System Description

The technologies for the conversion of biomass for electricity production are direct combustion, gasification, an d pyrolysis. As shown in Figure 1, direct combustion involves the oxidation of biomass with excess air, producing ho t flue gases which in turn produce steam in the heat exchange sections of boilers. The steam is used to generat e electricity in a Rankine cycle; usually, only electricity is produced in a condensing steam cycle, while electricity an d steam are cogenerated in an extracting steam cycle. Today's biomass-fired steam cycle plants typically use single-pass steam turbines. However, in the past decade, efficiencies and more complex design features, characteristic previousl y of only large scale steam turbine generators (> 200 MW), have been transferred to smaller capacity units. Today's biomass designs include reheat and regenerative steam cycles as well as supercritical steam turbines. The two common boiler configurations used for steam generation with biomass are stationary- and traveling-grate combustors (stokers ) and atmospheric fluid-bed combustors.

Flue.Gas ii

UNIT BOUNDARY

Substation

Flue.Gas ii

UNIT BOUNDARY

Substation

Dryer Exhaus^ ^

Make-up Water

Figure 1. Direct-fired biomass electricity generating system schematic.

Electricity

Electricity

Dryer Exhaus^ ^

Make-up Water

Figure 1. Direct-fired biomass electricity generating system schematic.

All biomass combustion systems require feedstock storage and handling systems. The 50 MW McNeil station, located in Burlington, Vermont, uses a spreader-stoker boiler for steam generation, and has a typical feed system for woo d chips [1]. Whole tree chips are delivered to the plant gate by either truck or rail. Fuel chips are stored in open pile s (about a 30 day supply on about 3.25 ha of land), fed by conveyor belt through an electromagnet and disc screen, then fed to surge bins above the boiler by belt conveyors. From the surge bins, the fuel is metered into the boiler' s pneumatic stokers by augers.

The base case technology is a commercially available, utility operated, stoker-grate biomass plant constructed in th e mid-1980's [2], and is representative of modern biomass plants with an efficiency of about 23%. Plant efficiency o f the stoker plant increases to 27.7% in the year 2000 through the use of a dryer, and in 2020 plant efficiency is increased to 33.9% due to larger scale plants which permit more severe steam turbine cycle conditions, e.g. higher pressure , higher temperature and reheat.

Solar Stirling Engine Basics Explained

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