Gasification

Gasification occurs when the fuel reacts with steam and oxygen under given conditions. In this case the fuel is consider as coal containing carbon, hydrogen, nitrogen, oxygen and sulfur.

Some gasification processes use air instead of oxygen. In this report air gasification is left out.

With air the fraction of inert N2 is high and it will require larger equipment to reach the same production rate as an O2 gasifier.

Figure 2.17. Gasification process.

Figure 2.17 gives an overview of what occurs in the gasification process. Coal reacts with steam and oxygen and creates syngas. Syngas is a mixture of carbon monoxide and hydrogen.

In addition to CO and H2 the reactions will form other products that will be described through this section, but the goal is to optimize CO and H2 production since these can be utilized in a gas turbine. For coals containing moisture there may be unnecessary to add steam.

The gasification process can be represented by equilibrium conditions and to describe gasification it is possible to set up an innumerable number of reactions between carbon, hydrogen, oxygen, water, nitrogen and sulfur.

2.2.2.1 Formation reactions

The first step to calculate the output composition from the gasifier is to look at the formation

reactions [18].

1 2 2

C+ O CO -111MJ/kmol (2.30)

2 2

C O+ →CO -394MJ/kmol (2.31)

Equation 2.30 and 2.31 shows the oxygenolysis where carbon reacts with oxygen to form carbon monoxide and/or carbon dioxide. In this case the oxygen feed will occur as pure oxygen and not in an air mixture. Equation 2.31 is the combustion reaction for pure carbon.

Sufficient amount of O2 gives complete combustion and creates CO2. For gasification the CO formation step is important. This occurs when there is deficiency of O2 in a high temperature

reactor, above 800˚C [18].The corresponding heat of reaction states if the reactions are

endothermic or exothermic. A negative sign indicates an exothermic reaction meaning that the reaction will liberate energy, and opposite for a positive sign. The values of the heat of the reaction are temperature dependent and are here determined for 25˚C (298K). The variation of the value will though not vary significantly with temperature, as an example the value for equation 2.30 will increase approximately 5MJ/kmol if the temperature is increased to 1500K.

With deficit of oxygen C may react with CO2 according to the Boudouard reaction [18].

2 2

C CO+ CO +172MJ/kmol (2.32)

Methane will also be formed when carbon reacts with hydrogen. This reaction is called hydrogenolysis. The hydrogen comes from the coal and from the water feed. The later explained hydrolysis reaction contributes to produce pure H2 [18].

2 4

2

C+ H CH -75MJ/kmol (2.33)

Reaction 2.30 and 2.33 react inverse according to temperature. The production of CO increases with increasing temperature while the CH4 production is on a maximum level around 300˚C and decreases with increasing temperature. For power applications with CO2

capture the formation of CO is of interests and reaction 2.30 plays the major role compared to reaction 2.33. The formation of CO is important because it can react with H2O in a water gas shift reaction and create H2. This reaction will be discussed in 2.2.2.4 Water gas shift

reaction.

Converting coal to syngas involves water, fed as steam or as a part of the coal. Reaction with water is called hydrolysis or water gas reaction and it is an endothermic reaction [18].

2 2

C H O+ CO H+ +131MJ/kmol (2.34)

The water gas reaction also produces more syngas with higher temperature. The partial pressure of water steam is proportional with syngas production as well.

Equation 2.30 and 2.34 describe the most relevant gasification reactions for this case. The heats of reactions show that reaction 2.30 is exothermic and reaction 2.34 is endothermic.

That indicates that most of the gasification will occur via reaction 2.30. On the other hand there will be more economical to maximize reaction 2.34. This reaction yields two syngas molecules per atom of carbon, while the first reaction only gives one syngas molecule.

Another issue is that steam is cheaper than oxygen, so one in this case one will anyway gain on maximizing the water gas reaction.

Formation of water as a result of hydrogen combustion appears in a well known equation [18].

2 1 2 2 2

H + O H O -242MJ/kmol (2.35)

The nitrogen and sulfur will react with the oxygen and form equilibrium with nitric oxide (NO) and sulfur oxide (SO2) [18].

2 2

1 2N +1 2O NO +90MJ/kmol (2.36)

2 2

S O+ SO -362MJ/kmol (2.37)

These substances are represented in small amounts, but they play an important role in the pollution calculation and they are therefore not neglected.

The sulfur will in a gasification process also react with the hydrogen and create the acid gas hydrogen sulfide (H2S) [18].

2 2

S H+ H S -85MJ/kmol (2.38)

The gasification process occurs as a set of exothermic and endothermic reactions. There will in a gasification process be an increasing in temperature during the process. This indicates that some of the chemical energy is transformed to heat. The amount of chemical energy in the product gas compared to the amount of energy in the feedstock can be expressed as the cold gas efficiency (CGE).

product

The gasification process will roughly appear under equilibrium conditions. Equilibrium states are helpful for deciding the composition of substances after the gasification. Equation 2.30, 2.31, 2.33, 2.35, 2.36, 2.37 and 2.38 can be rearranged at equilibrium form [18] [19].

[ ]

For the equilibrium equations Ki is the equilibrium constant, yi indicates the molar fraction of the substances and p the pressure. In fact Ki is not constant but temperature dependent. The values of Ki for a given temperature are tabulated. In a gasification system the temperature will vary a lot during the process and the equilibrium “constant” has to be modeled as a function of the temperature.

In a reactor producing syngas a lot of reactions occur at the same time. Substances will be created and broken during the process. Equation 2.30 to 2.38 shows the main reactions in converting coal, oxygen and steam into synthetic gas. The most interesting component in coal is the carbon. It is the main energy carrier and its heating value is of interests for the power production. In fact hydrogen has a higher heating value than carbon on mass basis, but there is normally significant less amount of it in the coal.

The main product of a gasification process is carbon monoxide and hydrogen. A typical molar product composition is about 60% CO, 30% H2 and the remaining 10% consist mainly of CO2, H2O, CH4, NOx, SOx and H2S. The amount of these substances will vary according to coal type, gasification conditions (temperature and pressure) and the amount of oxygen and steam in the feed.