initiates the gasification.  The reactor is closed for providing conducive environment with higher temperature and proper oxygen supply.  Instead of using charcoal, LPG torch system can be used to ignite the bed of feed material.   

2. Air requirement

The air requirement for gasification of fuel materials can be calculated by calculating the stoichiometric air requirement for complete combustion and equivalence ratio (ER) adopted.  Stoichiometric air requirement is calculated from the following formula.

                                                2.7 C + 8 H₂ + S – O₂

                        Q_C       =         --------------------------

                                                             0.23

where

Q_C – Stochiometric air supply, kg of air / kg of feed material

C, H, S and O - elemental composition of carbon, hydrogen, sulphur and oxygen, per cent

Q_G       =          ER x Q_C

where

Q_G  - Gasification air supply, kg of air / kg of feed material

ER – Equivalence ratio

A_F        =          Q_G x F

where

F  - Feed rate, kg h^-1

The producer gas was taken out from the reactor and passed through the cleaning systems.  The tar, char, dust and carryover particles were collected and the clean gas was brought down to the burner system for thermal applications or to the engines.

3. Instrumentation

The methods and instrumentation used for measuring various parameters in the study are given below.

3.1 Flow measurement

Airflow rate to the reactor was measured using rotameters.  The flow rate of outlet producer gas was measured using a sharp-edge orifice meter, the most widely employed flow-measuring element. 

               

where

C_d           -           Coefficient of discharge

A_1f         -           pipe cross-sectional area, m²

A_2f       -           orifice cross-sectional area, m²

p₁, p₂    -           static pressures, Pa

ρ_g         -           density of gas, kg m^-3

 3.2 Temperature measurement

Temperature readings at various zones such as drying, pyrolysis, oxidation and reduction zones are measured using K-type thermocouples, which can be connected with digital indicators or recorder with data loggers.

3.3 Gas composition

Composition of generated producer gas was analyzed using gas chromatograph.

4. Performance results

From the gasification results, parameters such as gas yield (Nm³ kg^-1), specific gas production (Nm³ m^-2 h^-1), calorific value (kCal/Nm^-3) and hot gas efficiency (%) can be calculated as per the methods given below. 

4.1 Gas yield

The gas yield (G_y) was found out from the flow rate of producer gas (Q_t) and feed consumption rate (F_r).  The formula used was given below.

                                                    Q_t

                        G_y, Nm³ kg^-1 =----------

                                                     F_r

4.2 Specific Gasification Rate

Specific gasification rate (SGR) was calculated using the weight of dry rice husk gasified for a run, net operating period and the cross sectional area of the reactor using the following relation.

            Weight of feed material used, kg h-1

SGR= ----------------------------------------------

          Cross sectional area of the reactor, m²

 

4.3 Specific Gas Production Rate

Specific gas production rate is the rate of producer gas generation at STP per unit cross-sectional area of the gasifier.

 

                    Rate of gas production, m³ h^-1

SGPR = ----------------------------------------------

             Cross sectional area of the reactor, m²

4.4 Calorific value of producer gas

Calorific value of producer gas (CV_PG) was calculated based on the composition such as CO, CO₂, CH₄, H₂, N₂, O₂ and water.  The procedure followed in calculating calorific value of producer gas is:

                                                         Σ X_i CV_i

                        CV_PG, kCal Nm^-3 =--------------

                                                              Z

where

X_i        -           mole fraction of i^th component of gas

CV_i      -           calorific value of i^th component of gas  

Z          =          Compressibility factor  =  

4.5 Hot gas efficiency

The performance of the gasifier was determined by calculating hot gas efficiency of the reactor system.  Hot gas efficiency was found out as follows.

                                        Q_t x CV_PG

                        η_H, %   =--------------   x 100

                                        F_r x CV_F

            where

CV_PG   -           calorific value of producer gas, MJ/Nm^-3

     CV_F     -           fuel calorific value, MJ/Nm^-3