Integration of a municipal solid waste gasification plant with solid oxide fuel cell and gas turbine

Abstract

An interesting source of producing energy with low pollutants emission and reduced environmental impact are the biomasses; particularly using Municipal Solid Waste (MSW) as fuel, can be a competitive solution not only to produce energy with negligible costs but also to decrease the storage in landfills. A Municipal Solid Waste Gasification Plant Integrated with Solid Oxide Fuel Cell (SOFC) and Gas Turbine (GT) has been studied and the plant is called IGSG (Integrated Gasification SOFC and GT). Gasification plant is fed by MSW to produce syngas by which the anode side of an SOFC is fed wherein it reacts with air and produces electricity. The exhausted gases out of the SOFC enter a burner for further fuel combusting and finally the off-gases are sent to a gas turbine to produce additional electricity. Different plant configurations have been studied and the best one found to be a regenerative gas turbine. Under optimized condition, the thermodynamic efficiency of 52% is achieved. Variations of the most critical parameters have been studied and analyzed to evaluate plant features and find out an optimized configuration.

Introduction

The word “Biomass” refers to vegetables and animals substances, not from fossil origin; these can be used as fuel in a power plant for the production of electrical energy. Biomasses derive from living or recently living biological organisms and can be considered as a particular kind of renewable energy source, because the carbon dioxide placed in the atmosphere by their use derives from the carbon amount absorbed during their life. In this way, the most important pollutants linked to biomass utilizations are related to transport, manufacture and transformation processes.

Municipal Solid Waste can be considered a valid biomass to use in a power plant. Some advantages can be obtained; the principal is the reduction of pollutants and greenhouse gases emissions. Another advantage is that by their use it is possible to reduce the storage in landfills and devote these spaces to other human activities.

It is also important to point out that this kind of renewable energy suffers significantly less availability which characterizes other type of renewable energy sources such as in wind and solar energy.

As proposed in Morris et al. [1], with a well management of waste, the following points should be considered:

• prevention of waste generation;

• recycling of waste materials;

• reduction at minimum of landfilling disposal;

• incineration with energy recovery at efficiencies comparable with alternative technologies and sophisticated exhaust gas cleaning equipment;

• gasification processes.

In a gasification process, waste is subject to chemical treatments through air or steam utilization; the result is a synthesis gas, called “Syngas” which is principally composed of hydrogen and carbon monoxide. Traces of hydrogen sulphide could also be present which can easily be separated in a desulphurisation reactor. The gasification process is usually based on an atmospheric-pressure circulating fluidized bed gasifier coupled to a tar-cracking vessel; the gas produced is then cooled and cleaned. Syngas can be used as fuel in different kind of power plant such as gas turbine cycle, steam cycle, combined cycle, internal and external combustion engine and Solid Oxide Fuel Cell (SOFC).

SOFC based power plants are known as efficient power generators not only as stands alone (about 50% thermal efficiency) but also in hybrid cycles, where SOFC is integrated with another plants, such as gas turbine or Rankine cycle (above 60% thermal efficiency). SOFC plants are also flexible in using different kind of fuels after minor fuel pre-treatment such as desulphurisation. SOFC based power plants have been studied in the past and some manufacturer are trying to realize such systems for CHP (combined heat and power) applications [2]. In literature it is possible to find studies about SOFC plant combined with CC (combined cycles) to achieve ultra-high electrical efficiencies [3], [4]. SOFC and gas turbine integrated systems have been extensively studied for CHP applications [5] and with biomass gasification [6], [7]. In Refs. [8], [9], the characterization, quantification and optimization of hybrid SOFC–GT systems have been studied. The dynamics and control concept of a pressurized SOFC–GT hybrid system has also been studied in Ref. [10]. In Ref. [11] modeling results were compared with measured data for a 220 kW SOFC–GT integrated plant, while details of the design, dynamics and startup of such hybrid power plants are studied in Ref. [12]. Part-load characteristics of an SOFC–micro GT were also studied in Ref. [13].

In the current study a gasification plant is considered which is modeled and compared with a gasifier named Viking gasifier. This biomass gasifier is an autothermal (air blown) fixed bed gasifier situated located at Technical University of Denmark, see e.g. Refs. [14], [15]. Hofmann et al. [16] had operated an SOFC on cleaned syngas from the Viking gasifier without noticing any degradation. They concluded that their gasification plant provided a biogas which could directly be fed into a solid oxide fuel cell. It shall be mentioned that the Viking gasifier has not been used with Municipal Solid Waste and its' functionality remains unknown. However, the mathematical model developed here is general and can be used with almost any type of gasifier unless the adjusting parameter to be decided, see below.

In the present study, a Municipal Solid Waste gasification plant integrated with SOFC is combined with a gas turbine to recover the energy of the off-gases from the topping SOFC cycle. In addition, a regenerative gas turbine has been introduced to recover more energy from the off-gases. The latter configuration has been compared with the one without regeneration; hybrid recuperator was shown to be very efficient and could increase significantly the plant efficiency. In addition, some relevant parameters have been studied to optimize the plant efficiency in terms of operating conditions. Compared with modern waste incinerators with heat recovery, the gasification process integrated with SOFC and gas turbine permits an increase in electricity output up of 50%, see e.g. Refs. [1]., thus solid waste gasification process can compete with incineration technology. Moreover waste incinerators require the installation of sophisticated exhaust gas cleaning equipment that can be large and expensive; these systems are not necessary in the studied plant.

No investigation on Municipal Solid Waste Gasification plant integrated with SOFC and GT has been found in the open literature, and therefore, the current investigation seems to be completely novel and might bring up new ideas on designing new energy system configurations for future applications.