ECN developments and results

SNG (Substitute Natural Gas, or Synthetic Natural Gas) is defined as a gas containing mostly CH4 (> 95% vol.), with properties similar to natural gas, which can be produced from thermochemical gasification of coal or biomass coupled to subsequent methanation. SNG can be cheaply produced at large scale, and is a storable energy carrier, thus enabling whole year operation independently of demand fluctuations.

Due to its interchangeability with natural gas, the use of SNG has a number of advantages over the direct use of coal or biomass. SNG can be injected into the existing grid and easily distributed for transport, heat, and electricity applications. SNG can also be efficiently converted in a number of well-established end-use technologies. Just as natural gas, SNG produces low emissions and has a high social acceptance.

The overall efficiency of conversion from biomass to SNG can be up to 70% on energy basis. SNG can be also used for storage of surplus power from renewable sources (e.g. solar, wind). This is a version of the so-called “power-to-gas” concept, where excess power produces H2, which can be added to an existing SNG-plant to convert additional CO2 into CH4.

ECN has been working in the last years on the development of a technology for the efficient production of SNG from biomass gasification. The MILENA indirect gasification and the OLGA tar removal system are the main achievements of this extensive research and development work. Recently, ECN has developed and patented a novel technology for the methanation of gas from biomass gasification. The ECN System for MEthanation (ESME) is designed especially for gas from fluidized bed gasifiers such as Bubbling Fluidized Beds, Circulating Fluidized Beds and allothermal gasifiers such as the ECN MILENA process or the FICFB process developed by the Technical University of Vienna. ESME allows the efficient conversion of producer gas from biomass gasification to SNG because the hydrocarbons contained in the producer gas (e.g. benzene, toluene) are not removed but converted, and are thus potentially available to be converted to methane. The main parts of the system were extensively tested downstream an atmospheric gasifier in a 500-hour test.

 

25 kW facility at ECN, 2004

        Steam or CO2 or ...

 

800 kWth pilot plant at ECN, 2008

 

4 MWth CHP plant, India, 2015

OLGA tar removal unit

The ECN approach for efficient SNG production

Bio-BTX

There are several options to separate BTX from producer gas, e.g. membranes, liquid absorption and cryogenic separation. Membranes posed a problem of selectivity and thus on the size of the membrane unit, and the use of cryogenic separation requires the prior deep removal of CO2 and H2O in order to avoid operational problems in the cold box. In addition, the energy consumption of the cryogenic unit is very high. Other existing processes are suitable for other gas compositions and concentrations, but cannot be directly applied to gasification producer gas. Eventually ECN developed a process based on the use of a liquid absorption medium, partly because ECN had already gathered a lot of experience in this field from the development of the OLGA tar removal system. For the development work, a first screening of liquid sorbents was carried out. After the selection of the optimal sorbent (a commercial silicon-based oil), stripping of the oil was modelled and experimentally studied. First encouraging results of BTX removal with nitrogen and steam as stripping medium led to the performance of an experimental plan to prove the BTX removal concept at lab scale. An important milestone for bio-BTX recovery at ECN was reached in November 2014, where > 1 kg BTX was harvested for the first time from biomass gasification producer gas at ECN laboratories. A BTX removal of ~94% was achieved. During the test, ~ 14 NL/min producer gas was fed to the BTX scrubber for almost 76 hours. The absorber operated at 35°C. The addition of a tracer gas and the online measurement of the inlet and outlet gas compositions allowed the performance of mass balances and the calculation of the removal efficiency. The removal efficiency kept around 94% when operating the stripper with 820 g/h steam. Reducing the stripping steam to ¼ of the initial flow made the efficiency drop to 82%.

Bio-BTX recovery during tests in November 2014. The upper yellow layer corresponds to bio-BTX, the lower layer is condensed water from the stripper

Total bio-BTX harvest during test in November 2014

BTX Scrubber setup at ECN laboratories

 

Composition of the bio-BTX, test November 2014

Total bio-BTX (g)

1091.36

Bio-BTX composition (wt.%)

Benzene

86.6

Toluene

6.54

Xylenes

0.2

Total BTX

93.34

Ethylbenzene

0.15

Styrene

1.14

Cresol

0.28

Naphthalene

0.57

Rest aromatics + unknowns

0.72

Thiophenes

0.12