Fired up for energy recovery
In part two of our seven-part Waste to Energy series, Robin Holder, Tal Golesworthy and Peter Davies of consultancy Environmental Development Technology explore the basics of burning waste with a look at current and future developments in firing systems.
The ciculating fluidised bed combustor is a hybrid that is capable of burning a wide range of waste materials
It is only in the last two decades that there have been significant developments in the burning of wastes. Until then, incinerators were crude combustion devices, typically comprising a refractory lined box connected to a chimney. The waste material was manhandled through a door on to a static grate and then heated by burners until combustion was self supporting. The operator would check the combustion visually, when completed the ashes were raked out manually and the incinerator recharged. There were many thousands of these incinerators operating in the UK. Some modifications were made in the 1950s and early 60s to automate the feeding of the waste particularly on large municipal waste incinerators with the advent of the moving grate.
What is wrong with this technology? If it has worked well for many years, why does it need to be changed? Why do we need these sophisticated burning systems? The answer lies in one of the techniques the operator employed to check on the state of the combustion – smoke!
When black, the waste was just starting to burn with the moisture and more volatile components within the waste feed being driven off – an example of incomplete combustion called gasification. If it was grey, then the waste was mid-way through its burning cycle with the volatile components already burned but with the main bulk of the waste still being burned. At this stage there is just about enough air in which to burn the waste and this is called combustion. If it was a very light grey, then combustion was nearly complete and there was an excess of air available to burn the waste
The operator had little control over the burning cycle. Nowadays, it is very important to accurately control the combustion/gasification of the waste to maximise the amount of energy removed from the waste and to minimise both solid and gaseous emissions (VOCs and PCBs).
The parameters that are particularly important in controlling combustion, and which form the bases of most incinerator designs are:
Waste residence time
Most modern incinerators are designed to operate at about 1,000°C and to heat the waste to at least this temperature for a minimum of two seconds. In fact, most legislation stipulates this as a minimum requirement. The excess air is a measure of the percentage of air required above the stoichiometric air level (which is the theoretical minimum amount of air required to burn a given fuel/waste) to achieve complete combustion.
What isn't so easily achieved is mixing of waste with combustion air so that all of the waste is burned. The problem is compounded when the waste is fed as large lumps, for example whole tyres, rather than in powder or granular form. It is clear, therefore, that one particular firing technique is not going to be suitable for all types of wastes. There are many different firing systems, some of the more common are described as follows.
The moving grate is used extensively in the burning of MSW. In the majority of cases, the waste is introduced via an "orange peel" grab into a chute at the front of the furnace. The waste is charged using a hydraulic ram through an air-lock system on to the grate. Modern grates tend to be inclined, multiple roller or hearth types. The rollers/hearth operate on a timer system, shuffling the waste between them as it gradually burns out. The main or primary air is introduced under the grate. Additional or secondary air is introduced into the furnace above the grate and is used primarily to burn out the volatile material. This technology is well established with few operational problems but is expensive which is why the units tend to be large (circa 30MW(th)).
Another popular category is the rotary kiln with secondary combustion chamber which is typically used in hazardous or clinical waste applications. The solid waste is loaded via an automatic airlock system and in many systems drops down a water cooled chute into the rotating kiln. Rotation of the kiln is fairly slow generally (no more than 6rpm) so that all of the organic material is gasified. The kiln can be considered as a pre-treatment zone, gasifying the waste at temperatures of up to 1,100°C. It is also inclined so that the molten slag produced from the inorganic material in the waste runs out into a water quench system.
The gases produced during the initial gasification step in the kiln are burned separately in a secondary combustion chamber. Both the kiln and combustion chamber are lined with refractory to protect the metal shell from high temperatures and chemical attack. Often the gases are introduced tangentially to ensure good mixing with the air so that all of the gases are burned completely before they leave the chamber.
One of the more recent techniques introduced to burn waste streams is fluidised bed combustion. This technique differs from conventional incinerator firing systems in that the waste is introduced into a bubbling bed of inert material such as sand operating at temperatures between 800 - 1,000°C. The waste becomes extremely well mixed with the air ensuring reasonably good combustion. The technique is limited as the waste needs to be crushed or macerated into small lumps or granules of less than 25mm diameter. There is, therefore, a limit to the type of waste that can be burned, typically wood waste, chicken litter and bone meal.
From an energy recovery point of view, another disadvantage of fluidised bed combustion is that to maximise the efficiency of heat removal, the boiler tubes should be immersed within the bed itself. In the past, this has led to premature failure of these tubes through a combination of erosion and corrosion. To avoid this problem, hybrid designs have evolved.
One of these hybrids is the circulating fluidised bed combustor. This has proved particularly successful in burning a whole range of waste materials often with a support fuel (normally coal). It differs from conventional designs in that the bed of inert material is fluidised at a higher velocity so that some material is carried over into a separate heat transfer zone avoiding most of the erosion problems. The heat is removed from the particulates before they are recycled to the first chamber. This repeated recycling ensures that a very high percentage of the waste material is burned completely.
There are several waste streams which are currently being burned in these systems including bark, paper and various waste sludges. While some pre-treatment may be necessary, the quality control does not have to be as tight as for conventional fluidised beds. The higher fluidising velocity of the circulating bed enables a wider size range to be handled. Another advantage is the low gaseous emission levels particularly of nitrogen oxides and carbon monoxide. The former is as a result of the relatively low combustion temperature, the latter from effective mixing and long residence time. The main disadvantage is the relatively high capital cost of these systems which means that there is a minimum size below which they are uneconomic. Typical sizes range from 30 to 150MW(th).
As waste to energy will almost certainly form an increasing part of the UK's electricity production, maximising the useful energy from the waste is becoming a key part of the process. The efficiency of electricity produced by conventional steam cycle technology (boiler, superheater, steam turbine, alternator) is limited by steam temperature and pressure. Therefore alternative solutions are being sought which give an improved efficiency.
Most of the solutions currently being investigated include gasification followed by combustion steps normally termed combined cycle technology. The basic principles are that the waste is gasified to produce primarily carbon monoxide and hydrogen both of which can be burned and expanded through a gas turbine to produce electricity. For the system to work, the gases need to be at pressure as well as temperature. The solids from the gasifier which have a high carbon content can then be burned in a combustion system with steam cycle technology to produce electricity. It is estimated that producing electricity in this way will increase the efficiency of its production by 10-15%. This technology is still a few years from the market place.
Waste to energy
Firing technologies have moved a long way in the last 20 years. There is now a range of systems available in the market place able to burn waste materials with varying calorific values, moisture and ash contents. The economic viability of a waste to energy system (particularly if NFFO applies) is dependent on the reliability and availability of the entire plant. Burning the waste is the first and most important step in the process. If this is done effectively, efficiently and cleanly, then there is a good chance that plant operation will be successful.
Environmental Development Technology (EDT) specialises in combustion and air pollution control within the industrial energy market. It aims to assist industry in improving energy efficiency and minimising the environmental impact of combustion equipment.