Бурение грунтовых зондов, установка энергетических колодцев

The energy crisis of the 1970s and the accompanying concerns about a shortage of natural gas gave rise not only to intensive research into hydrogenating gasification systems but also a large number of projects for the manufacture of synthetic natural gas (SNG). Of these only one was ever built, in Beulah, North Dakota. The plant still operates today and in 2000 has broken new ground by making the C02 from the acid-gas removal unit available for enhanced oil recovery (Dittus and Johnson 2001). Given the current availability of natural gas it is unlikely that another SNG facility will be built in the near — or even medium-term. Nonetheless it is instructive to look at a number of issues connected with its manufacture from coal.

SNG consists primarily of methane, which is synthesized by the reaction of car­bon oxides with hydrogen over a nickel catalyst according to the equations:

CO + 3 H2 ^ CH4+H20 -206 MJ/kmol

C02+4 H2 ^ CH4+2 H20 -165 MJ/kmol

Specifications for SNG require a maximum hydrogen content of 10% and, depending on the heating-value requirement, a limitation on C02. Typically, this results in a requirement on the stoichiometry of the synthesis gas such that the stoichiometric number (SN = H2/(3 CO+4 C02) has a value between 0.98 and 1.03.

The selection of the coal gasifier cannot be made in isolation from the quality of the coal itself (see Chapters 4 and 5). Looked at from the point of view of the application alone, there is a distinct advantage to processes that produce a high meth­ane content ex gasifier, since this reduces the volumes of gas to be treated and, if neces­sary, compressed between gasification and synthesis. For this reason, a Lurgi dry-ash gasifier has been selected. (See Figure 7-9.)

With the selection of a Lurgi dry-ash gasifier a decision has to be taken on the matter of tar handling. The tar can be used as a raw material for the manufacture of tars and phenols. Alternatively, it could also be gasified to generate additional synthesis gas, thus reducing the coal throughput requirement. The former solution has been chosen in the example. Phenosolvan and CLL units have been incorporated to recover ammonia from the waste water.

The raw gas contains more CO than that required by the methane synthesis, so a partial stream is shifted on a raw gas shift catalyst. In the downstream acid-gas removal, the residual hydrocarbons, sulfur and nitrogen compounds (NH3 and HCN), must be removed. To achieve the correct stoichiometry, a partial C02 removal is also required. The sulfur specification for the nickel catalyst is maximum lOOppbv or even lower. The only system capable of ah these tasks is Rectisol, and

this is also included in the example flowsheet. In order to achieve the low C02 specification of the SNG, C02 may be removed in a final Rectisol stage, which also provides the necessary drying function.

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