Natural gas preheating

In the relaxation of natural gas results in the cooling thereof, which is why it first by means of a preheating system is heated, in order impairment of piping systems to prevent.

General

Natural gas is compressed to up to 80 bar for transport over long distances in order to keep pipeline diameters small. The pressure is reduced to a few millibars again by the end customer. When the gas is expanded, the Joule-Thomson effect cools down. In order to prevent the gas pressure control system from being impaired, the natural gas must therefore be preheated before the pressure is reduced.

Possible effects through cooling are: condensation of water contained in natural gas → corrosion, condensation of humidity on the outside of the gas pipe → corrosion, methane hydrate formation → clogging of the gas pipe and fittings, icing → impairment of the function of the fittings.

Preheating systems

Indirect preheating systems

Today, natural gas is almost exclusively preheated indirectly, i.e. using heated water and a water / natural gas heat exchanger. The water, in turn, is usually heated by a gas boiler , as is familiar from the household. As a heat exchanger is used for both lying and standing shell and tube heat exchanger in which the gas stream is divided into many smaller lines, all of which are immersed in the hot water.

Another possibility of indirect preheating, with correspondingly large gas pressure regulating systems, is the coupling of a gas-engine block- type thermal power station with a top gas boiler. The advantage of this variant lies in the fact that the process is made more effective than conventional preheating through the principle of combined heat and power . The electricity generated here covers the electricity required by the gas pressure regulating system (required by measuring instruments , regulators , electrical installations, etc.) and the excess electricity can be fed profitably into the public electricity supply network.

Direct preheating systems

Another possibility is to use a natural gas heater, in which the combustion chamber is practically located in the heat exchanger.

Control and heat demand for natural gas preheating

Scheme of natural gas preheating with subsequent throttling

Course of physical quantities during preheating and throttling

The idealized curves of the state variables gas pressure and gas temperature are plotted over the processes of natural gas preheating and the subsequent throttling.

The term OP stands for operating pressure . The index u for upstream to German upstream, on the input side, and the index d for downstream to German downstream, on the output side.

These terms, along with others, are used in the following calculation process to determine the heat output.

Determination of the required heat output

This can be calculated using the following equation:

${\displaystyle W_{\text{Gas}}={\frac {{\dot {V}}_{n}\cdot \rho _{\text{n,Gas}}\cdot c_{\text{p,m,Gas}}\cdot (T_{\text{soll}}-T_{\text{e}}+(p_{\text{e}}-p_{\text{a}})\cdot \mu _{\text{m,JT}})}{3600{\frac {s}{h}}}}}$

Here are:

${\displaystyle \ W_{\text{Gas}}[kW_{\text{th}}]}$: required heat requirement for natural gas preheating ${\displaystyle {\dot {V}}_{n}[{\frac {m^{3}}{h}}]}$: Gas volume flow in the normal state ${\displaystyle \ \rho _{\text{n,Gas}}[{\frac {kg}{m^{3}}}]}$: Standard density of the gas ${\displaystyle \ c_{\text{p,m,Gas}}[{\frac {kJ}{kg\cdot K}}]}$: mean specific heat capacity of the gas

${\displaystyle \ T_{\text{soll}}[^{\circ }C]}$: Setpoint gas temperature downstream of the gas pressure control valve ${\displaystyle \ T_{\text{e}}[^{\circ }C]}$: Gas inlet temperature upstream of the heat exchanger ${\displaystyle \ p_{\text{e}}[bar]}$: Inlet pressure upstream of the heat exchanger ${\displaystyle \ p_{\text{a}}[bar]}$: Outlet pressure after the gas pressure control valve ${\displaystyle \ \mu _{\text{m,JT}}[{\frac {K}{bar}}]}$: mean integral Joule-Thomson coefficient of the gas (= 0.532)

literature

• Günter Cerbe: Basics of gas technology Carl Hanser Verlag, Munich / Vienna 2004, ISBN 3-446-22803-9