Technologies for electricity production with biomass

Co-combuistion of biomass in conventional power plants

Austrian utilities have realised two projects testing the possibilities of the co-combustion of biomass in pulverised coal power plants. In the plant at St. Andrä a biomass grate was installed at the base of an existing coal boiler, allowing a 10 MWth power contribution from biomass combustion. Due to lack of space, the contribution of biomass cannot be extended however. The project at Zeltweg solves this problem by gasifying the biomass in an external fluidised bed gasifier and feeding the gas directly into the boiler. There are a number of advantages to this strategy: the biomass need not be completely gasified as small char particles in the product gas can also be burned in the coal boiler. Consequently the Circulating Fluidized Bed reactor can be much smaller than for complete gasification which requires longer residence times. There is no need for predrying of biomass or for gas cleaning as the gas is burned under optimal conditions in the coal burner. The product gas can be used as reducing gas in the reburning zone and could reduce or avoid other measures to lower NOx emissions. Finally, gasification allows to simply add a module to existing or newly planned large power plants which is sized according to local biomass availability and allows conversion at the high efficiency and low operation costs of large scale plants.

Contact: Österreichische Draukraftwerke AG

Gasification in a solid bed reactor and CHP gas motor

Experiments with gasification in a solid bed reactor for heat and power production in a gas motor are conducted at the Technical University of Graz. Gas from a two zone solid bed gasifier (250kWth) is cleaned and cooled in a washer and used in a gas engine. Gasification promises the realisation of cheaper and more effective CHP plants in the low power range typical for many biomass district heating plants. The experiments are designed to clarify the technical feasibility of long term operation of a gas motor with a solid bed gasifier. A pilot project based on a solid bed gasifier with sawdust filters for gas cleaning was set up in 1997 by a farmer in Styria.

Contact: University of Technology Graz, ÖAR

Gasification of biomass with steam in a novel fluedised bed system

Usually gasification of biomass is carried out in fixed or fluidized bed reactors with air as gasification agent. In this case the product gas has a low calorific value (around 4-6MJ/m³) and a high nitrogen content.

Fig. 9: Scheme of a Gasification of biomass with steam in a novel fluedised bed system Source: Energy from Biomass - R&D in Austria (Ministry of Science and Transport)

A novel fast internally circulating fluidized bed gasifier (FICFB) developed in a cooperation by the Vienna Technical University and Austrian Energy and Environment Inc. yields a high calorific gas with up to 15 MJ/m³ and very low nitrogen (<3%) and tar content (<0,8g/m³).

The gasification process is based on an internally circulating fluidised bed system and consists of a gasification zone fluidized with steam and a combustion zone fluidized with air. In the combustion zone char is burned and produces the heat necessary for gasification. The circulating bed material acts as a heat carrier from the combustion to the gasification zone. Gas mixing between these zones is avoided by construction measures so the flue gas from combustion is not mixed with the product gas. The resulting high quality of the product gas can be further enhanced with catalytic bed material. Olivine has proved highly efficient for this purpose.

After two years of successfully operating a 100 kWth laboratory unit a 4 MWth gasifier and a gas engine will be integrated into an existing district heating system and deliver heat and electric power. In an international pilot project catalysts will be used to achieve up to 55% hydrogen content in the product gas which could subsequently be used in a fuel cell for electricity production.

Contact: Technical University of Vienna, AE Energietechnik

State of the art gas engines for CHP

Jenbacher Energiesysteme Ltd. has developed gas engines which can utilise an unmatched range of different gases such as biogas, pyrolysis gas and almost any other combustible gas. The heating value of gases that can be turned into electricity at an efficiency of up to 40% lies between 0.5 kWh/m³ and 34 kWh/m³. Jenbacher systems apply combustion technologies which have been patented world-wide and achieve guaranteed emission levels below 250 mg/m³ of NOx and below 300 mg/m³ of CO when operated with natural gas. The overall energy efficiency of the systems is as high as 88%.

Contact: Jenbacher Energiesysteme

Fig. 12: Gas engines Source: Jenbacher Energiesysteme

Stirling Engine

The Stirling engine is another promising concept for small scale heat and power production. Its advantages are low operation cost, simple design and relatively high efficiency (approx.25%). A 3 kW alpha type Stirling model was constructed and successfully tested. The advantage of this Stirling type is that it can be constructed with standard industrial parts, in this case a Ducati 500cm³ motor cycle cankcase. The heat exchanger, a bundle of polished metal pipes, is placed into the hot stream of exhaust gas. There is no necessity for hot gas cleaning. Stirling engines in district heating plants could be used primarily for electricity production for plant operation. A Stirling engine could also be used to power the compressor of a flue gas condensation heat pump.

At present Joanneum Research is developing a small Stirling engine CHP production plant with an electric output of 30-100 kW that meets the criteria for series production.

Contact: Joanneum Research

Fig. 13: Stirling Engine

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