Steffen Heidenreich

Gas cleaning, gas conditioning and tar abatement by means of a catalytic filter candle in a biomass fluidized-bed gasifier

A bench-scale fluidized-bed biomass gasification plant, operating at atmospheric pressure and temperature within the range 800-820 degrees C, has been used to test an innovative gas cleaning device: a catalytic filter candle fitted into the bed freeboard. This housing of the gas conditioning system within the gasifier itself results in a very compact unit and greatly reduced thermal losses. Long term (22h) tests were performed on the gasifier both with and without the catalytic candle filter, under otherwise identical conditions. Analysis of the product gas for the two cases showed the catalytic filtration to give rise to notable improvements in both gas quality and gas yield: an increase in hydrogen yield of 130% and an overall increase in gas yield of 69% - with corresponding decreases in methane and tar content of 20% and 79%, respectively. HPLC/UV analysis was used to characterize the tar compounds.

Study of the behaviour of a catalytic ceramic candle filter in a lab-scale unit at high temperatures

Solid particles and tars are among the non-desirable products of synthesis gas produced during biomass gasification. Removal of fly ashes is necessary in order to comply with emission limits as well as avoid their deposition in downstream units. Condensation of tars, on the other side, can cause clogging. A catalytic hot gas filter can remove both solids and tars, when operating at temperatures as high as 850°C. Catalytic hot gas filter elements are under development in order to solve this issue.A lab-scale filtration unit has been designed and constructed at Delft University of Technology. The unit contains one ceramic hot gas filter candle which is made of a SiC porous structure coated with a mullite membrane. The integration of a Nickel-based catalyst layer allows the dual function of particle filtration and tar cracking. The filter vessel is part of a set-up that is equipped with a tar evaporator and a pre-heater, both located upstream of the filter unit.This paper presents the results of the first set of experimental tests that have been performed with this unit. A dust-free model gas was used and consisted of a mixture of CO (14%), CO2 (14%), H2 (7%), CH4 (5%), and varying concentrations of N2 (30, 40, 50%) and H2O (30, 20, 10%). Naphthalene (varying concentrations up to 9 g/Nm3) was adopted as model compound in order to study the catalytic conversion of heavier hydrocarbon species to H2. A gas face velocity of 2.5 cm/s and 3 cm/s was selected for tests performed at atmospheric conditions and at operating temperatures varying between 700 °C and 850 °C. The pressure drop through the filter candle was continuously monitored during the process. The gas composition was measured upstream and downstream of the filter unit by means of an on-line micro-GC, while naphthalene concentration was attained with the SPA method.The following findings were obtained: higher naphthalene conversion with increasing temperatures and better conversion at any temperature with lower concentrations. Tests at 850°C and 30 vol% H2O produced a conversion of 99.4% with 2.5 g/Nm3 while 98.5% with 7.8 g/Nm3. Experiments with higher steam content showed higher conversion values. Methane concentration was also affected thus indicating that reforming reactions took place as well. Low toluene concentration was detected as a product of the reactions while no benzene was identified.

Characterisation of Tar produced in the Gasification of Biomass with in situ Catalytic Reforming

This paper concerns the cleaning of the hot gas produced by steam gasification of biomass in a fluidized bed. The cleaning takes place in a catalytic filter candle device placed directly in the freeboard of the bed. Such integration results in a compact processing unit and increased thermal efficiency; the result of the cleaning being carried out directly at the reactor outlet temperature. It thus lends itself to exploitation in distributed power generation systems utilizing renewable energy sources. Results are reported for runs performed in a bench scale fluidized bed steam gasifier fitted with a single full-size catalytic candle filter. Tar and particulates in the product gas were sampled in accord with technical specification CEN/TS 15439 with analysis by UV and fluorescence spectroscopy.

Advanced Process Integration: The UNIQUE Gasifier Concept—Integrated Gasification, Gas Cleaning and Conditioning

To promote the utilization of biomass gasification, advanced concepts are required which have to maximize the syngas yield, optimize the gas quality, increase the gas purity, increase the overall process efficiency, and improve the economic viability by decreasing system and production costs. In this chapter, an example for a promising process integration is described—the recently developed UNIQUE gasifier concept. The UNIQUE gasifier integrates gasification, gas cleaning, and conditioning in one reactor unit. This concept offers a compact design with decreased requirements for footprint as well as reduced investment costs.

Chapter 7 – Polygeneration Strategies

Polygeneration is a concept to improve the economic viability and sustainability of the utilization of biomass via gasification by combining the production of at least two products. The goal is to maximize the efficiency to transform energy and material of the feedstock into products. Syngas from biomass gasification can be converted into a broad range of energy products, for example, electricity, heat, gaseous or liquid fuels, or chemicals. Polygeneration offers high flexibility with regard to changes of market demands. For example, a polygeneration process for combined production of biofuels, heat, and power could be used as a back-up power plant. If the current electricity consumption increases, the production of biofuels can be switched to the production of power. Higher flexibility requires higher capital investments as well as higher costs for operation and maintenance. In this chapter, some polygeneration strategies are described starting with the classical example of combined heat and power production followed by newer approaches, such as combined synthetic natural gas, heat and power production, combined biofuels, heat and power production, as well as combined hydrogen and heat production.

Chapter 2 – Fundamental Concepts in Biomass Gasification

Gasification is a thermochemical process to convert fuels into a combustible gas, the so-called “producer gas.” This chapter deals with the basic chemistry and technology of gasification processes.

Chapter 5 – Advanced Process Combination Concepts

Gasification is a process to convert carbonaceous material into gas by using a gasifying agent. If this thermochemical process is performed in a conventional one-stage biomass gasifier, several process steps are overlapped, such as heating and drying of the fed biomass, pyrolysis of the volatiles, oxidation and gasification of the charcoal. To improve the gasification process, modern, advanced gasification concepts discussed in Section 5.1 combine separated pyrolysis and gasification stages. Two different approaches are shown in this chapter: (1) combination of pyrolysis and gasification directly in a multistage gasification process; (2) pyrolysis and gasification at different locations. Gasification of biomass can advantageously be combined with a combustion stage. In Section 5.2 three concepts are described: (1) dual fluidized bed process with internal combustion; (2) gasification with partial oxidation for the reduction of tar in the product gas; (3) gasification for cofiring of the product gas in a coal combusted power plant. Also the combination of downstream processes can simplify the overall processes and increase their efficiency. One concept, which will be discussed in Section 5.3, is the combination of the water gas shift reaction and the following separation of H2 and CO2 in one single catalytic membrane reactor.

Chapter 6 – New and Improved Gasification Concepts

Even though gasification is an old technology, there is still much potential for improvement, for example, related to efficiency, complexity, and flexibility. Beside the technical aspects, economic aspects also drive the development of new and improved gasification technologies. Some new and improved gasification concepts are described in this chapter. Oxygen–steam gasification, being state-of-the-art in coal gasification, is still under investigation for biomass gasification and needs further innovation for an economically feasible oxygen supply, for example, by membranes, as discussed in Section 6.1. Section 6.2 deals with an alternative for oxygen supply in gasification, that is, the chemical looping (oxidation and reduction) of oxygen carrier materials. Section 6.3 gives an overview on supercritical water gasification which has particular advantages in the gasification of wet biomass.

Chapter 3 – Biomass Pretreatment

Biomass feedstock can have a wide variety of chemical and physical properties. In contrast to that, conversion processes like gasification usually require specific physical and chemical properties of a fuel. Several pretreatment methods are used to adjust the fuel properties to the process requirements. This chapter will give a brief overview on relevant physical, thermal, and thermochemical pretreatment methods, which are partly already state-of-the-art or promising new processes.