Major Technologies for the Exploitation of Renewable Energy Sources

Major Technologies for the Exploitation of Renewable Energy Sources

 关于生物质燃烧技术及气化技术概要

According to the degree of maturity, biomass conversion technologies can be classified into three categories:

- Traditional technologies

- State-of-the-art technologies

- Emerging technologies

Traditional technologies

Traditional technologies are conventional technologies which are used for a long time without any technological barrier.  The major traditional technologies are:

Pile burners.  Approximately fifty years ago, all common ways of burning wood waste involved some form of pile burning.  The pile burning combustor was typically composed of number of cells.  Each of the cells consisted of a lower, refractory lined combustion chamber with a grate floor to support fuel pile and an upper, second combustion chamber.  The furnace and the boiler were separated with the furnace generally located above the secondary combustion chamber.  The pile burning boiler designs were  simple to design and inexpensive.

In pile burning system, wood waste is normally piled as high as ten to twelve feet.  Combustion air flow upward through the grates from underneath the pile and inward from the cell walls providing oxygen for combustion, cooling air for the grates and promoting turbulence and fuel drying.  Most of the fuel will be burned  on the grates.  Burning of the volatiles will be completed in the secondary combustion zone where overfire air is introduced.  Operating temperatures typically ranged from 1100°C to 1400°C.  Typically, pile burners had a slow response time to demand fluctuations.

The combustion process is very difficult to accurately control and the cells had to be shutdown periodically for cleaning.  The load and the amount of excess air are difficult to control.  Due to high operating temperatures, the wood ash actually slagged and pooled on top of the grate.  The ash is then manually broken up and removed from the furnace.

Pile burners are capable of handling wood fuel with a high moisture content with large quantities of dirt and contaminants mixed in with the fuel.  Fuel sizing is less critical with pile burners than with other combustion systems.

Relative to other combustion technologies, the efficiency is low.  Especially in older designs, boiler efficiency is low, generally 50% to 60%, due to large surface area of the furnace and the absence of radiant air heating.  The high operating temperatures will basically be a disadvantage with respect to thermal NO_x  creation.

Combustion on grate.  There are three kinds: stationary sloping grate system, travelling grate system, and vibrating grate system.

Stationary sloping grate system.  The concept of the stationary sloping grate was already developed in the late 1920s.  Fuel is introduced at the top of the grate and slides down the grate.  The fuel burns as it proceeds the bottom.  This system is also referred to as semi-pile burning.  Characteristic problems with early designs included avalanching of the fuel on the grate and difficulty in controlling both the steam load and the rate of combustion.

Travelling grate system.  In travelling grate spreader stoker designs, the entire bottoms of the furnace is a slow moving platform or conveyor forming the grate.  The grate is cooled by air fed from under the grate.  In this way, the grate mechanism and its cooling system defines the maximum acceptable undergrate air temperature which, correspondingly, defines the moisture content of the fuel that can be burned.  Water cooled walls could be used to prevent slag formation adjacent to the stoker.  Fuel is fed from a pneumatic spreader stoker system located on the front of the furnace.  Smaller and dryer fuel particles are burned in suspension, while the larger particles fall in a thin layer on the moving grate.  The fuel has to burn at a uniform rate and a sufficient speed.

Vibrating grate system.  The vibrating grate system offers the benefit of spreading the fuel so that small piles that might form on the grate are levelled out.  There are less moving parts than with the moving grates and therefore less maintenance is required.  Fuel can be mechanically distributed by screw feeders located at the top of the grate.  Recent boilers use water-cooled vibrating grates, allowing the use of high temperature undergrate air and a higher percentage of overfire air.  This at its turn enables lower combustion temperatures and therefore better control of NO_x  formation.  Another advantage of lower quantities of underfire air is lower unburned particle carry-over.  Other advantages of the system are:

-           load control capabilities comparable to those of an oil burner, because of the fact that a large amount of the fuel is burned in suspension; and

-           possibility to switch to 100% firing of alternative fuels such as oil or gas without any further protection of the grate.

Usually, fuel switching capacity of grate systems is limited.  Moisture content should typically be kept within about 10% of the design rate.  Fluctuations in moisture content outside this range result in significant changes in flue gas flows and in heat transfer rates.  Fuels with low melting ashes, like many agricultural wastes, are typically kept below approximately 15% (heat input) of the total boiler fuel.  The simplicity and flexibility of the grate system makes this design one of the most adaptable units to co-fire solid fuels.  The difference in bulk density of fuels may create difficulties in using the same spreader or distributor for both fuels.

Efficiencies of recent designs range up to about 84% (LHV) for travelling grates and 96% (LHV) for vibrating grates.  In the 1980s many of these systems adopted a staged combustion process  in order to meet with NO_x  emissions standards.

Examples of combustion on grate systems are presented in Table 21.

Table 21.    Combustion on grate - examples