Renewable Energy Technology 3(2+1)

Lesson 9. Energy analysis of gasification

i. Mass balance

Mass balance study can be conducted for the gasification of biomaterials.  The various mass input and out put to be considered and the corresponding calculation procedure is given below.

Mass input

1. Feed rate, kg h^-1                             :           F_r
2. Air flow, kg h^-1                               :           F_a
3. Charcoal, kg                      :           Mc

            Total mass input (M_I)              =          F_r + F_a + Mc   

Mass output

1. Product gas, kg h^-1             :           Q_t x F_r x ρ_G
2. Tar, kg h^-1                                          :           Mt
3. Char/ dust, kg h^-1                         :           Mch
4. Ash, kg                             :           M_A
5.  Char coal, kg                    :           Mco

            where

            Q_t        -           quantity of producer gas generated, Nm³ h^-1

            ρ_G        -           density of producer gas, kg Nm^-3

            Total mass output (M_O)           =          Q_t x F_r x ρ_G + Mt + Mch + M_A + Mco           

Mass balance is given as,

Total mass input (M_I) - Total mass output (M_O) = 0

F_r + F_a + Mc – (Q_t x F_r x ρ_G + Mt + Mch + M_A + Mco) = 0

From the mass difference worked between input and out put, accuracy of the method adopted for calculation can be determined by mass closure procedure.   

ii. Energy balance

The procedure adopted for the calculation of energy balance for fluidized bed gasification of all biomaterials are given below. 

Energy input

1. Feed, MJ h^-1                                     :           F_r x CV_F
2. Charcoal, kg                      :           Mc x CV_C

where

CV_F     -           calorific value of feed material, MJ kg^-1

CV_C     -           calorific value of char, MJ kg^-1

3. Air (if preheated)                    :           m_a cp_a dt_a

            where

                        m_a        -           mass of air, kg

                        cp_a       -           specific heat of air, MJ kg^-1ºK^-1

                                dt_a          -           temperature difference in air, ºK

Total energy input (E_I)            =          (F_r x CV_F) + (Mc x CV_C)       

Energy out put

1. Gas energy output, MJ h^-1 (Eg)

=(Q_t x F_r x ρ_G x CV_G) + (Q_t x F_r x ρ_G x cp_G x ∆Tg)

where

ρ_G           -           density of producer gas, kg Nm^-3

cp_G       -           specific heat of producer gas, MJ kg^-1

∆Tg      -           temperature difference of producer gas to atmosphere, ºC

2. Char energy out put MJ h^-1 (Ec) =      (Mch x CV_C) + (Mch x cp_ch x ∆Tc)

            where

cp_G       -           specific heat of char, MJ kg^-1

∆Tc      -           temperature difference of char to atmosphere, ºC

3. Energy loss,  MJ h^-1 (Ec)         =          U x S_re x hr x ∆Tr

            where

U         -           overall heat loss coefficient

S_re        -           Surface area of the reactor, m²

hr         -           heat transfer coefficient, kCal m^-2 ºC

∆Tr      -           temperature difference of reactor to atmosphere, ºC

Total energy out put (Eo)        :           Eg + EC + EL

Energy balance is given as,

Total energy input (E_I) - Total energy output (M_O) = 0

(F_r x CV_F) + (Mc x CV_C) – [(Q_t x F_r x CV_G) + (Q_t x F_r x ρ_G x cp_G x ∆Tg) + (Mch x CV_C) + (Mch x cp_ch x ∆Tc) + EL] = 0

The difference in energy between input and out put of the study gives the accuracy of the method adopted for calculation of energy balance and the corresponding energy closure calculation.