Bio-SNG - CNG Services

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BIO-SNG FEASIBILITY STUDY – ESTABLISHMENT OF A REGIONAL PROJECT5 Process and Technology ReviewThe focus of this section is not to undertake a panoramic review of potential technologies in variousstates of maturity; that has been done elsewhere 34 . Rather it is to focus on a rationale for theconfiguration of a practical plant that could, subject to commercial considerations be deployed now at anindustrial scale.Experience reveals that process developments are rarely founded on technological break-throughs;rather it is normally the case that process developments are incremental and founded upon existingproven techniques. The guiding principle in this review has been therefore, to establish whether existingtechnologies could be employed for the entire process chain from fuel reception and preparation throughto Bio-SNG compression and delivery, and in a way that gives a good level of performance in comparisonwith alternatives and with respect to efficiency, technical risk, commerciality and speed to market.The development of a processing scheme should be dominated by an understanding of the desiredoutput stream as well as the properties of the feedstock; including a precise understanding of the levels ofcontrary elements in the fuel such as heavy metals, sulphur and halogens. This drives the requirementsand specification for the intervening processing stages. An overall appreciation of the principal processoperations required for the production of Bio-SNG is shown in Figure 5.1 below.BALANCE OF PLANTFUEL PREPTHERMO-CHEMICALBREAKDOWNINTERMEDIATEPURIFICATIONINTERMEDIATECONDITIONINGMETHANATIONPOLISHING“PACKAGING”includingCOMPRESSIONPRODUCTS: HEAT, ELECTRICITY, OTHER CHEMICALS AND FUELSPRODUCTBio-SNGFigure 5.1Principal Process operationsA systematic process review therefore will begin with the fuel handling facilities - reception, storage,preparation and feeding arrangements.5.1 BIOMASS RECEPTION, PREPARATION AND HANDLING.The operational effectiveness of the gasification process plant will depend upon the continuous supply offuel exhibiting regular properties – particle size, density, humidity, calorific value, chemical analysis, etc.34 e.g. NNFCC project 09/008: Review of Technologies for Gasification of Biomass and Wastes: E4Tech June 200932
BIO-SNG FEASIBILITY STUDY – ESTABLISHMENT OF A REGIONAL PROJECTA key design consideration therefore is whether to import material of a defined specification and quality orto manufacture the fuel on site from raw biomass or residues. On the one hand manufacture on site willdemand more space, more plant, a larger workforce and a significant parasitic energy consumption,however on the other hand, bought-in ready to use fuel will be more costly, and could subject the plant togreater supply chain vulnerability. Moreover biomass drying is likely to be a significant feature of the fuelpreparation process and could represent an economically effective use of waste heat from the gasificationprocess. A balanced judgement needs to be taken, therefore, on the fuel supply philosophy, taking intoaccount, the type of raw feedstock (lumber, waste wood, wood chip, pellets, miscellaneous biomassresidues, commercial / industrial waste etc.), the plant location, the space available, and the fuel supplychain arrangements.There is extensive expertise in the area of fuel reception, preparation and handling, however, it is acommon location for serious process malfunctions; due diligence experience reveals a consistent andrecurrent problem with fuel preparation, quality and feeding. It is vital, therefore at the design stage touse proven and reliable designers, and equipment suppliers and to confirm that the process plant willoperate with the particular material specifications envisaged for fuelling the gasifier. Solids handlingsystems must be designed in consideration of the particular properties of the materials in question; forexample it would be unwise to assume that woodchip will behave in a handling system in the same wayas wood pellets. CEN/335 goes some way to describing standard test methods that can be employed todetermine the critical handling attributes of particular solid biomass fuels.5.2 GASIFICATIONThere are fundamentally three main types of gasifier:Fixed bed (down-draft and up-draft)Entrained flowFluidised bed (direct and indirect heating)Fixed bed biomass gasifiers are used extensively in some parts of the world in small, relatively crudeapplications producing a low quality gas for small scale power generation. Fixed bed gasifiers are alsoused at a large scale, however, the low carbon intensity of biomass fuels makes them unsuitable for usein large scale fixed bed gasifiers, unless they are co-fired with coal. On this account it is proposed thatfixed bed gasifiers should be excluded from further consideration where the intent is to produce biosyngasat a moderate industrial scale.33
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