Syngas from Waste

This chapter briefly introduces the main components that play a role in syngas production technologies. First, the thermochemical principles underlying the syngas production process are enunciated. Second, the suitable raw materials for gasification, the products obtained, and their end uses are identified. Third, the advantages, opportunities, present commercial status, and future perspectives of gasification are discussed. Finally, the need for enhanced technologies supported by the development of an appropriate framework for a robust design and optimization is justified. These issues are covered in detail in this book and should prove useful for assessing current technologies, conveying novel concepts and providing the basis for the future technological advancement of sustainable clean syngas production at a competitive advantage.

Among the different energy sources, biomass wastes hold most promise for the near future. Biomass is considered a neutral carbon fuel because the carbon dioxide released during its use is an integral part of the carbon cycle. Increasing the share of biomass in the energy supply contributes to diminishing the environmental impact of CO₂ and to meeting the targets established in the Kyoto Protocol. The use of biomass waste material as a fuel, however, has certain drawbacks related with its high-moisture content, low-energy density and the problem of reducing the size of the biomass, especially in the pulverized range of entrained flow gasifiers. Currently, there is increasing interest in developing new processes for the pre-treatment of biomass wastes, through the modification of their properties prior to gasification, so as to make them more attractive for their subsequent use. Pelletization is a proven technology for improving biomass properties, whereas torrefaction is considered a plausible alternative for decreasing the moisture content, increasing the energy density and greatly facilitating the handleability and grindability properties of the torrefied material.

The pressure on reducing environmental footprint is facilitating the emergence of energy supply chains that have biomass as main feedstock. For the development of efficient energy supply chains from biomass it is required to properly integrate the various elements that comprise such systems (e.g., biomass supply, pre-treatment facilities and technologies for biomass to energy and/or fuels conversion). Additionally, it is recognised that a concerted effort is required, embracing the different supply chain entities, in order to correctly estimate environmental burdens and to propose effective environmental strategies. This chapter proposes the use of a mathematical modelling approach as an analytical tool that can support decision-making towards accomplishing the design and planning of efficient multiple source—multiple product bio-energy supply chains. The mathematical formulation of this problem becomes a multi-objective MILP (moMILP). Criteria selected for the objective function are the net present value (NPV) and the overall environmental impact, which is computed using the Impact 2002+ indicator. The main advantages of this approach are highlighted through a case study of a biomass-based supply chain that comprises different components geographically distributed over Spain. For comparison purposes, such a supply chain is contrasted to one embracing coal usage.

Syngas generation refers to the production of a synthetic or synthesis gas that is mainly composed of CO and H₂, in different proportions according to the generation process used and the raw material composition. Gasification is the referred technique to produce syngas. It can be used for different purposes, such as power and/or heat generation or for chemicals and fuels production. This chapter describes, we comment the generalities of syngas and its main characteristics and properties, also discuss its possible sources and focus on biomass waste and its co-gasification with coal and petcoke. Then, gasification modelling most common approaches are mentioned. A thermochemical equilibrium model is presented here as the model used for gasification plant conceptual design. Through sensitivity analysis technique, the effects of the reactor temperature and pressure are seen in syngas composition. This chapter enumerates the major hypothesis assumed in this syngas generation step, which must be inevitably applied in modelling and optimizing the entire gasification plant.

Syngas final usage requires a previous step of cleaning and conditioning to meet with the requirements of its final use which might range from chemicals and fuels production to power and/or heat. This chapter deals with the description and the modelling of the required syngas treatment units before electricity production or before hydrogen generation, specifically in an IGCC power plant. In the case of electricity generation application, the pursued objective is to avoid as much as possible nitrogen and sulphur oxide emissions to the atmosphere. In a first step, the gas is cleaned from solids. Secondly the gas before its combustion goes through an acid and basic species removal train of units. In the case of hydrogen generation, besides syngas cleaning from other species, the main pursued objective is to separate CO from H₂. In order to accomplish the former, CO should be converted into CO₂ and then separated from the main stream. Hydrogen can be further purified to be sold to the market, or used in a combined cycle, in an analogous way as the syngas. Modelling calibration and validation are shown, and the chapter finishes with a model utilisation to evaluate the behaviour of the already built up superstructure to produce hydrogen or syngas for electricity generation section, or hydrogen for other applications.

Syngas normally contains a series of contaminating gases, depending on the raw materials used, the most abundant one usually being H₂S, accompanied by COS and, also, HCl, HF, etc. Normally, purification should be performed before its combustion in the gas turbine (in the case of combined cycle plants) and the classic procedure, as performed at present in some installations, uses the wet process, which demands a reduction in the temperature of the gas to be purified and, therefore, gives rise to a series of thermodynamic losses. The trend is to research high-temperature purification processes that avoid or reduce this loss in performance. In particular, there are two research lines for sulphur compounds: (i) The use of low-value metal adsorbents that may be discarded once they have been stabilised and without contaminating properties, such as calcium compounds that may produce CaO that captures the hydrogen sulphide and (ii) the ‘important value’ adsorbents that therefore require the ability to be regenerated and reused, and whose base are metals with high affinity with sulphur, such as Zn, Fe and Cu but whose cost is much higher than the previously mentioned calcium compounds.

A promising technology for H₂ production and CO₂ separation is based on water gas shift reaction operated in water gas shift membrane reactor (WGSMR). In such a reactor the synthetic gas reacts with steam in a catalytic bed to produce additional hydrogen and CO₂. A H₂ selective membrane allows the simultaneous production of hydrogen at a high purity level and a stream of concentrated CO₂. The performance of such a reactor is defined in terms of CO conversion fraction, H₂ recovered fraction and produced H₂ flow rate. The chapter deals with the modelling of a WGSMR. A model developed to assist the design of a pilot scale, tube-in-tube reactor, is described. Simulations with the model are presented and discussed. The simulations were performed to analyse the effect of operating conditions (H₂O/CO ratio, temperature, pressure and syngas flow rate), catalyst characteristics (catalytic bed efficiency, void fraction) and membrane length, on the reactor performance. The results provide quantitative information to define the set of conditions to obtain the target value of the H₂ flow rate, with high values of CO conversion fraction and H₂ recovered fraction, minimising the length of the H₂ selective membrane. A last paragraph is dedicated to a short analysis of the main issues and foreseen solutions for the industrial application of the technology.

In this chapter a description of how the process synthesis problem can be casted as a superstructure optimisation problem is done. The first section draws on how the superstructure can be built, while the second section depicts the different techniques that can be used to reduce the computational time required to run a superstructure optimisation. Section 3 describes the different integration and control considerations embedded in a possible superstructure for analysing syngas generation and treatment. This last section also shows the results of different scenarios of this superstructure model.

This chapter provides a brief description of thermodynamic analysis, the maximization of heat-recovery and power generation and the synthesis of IGCC’s heat exchanger network. Trade-offs between the income from power generation and utility costs plus the investment in HEN are studied with respect to the generation of different pressure-levels of steam and temperature driving force losses within the heat-recovery network. A combined pinch analysis/mathematical programming approach is applied and the optimization models are described for (i) the maximization of heat-recovery and power generation, and (ii) a synthesis of the heat-recovery and steam/power generation network. A sensitivity analysis for the synthesis is performed in order to show how optima are sensible for the expected increase in future electricity and utility prices, and the project’s lifetime. The results obtained from the studied IGCC process indicate that detailed optimization has to be performed during the network synthesis step, otherwise optimal trade-offs are missed that may result either in serious power generation losses or in obtained over-designed networks.

This chapter begins with an introduction to the different possible metrics related to clean gas process synthesis and its subsequent usage. Latter, the different techniques for tackling with multiple criteria are presented, emphasising the use of multi-criteria decision analysis (MCDA) and multi-objective optimisation (MOO). The different criteria elected here for optimisation are described and later used as key performance indicators (KPI) for the proposed scenarios, in chapter “Selection of Best Designs for Specific Applications”. Finally, a case study related to the operation of an IGCC plant considering coal–petcoke or natural gas as a fuel is assessed applying the optimization concepts introduced here and taking into account the operation considerations developed in this and in previous chapters.

The present chapter assembles all the previous chapter’s concepts and methodologies with the aim of selecting a given process design. The developed superstructure is used to simulate scenarios with different feedstocks and process topologies for co-production of electricity and hydrogen. The most representative output data are shown, and the described points of view, discussed in chapter “Global Clean Gas Process Synthesis and Optimisation”, are here evaluated under a techno-economic and environmental assessment. Sixteen scenarios are considered encompassing four different feedstocks combined with four different plant topologies; electricity generation with syngas, electricity generation with hydrogen, hydrogen production and hydrogen production with PSA flue gas profit in the CC.

The description of the 335 MWe_ISO coal-based Puertollano IGCC power plant as example of industrial application of IGCC technology together with its main lessons learnt are summarised in this chapter. This chapter includes process description, syngas analysis, main operational real data (power production and emissions), and main causes of unavailability as well as R&D lines. These are mainly focused on improvement of IGCC technology taking into account efficiency increase and emissions reduction. So, first results of the CO₂ capture and H₂ co-production 14MW_th pilot plant installed in the Puertollano IGCC are included.

In this chapter, a general description of data-mining techniques is done in the context of IGCC operation. The different control philosophies applicable to IGCC operation are discussed together with different examples of data reconciliation based on process simulation. The problem of process monitorisation, as an example of data-mining application, is extensively discussed and an approach based on PCA is presented.