Caso 3: Mellizos 2-3 años

Designs for the gasification of biomass tend to focus on fluidised bed gasifiers for lignocellulosic biomass[36,37], some of which are operated in autothermal gasification (AUT-G) mode, where the gasification medium includes both steam and an oxidant in sub-stoichiometric proportions, to initiate the combustion reactions to supply the reaction heat required by the endothermic gasification (reduction) reactions[38,39]. If a high calorific value of the synthesis gas (syngas) is desired for purpose of Fischer-Tropsch synthesis, then the oxidant is required as pure oxygen that is obtained by an Air Separation Unit, which bears substantial energy and capital costs[7,40]. AUT-G systems can also be pressurised to prevent the costs of the compression of syngas further downstream[8,41], though it requires higher gasifier capital cost[42] and reduces the content of hydrogen and carbon monoxide in the syngas[41]. Fluidised bed gasifiers can also be operated in allothermal gasification (ALO-G) mode, where the gasification agent consists only of steam, to generate a syngas with a calorific value similar to that of the oxygen-fed autothermal systems[40,43]. In the ALO-G system the energy required for the endothermic steam gasification reactions is supplied by a heat transfer medium from an external heat generating source, such as the combustion heat from a fuel and char burner[40,44]. This however, can have a substantial effect of the overall efficiency with which the biomass is converted, notwithstanding the additional capital required for the combustor[40,44].

As syngas production (Figure 2) bears the primary energy and capitals costs in a gasification- synthesis process [9], previous studies have studied the effects of the gasification operating parameters, such as moisture content, temperature, steam-to-biomass ratio (SBR), equivalence ratio (ER) and pressure on the syngas composition and the energy requirements[9,37,44]. The primary predictive tool utilised in such parametric orientated studies is Equilibrium (or Thermodynamic)

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Modelling, which has the basic assertion that the composition of the syngas leaving the gasifier unit is similar, or very close to the composition predicted through thermodynamic calculations under equilibrium conditions. This assertion however, bears significant inaccuracies at temperatures of 750-1000o_{C, which are typical for the gasification of biomass in a CFB[40,45]. This is primarily due to}

the incomplete cracking of tars and the slow kinetic rate of the reaction of reforming methane with steam at these temperatures [40,45]. However, the application of equilibrium modelling for biomass/derivative gasification at temperatures of 800-1000o_{C is readily justifiable when a}

gasification or steam reforming catalyst (such as dolomite or nickel-based), which promotes syngas compositions that are very close to equilibrium and reduces the tar content to acceptable levels [38,46,47], are considered.

The impact of the operating parameters on the gasification performance, predicted with the aid of equilibrium models, is typically evaluated with the gasification efficiency – a term that has slight variances in its definition, depending on the system under consideration[9,40,48]. Schuster et al[40] for example, defined the chemical (gasification) efficiency as the ratio of the flow of calorific energy of the syngas (numerator)to the flow of calorific energy in the biomass feed and the steam enthalpy (denominator). Leibbrandt et al[9]. defined the gasification efficiency for an oxygen-fed autothermal system similar to Schuster et al[40], but also included the energy costs of producing pure oxygen in the denominator. The gasification efficiency was used by Leibbrandt et al[9] as an optimisation variable in a statistical model, to determine the preferred operating conditions for producing a synthesis gas from sugar cane bagasse (and its pyrolysis products). The target syngas composition contained hydrogen (H2) and carbon monoxide (CO) at a ratio of 2 for Fischer-Tropsch synthesis. A

similar approach where the capabilities of equilibrium models was combined with a statistical model was developed Silva and Rouboa[48], where the operating conditions for producing a H2 rich product

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Previous reports on the gasification efficiency, as determined from equilibrium modelling, did not include the impacts of the energy recovery units (such as the steam generator) and clean-up units (such as the Rectisol, see Figure 2) in the definition of gasification efficiency. These additional energy components are affected by the energy balance and operating parameters of the gasifier, as noted by the work by Van der Meijden et al.[49], whom compared various synthetic natural gas (SNG) production systems, defined by the mode of gasification. The steam generating capacity of the unit that cools the synthesis gas (syn-cool generator) is dependent on the enthalpy of the product gas[7,10], which in turn is dependent on the temperature of the gasifier and the flow of gasification medium. Furthermore, the energy demands of the Rectisol unit for syngas clean-up are dependent on the content of acid gases (primarily carbon dioxide (CO2) and hydrogen sulphide (H2S))

in the syngas[7,50], which in turn affected by the SBR, ER and moisture[36,40,51]. Thus, the overall efficiency of a synthesis gas production should be inclusive of the energy effects of the downstream units, up to the Rectisol unit. This gives a comprehensive view on the gasification efficiency in response to variations in the gasification operating conditions.

Figure 2: Generic Flow-sheet of Syngas Generation

Selecting a process technology, or set of operating conditions, for gasification based solely on a measure of energy efficiency does not account for the economics of variable capital and operating costs, incurred by changes in operating conditions [52,53]. Generally speaking, processing units that

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are more efficient at generating energy (and consuming energy), have greater capital and operational costs, compared to less efficient counterparts[52,53]. As an example, oxygen-fed autothermal gasification has higher conversion efficiency than an allothermal system, since the entire amount of biomass fed is used to produce syngas[39]. However, AUT-G systems that are oxygen blown, involve significant operating and capital expenditure, due to production of pure oxygen[40,44], thus indicating a trade-off between costs of efficiency. Thus, the optimisation of a gasification system should also aim to minimise the overall costs of producing syngas by accounting for both the capital and operational cost implications for the overall syngas production train, up to purified syngas suitable for synthesis, when varying the operating conditions.