Gasification of Biomass using Solar Energy

by Angelo Maria Sepe

Supervisor: Dr. Manosh Paul

Introduction

Objectives

Modeling the gasification

process

Reactor Concept

Chemical Equilibrium Analysis

Solar flux sensitivity analysis

Main results

Principal objectives of this work are:

• To perform an analysis of the state of

the art of Solar Gasification technology.

• To design an efficient solar reactor

configuration.

• To develop a computational model to

asses and evaluate reactor

performance.

• To asses the solar technology benefits

respect to conventional gasifiers.

• To compare different type of suitable

feedstock to be used in a solar-assisted

process

Nowadays, one of the most important

challenge is the development of

sustainable energy processes to satisfy

the increasingly demand of energy but also

able to slash carbon emissions. The

majority of greenhouse gases emission is

coming from combustion of fossil fuels,

which is still the predominant way to

produce energy. The gasification of fossil

fuels is a well-studied and established way

to generate quality gas products. A

promising alternative to the conventional

gasification process is represented by

Solar-assisted steam-only gasification,

which can convert biomass or fossil fuels

into a quality syngas, high in H2 content

and CO2 free. Concentrated solar energy

is used to drive the highly endothermic

gasification reactions.

A 1D multi-phase steady-state model has

been created and implemented in

MATLAB. The gasification process is

modelled by means of heterogeneous and

homogenous gasification reactions that

were coupled with a radiative transfer

method based on the radiative flux density.

Pyrolysis was modeled as a one-step

mechanism. A system of highly coupled

differential equations composed by Mass

and Energy balances for both solid and gas

phases were solved with a finite volume

method and using an implicit solver.

Species concentration, temperature profile

and gas production rates are examined.

Heterogenous (Solid-Gas) reactions:

Rs1 C + 1/2O2 → CO

Rs2 C + CO2 → 2CO

Rs3 C + 2H2 → CH4

Rs4 C + H2O → H2 + CO

Homogeneuous (Gas-Gas) reactions:

Rg1 CO + ½ O2→ CO2

Rg2 H2 + ½ O2 →H2O

Rg3 CH4 + 2 O2 →CO2 + 2H2O

Rg4 CmHn + (m/2 + n/4) O2→ mCO2 + n/2 H2O

Rws CO + H2O ←→CO2 + H2

Solid energy balance:

Irradiance given by Rosseland approx.:

A zero-D model has been developed to provide an overview of

syngas production in function of key parameters as

Temperature, ER, Steam/Carbon ratio.

1. Solar gasification using steam as gasification

agent proved to be an extremely efficient

way to convert different type of fuels into high

quality gas.

2. Fixed bed technology seems to be the most

suitable for this scope since it is more efficient

than other reactor types.

3. Increasing the solar input increases syngas

production rates.

4. An optimal value of the solar input can be

found, after what results stall.

5. Biomass is the feedstock with major gains for

highly concentrated solar inputs.

6. The most critical zone in a solar reactor is the

bed entrance, which is where important

reactions and heat&mass transfers take place.

7. Hybrid solar/O2 systems can be used to

prolong the operative time of a solar gasificator.

A new solar-assisted gasificator has been proposed in fixed

bed, downdraft configuration. This type of reactor was

chosen for its documented high efficiency and versatility.

Solar-assisted gasification process

Source: B.J. Hathaway,Integration of solar gasification with conventional fuel production.

Autothermal vs Solar