# Synthesis gas

In the broadest sense, synthesis gas is a gas mixture that is used for a synthesis , e.g. B. also the mixture of nitrogen and hydrogen for ammonia synthesis . In the narrower sense, synthesis gas is understood to mean industrially produced gas mixtures that mainly contain carbon monoxide and hydrogen in addition to varying amounts of other gases. Depending on the manufacturing process or intended use, some other terms for synthesis gas are also in use: If synthesis gas is obtained from water and coal, it is called water gas , with methane as a source, cracked gas . Methanol synthesis gas is synthesis gas for the production of methanol , Oxogas for hydroformylation (or Oxo synthesis).

## Manufacturing

Synthesis gas can in principle be produced from solid (s-solid), liquid (l-liquid) and gaseous (g-gaseous) starting materials (starting materials).

### Synthesis gas from solid educts

In the production of synthesis gas from solid educts, coal gasification is particularly important . Coal - C (s) is here in a mixture of partial (partial / incomplete) oxidation with air or pure oxygen - O 2 (g) and gasification with water vapor - H 2 O (g) to a mixture of carbon monoxide - CO ( g) and hydrogen - H 2 (g) reacted. Due to the Boudouard equilibrium , CO (g) is still in equilibrium with C (s) and carbon dioxide - CO 2 (g) :

{\ displaystyle {\ begin {alignedat} {2} \ mathrm {2 \; C + O_ {2}} & \ longrightarrow \ mathrm {2 \; CO}, & \ quad & \ mathrm {\ Delta} H = - 221 \; \ mathrm {kJ / mol} \\\ mathrm {C + H_ {2} O} & \ longrightarrow \ mathrm {CO + H_ {2}}, && \ mathrm {\ Delta} H = + 131 {, } 3 \; \ mathrm {kJ / mol} \\\ mathrm {C + CO_ {2}} & \ rightleftharpoons \ mathrm {2 \; CO}, && \ mathrm {\ Delta} H = + 172 {,} 4 \; \ mathrm {kJ / mol} \ end {alignedat}}}

Furthermore, the water gas balance must be taken into account:

${\ displaystyle \ mathrm {CO + H_ {2} O \ rightleftharpoons CO_ {2} + H_ {2}}, \ quad \ mathrm {\ Delta} H = -41 {,} 2 \; \ mathrm {kJ / mol }}$

The reaction with oxygen provides the energy necessary to achieve the high reaction temperature for the endothermic gasification reaction of coal with water vapor through the exothermic reaction .

The composition of the synthesis gas can be controlled by carefully selecting the starting materials (depending on the desired carbon monoxide and hydrogen content).

Since coal contains other elements in addition to carbon ( sulfur , nitrogen , vanadium , ...), the synthesis gas obtained after the reactor has to be cleaned and processed at great expense. Above all, water, CO 2 , soot and H 2 S must be removed.

In addition to coal, the use of other solids such as B. biomass (wood, straw) is conceivable, but a pretreatment of the input materials and a post-treatment or purification of the synthesis gas is necessary.

### Synthesis gas from liquid educts

Different crude oil distillates can be used as liquid starting materials for synthesis gas, both low-boiling and high-boiling fractions. Low-boiling distillates can be converted after removal of sulfur by reaction with steam according to the steam reforming process. The steam reforming process is an endothermic reaction which is carried out on a heterogeneous catalyst (reaction using pentane as an example):

${\ displaystyle \ mathrm {C_ {5} H_ {12} +5 \; H_ {2} O \ longrightarrow 5 \; CO + 11 \; H_ {2}}, \ quad \ mathrm {\ Delta} H = + 802 {,} 9 \; \ mathrm {kJ / mol}}$

When using high-boiling oil fractions ( flashed visbroken residue , see cracking ), the partial oxidation is carried out, which works without a catalyst (reaction using the example of pentane):

${\ displaystyle \ mathrm {2 \; C_ {5} H_ {12} +5 \; O_ {2} \ longrightarrow 10 \; CO + 12 \; H_ {2}}, \ quad \ mathrm {\ Delta} H = -406 {,} 3 \; \ mathrm {kJ / mol}}$

### Synthesis gas from gaseous starting materials

The most important gaseous starting material for the generation of synthesis gas is natural gas . Compared with the other educts, natural gas provides the highest proportion of hydrogen in relation to carbon monoxide.

#### Steam reforming

The natural gas is converted with steam according to the steam reforming process:

${\ displaystyle \ mathrm {CH_ {4} + H_ {2} O \ longrightarrow CO + 3 \; H_ {2}}, \ quad \ mathrm {\ Delta} H = + 206 {,} 2 \; \ mathrm { kJ / mol}}$

#### Plasma converter

A two-stage process developed in 2012 produces synthesis gas, which consists only of carbon monoxide and hydrogen. In the first step, methane is decomposed into a mixture of carbon and hydrogen with the help of plasma at more than 1000 ° C (reaction: CH 4 + energy -> C + 2 H 2 ). In the second step, CO 2 is added to the mixture of carbon and hydrogen. The carbon and the CO 2  react at high temperatures to form carbon monoxide (reaction: C + CO 2 -> 2 CO). Alternatively, water can be used instead of CO 2 in order to obtain a higher concentration of hydrogen in the synthesis gas. Together with the hydrogen from the first step, a high-purity synthesis gas is obtained, which consists only of CO and H 2 .

#### Partial oxidation

In addition to the steam reforming or steam reforming process, natural gas can also be converted into synthesis gas through partial oxidation (POX):

${\ displaystyle \ mathrm {2 \; CH_ {4} + O_ {2} \ longrightarrow 2 \; CO + 4 \; H_ {2}}, \ quad \ mathrm {\ Delta} H = -35 {,} 7 \; \ mathrm {kJ / mol}}$

Synthesis gas for ammonia synthesis is also produced by partial oxidation, in which case air is used instead of pure oxygen. The resulting carbon monoxide is converted (reacted) in a second reaction stage with steam to CO 2 and further hydrogen:

{\ displaystyle {\ begin {aligned} \ mathrm {2 \; CH_ {4} + O_ {2} \; (+ \; 4 \; N_ {2})} & \ longrightarrow \ mathrm {2 \; CO + 4 \; H_ {2} \; (+ \; 4 \; N_ {2})} \\\ mathrm {CO + H_ {2} O} & \ longightarrow \ mathrm {CO_ {2} + H_ {2} } \\\ hline \ mathrm {2 \; CH_ {4} + O_ {2} \; (+ \; 4 \; N_ {2}) + 2 \; H_ {2} O} & \ longrightarrow \ mathrm { 2 \; CO_ {2} +6 \; H_ {2} \; (+ \; 4 \; N_ {2})} \ end {aligned}}}

After removal of CO 2 a mixture of N is then 2 , and H 2 obtained, which then still on the desired N 2 / H 2 must be set molar ratio.

### Synthesis gas from air and electricity

Using amine scrubbing, oxidized carbon (CO 2 ) can be extracted from the ambient air via direct air capture , or more concentrated from flue gas , which can be converted into methane in the Sabatier process . Electricity is required for heating the catalytic converters, operating the amine scrubber and electrolysis of the water to produce hydrogen . This production of gas takes place with an efficiency of up to 80 percent. This gas is also the preliminary stage for the production of e-fuel ( power-to-liquid ), which is then produced using the Fischer-Tropsch process and hydrocracking .

### Synthesis gas cleaning

Most of the manufacturing processes mentioned are followed by more or less time-consuming and complex cleaning and treatment processes after the reactor. These are essentially:

• Soot separation
• Water removal and drying
• Separation of sulfur compounds
• Adjustment of the desired CO: H 2 ratio
• CO 2 separation.

## use

Synthesis gas chemistry products

The most common synthesis gases are used:

1. in methanol synthesis
• ${\ displaystyle \ mathrm {CO + 2 \ H_ {2} \ longrightarrow CH_ {3} OH}}$
2. in ammonia synthesis using the Haber-Bosch process
• ${\ displaystyle \ mathrm {N_ {2} +3 \ H_ {2} \ longrightarrow 2 \ NH_ {3}}}$
3. in oxo synthesis
• ${\ displaystyle \ mathrm {R {-} CH {=} CH_ {2} + CO + H_ {2} \ longrightarrow R {-} CH_ {2} CH_ {2} CH {=} O}}$
4. in the Fischer-Tropsch synthesis
• ${\ displaystyle n \; \ mathrm {CO} + (2n + 1) \; \ mathrm {H} _ {2} \ longrightarrow \ mathrm {C} _ {n} \ mathrm {H} _ {2n + 2} + n \; \ mathrm {H} _ {2} \ mathrm {O}}$

In addition to these chemical-technical areas of application, synthesis gas can also be used biotechnologically via fermentation . Products of this option can e.g. Alcohols such as ethanol , butanol and 1,2-propanediol , acetone and organic acids.