Aliphatic as well as cyclic hydrocarbons CxHy are thermally decomposed (cracked) in a limit case all the way down to elemental (pyrolytic) carbon and gaseous hydrogen. The reactions can be schematically expressed by the following equation
CxHy→ C þ1=2yH2 ð1Þ
At the same time, hydrocarbons with shorter chains are formed in the way described by the equation
CxHy→CmHn þ Cðx−mÞHðy−nÞ ð2Þ
Aliphatic hydrocarbons with one to four carbon atoms in a molecule (m=1 to 4) are contained in pyrolysis gas.
When hydrocarbons containing chemically bound oxygen in their molecule are decomposed, the oxygen in the cracking module is transformed, limitedly all the way down to elemental oxygen, which in turn readily reacts with the other products of the thermal degradation. Exothermic reactions with carbon take place [11]
2C þ O2→2CO ΔH ¼ −246:4 kJ mol−1 ð3Þ
C þ O2→CO2 ΔH ¼ −406:4 kJ mol−1: ð4Þ
Under certain reaction conditions (temperature, pressure), thermodynamic equilibrium is established between the carbon oxides:
2CO↔C þ CO2: ð5Þ
The higher the temperature is, the more the equilibrium concentration shifts in favour of carbon monoxide. Whereas at 550 °C, it yields 10% carbon monoxide, at 1000 °C the percentage is already 99% [12]. At any pressure, there is a quite wide range of temperatures where carbon dioxide dissociates into carbon monoxide and oxygen without carbon being
released [13]
2CO2↔2CO þ O2: ð6Þ
At approximately atmospheric pressure (at 101.32 kPa), this temperature range is between 320 and 480 °C.
The oxygen reacts exothermically with the present hydrogen, yielding water
2H2 þ O2→2H2O ΔH ¼ −482:2 kJ mol−1: ð7Þ
The cracking module has a sufficient amount of energy for endothermic gasifying reactions producing hydrogen to occur between pyrogenetic water and carbon
C þ H2O→CO þ H2 ΔH ¼ þ118:6 kJ mol−1 ð8Þ
or
C þ2H2O→CO2 þ2H2 ΔH ¼ þ16:2 kJ mol−1: ð9Þ
An exothermic reaction (water-shift reaction) with carbon monoxide can subsequently take place, again yielding gaseous hydrogen
CO þ H2O→CO2 þ H2 ΔH ¼ −42:4 kJ mol−1: ð10Þ
At a temperature of 1200 °C in the presence of carbon, thermodynamic equilibrium is established between the carbon oxides again in favour of carbon monoxide (5).