Exergy Analysis of Direct and Indirect Combustion of Methanol by

Utilizing Solar Energy or Waste Heat

H. P. Nuwan S. De Alwis, Abdulmajeed A. Mohamad, and Anil K. Mehrotra

The paper compares the Direct and indirect (conversion to syn gas before subsequent

combustion) of methanol using exergy analysis. In the indirect process, waste heat or

solar energy is being used to decompose methanol in Syn gas (CO and H2) before

combustion.

Exergy can be defined as the available or net extractable heat of a system. According to

the author's analysis, the indirect route not only extracts more energy but also has a

higher exergetic efficiency (6-7% higher) compared to direct combustion of methanol.

The main idea of the paper is to be able to convert and store waste heat or renewable

energy in the form of chemical energy, thereby reducing consumption of petroleum fuels.

Direct Combustion of Methanol:

The overall exergetic efficiency of the direct combustion of methanol is defined as the ratio of net exergy output of the system to the exergy of methanol feed. That is,

= + 1

0

100%

Fig.1: Schematic of combustion system (direct combustion of

methanol) showing the exergy transfer

Indirect Combustion of Methanol :

The overall exergetic efficiency of the indirect combustion of methanol is defined as the ratio of net exergy output of the system to the total exergy input of the

system, which consists of exergy of the total heat input and the exergy of ethanol

feed. That is,

= + 1

0

10 +

100%

Results and Analysis:

Maximum exergy upgrade was possible between 400-500K which also corresponds to max decomposition efficiency.

Exergy loss of Direct combustion is higher, as methanol has a tendency to convert to syn gas between 375-500K. Exergy losses in indirect process on the other hand

are solely because of losses in decomposition and combustion processes.

Fig.3: Exergy destruction of direct and indirect combustion with

decomposition temperature.

Fig.2: Schematic of reactor and combustion system (indirect combustion)

showing the exergy transfer.

Nomenclature:

out Exergy of the exit stream H2O Exergy of H2O CO2 Exergy of CO2 in Exergy of input stream (methanol) CH3OH Exergy of Methanol Qdirect Heat from combustion

T0 Ambient temperature

T Operating temperature

Exergetic efficiency of indirect combustion Exergetic efficiency of direct combustion

Q Heat Transfer

Fig.4: Overall exergetic efficiency with decomposition temperature