1 Introduction
The methanol economy has been advocated since the 1990s. Methanol has been suggested as a potential replacement for fossil fuels in energy storage, as fuel and as raw material for synthetic hydrocarbons and their products. Methanol price also could be potentially lower than distillate when it is produced based on coal gasification [1].
One of the fuel uses for methanol is to produce power through gas turbines (GTs). Tests have shown that, with minor system modifications, methanol is readily fired and fully feasible as a gas turbine fuel. Effective utilization of methanol as a fuel in power systems will become an important issue in the future [2].
Methanol indirect combustion typically includes two kinds of chemical reactions: methanol decomposition (dry methanol cracking) and methanol steam reforming. Gaseous methanol can be catalytically decomposed to carbon monoxide and hydrogen according to the reaction:
CH3OH = CO + 2 H2
Equation 1
Energy-Tech Magazine
May 2013 · Energy-Tech Magazine
|
|
|
|
|
|
|
|
|
|
Advertisement |
| | ||||||||||
This takes place rapidly with respect to temperature. Pressure hinders the decomposition process so that a higher temperature is required when pressure increases.
The chemical reaction, whereby methanol is catalytically steam-reformed, is the following:
CH3OH + H2O = CO2 + 3 H2
Equation 2
This reaction takes place through a catalyst being active above 200°C and at the same time, the catalyst is not active for the reverse reforming reaction (methanation). It is possible to achieve full conversion at relatively low temperature [3, 4].
There is an assortment of techniques that take advantage of the methanol indirect combustion, resulting in a variety of power cycle configurations. [5-7].
In recent years, concerns about global warming have driven the potential for imposing limits on greenhouse gas emissions from conventional fossil fuel power plants. However, greenhouse gas issues for methanol fueled power systems are not addressed in the noted studies.
This paper develops two advanced combined-cycle systems based on methanol indirect combustion:
- Methanol decomposition combined-cycle system without CO2 capture
- Pre-combustion CO2 capture combined-cycle system based on methanol steam reforming
This study is based on process modeling using Aspen Plus and GTPro software tools. In order to carry out a comprehensive study, two conventional combined-cycle configurations with/without CO2 capture are modeled and compared. Based on the simulation results, exergy analysis is carried out to identify advantages of the new systems.
2 Methanol indirect combustion combined cycle
2.1 System description
Figure 1 shows a flow schematic of a methanol decomposition combined-cycle system. The liquid methanol as a fuel, at pressure above 22 bar, is first heated and converted into gas through 3 stages (preheating, evaporation and superheating) to a temperature of 220°C. The superheated methanol proceeds to an endothermic reaction with a catalyst in the reactor. The reaction takes place at constant temperature of 220°C, and the majority of the methanol gas is decomposed into syngas (CO and H2) as per Equation 1 [7]. The sources for methanol heating and decomposition are:
- Intermediate pressure (IP) steam from high pressure (HP) steam turbine exhaust, cold reheat steam
- Low pressure (LP) steam from IP turbine
The combined cycle in Figure 1 consists of a gas turbine, HRSG and steam turbine generator. The minor differences from conventional combined-cycle configurations are:
- Gas turbine burns syngas;
- Steam turbine has two stages of extraction steam.
| Go to Page 1 2 3 4 |  |
ADVERTISEMENTS