District Heating - a Clean Solution

Introduction

The environmental risks and uncertainties of' a high-energy future are disturbing and give rise to several reservations concerning the use of fossil fuels.

A number of technologies will help to reduce atmospheric pollution. In Denmark special importance is attached to the following lowing:

- Energy conservation

- Efficient energy conversion

- Renewable energy sources

- District heating, combined production of heat and power.

Many agree that district heating (DH). produced by the traditional heat-only plant. and combined heat and power (CHP) have enormous potential when considering thermal efficiency and lowered environmental impacts: The basic technology of each Is proven, it would be relatively simple to satisfy a substantial part of the energy demand. and their high efficiencies mean reduced pollution including greenhouse gas emissions. This is especially important in high population density areas - the obviously preferred sites for such energy generation.

Compared with individual heating DH can provide a community with an operationally efficient and most often also an economically competitive heat supply. This is particularly true under the circumstances where the DH system is supplied from CHP plants. Their use results in very substantial improvements in overall efficiency. Further environmental improvements arise from the reduced air pollution obtainable in reason ably large CHP plants equipped with flue gas cleaning to remove particles, sulphur dioxide, and nitrogen acids.

As a consequence of these considerations. DH plays an important role in fulfilling the space and water heating demand in many countries. This is especially the case in Den mark where this technology is utilised to a very great extent. Indeed. DH is one of the reasons why Denmark has relatively good air quality in the cities. The following table Indicates the extent to which this technology Is used in various countries.

Potential Environmental Advantages - how?

It is noted above that DH and CHP plants operate at high efficiency and therefore use less fuel for the same amount of electric and heat energy produced, in turn resulting in conserving fossil fuel and lowering atmospheric pollution. The advantages of CHP in comparison with separate production is clearly demonstrated In Fig. 2, which compares the CO: emission from CHP production with that from separate production based on coal, oil, or natural gas.

The centralised nature of DH and CHP production plants results in a concentration of the emissions at one or more distinct sources. Introducing several direct benefits. As a result of scaling advantages large facilities are much more able to economically incorporate sophisticated state-of-the-art pollution control technologies than individual buildings (particularly households, commercial establishments, and small Industrial complexes). Large-scale DH systems, which In many cases have Included the best avail able control technology as part of compulsory regulation, are thus capable of reducing emissions on an equivalent energy production basis.

Controlled Operation

Large, centralised plants, such as DH and CHP facilities, typically utilise better operating and maintenance practices than do small individual heating systems. Larger facilities usually have trained staff as well as sophisticated, often computerised monitoring equipment for continuously monitoring system operations, ensuring that performance specifications are being met. When such specifications are not met, prompt maintenance can be administered or operating changes or improvements introduced as necessary. Regularly scheduled maintenance is a normal function of facilities of this scale. The incentives to maintain a high level of operability. with little downtime or drop in operating efficiency, are usually economic ally based and often play a crucial role in the overall viability of a plant. Individual building systems, on the other hand, cannot always afford sophisticated continuous monitoring equipment or a permanent maintenance staff. The result of this is that many such operations deteriorate with poor maintenance, so that operating efficiencies subsequently drop well below optimum levels. The higher operating efficiency afforded by large, well-maintained, facilities results directly in reduced fuel consumption which in turn results in fossil-fuels conservation and reduced emissions. Higher operating efficiency levels of the combustion process (where parameters such as temperature, combustion air and fuel input levels, and residence time are closely monitored) lead to a reduced emission of certain pollutants produced, particularly CO₂ and NO_x.

Technical Development

Centralised DH and CHP facilities permit developing technologies to be adopted at the earliest possible date In order to lower environmental impacts. Examples of such adoptions include retrofitting boilers with low NO, burners, flue gas recirculation and selective catalytic reduction techniques to reduce NO, levels, and flue gas scrubber systems to minimise SO₂ and HCL emissions. Such adaptions can be made at a lower cost compared to the same effort at a large number of small facilities.

Other Benefits

There are many indirect environmental benefits of DH and CHP plants which may not have as much impact as those described above but which are worth noting.

For example, noise associated with the operation of DH plants is concentrated to a single boiler house. Here the two main sources of noise, the boiler blowers and the pumps, can be placed more practically and cost effectively behind thick brick or concrete walls fitted with mineral wool noise suppressors.

With the concentration of fuel storage at central oil-fired facilities, the potential risks associated with leakage or emission of pollutions other than fuel are minimised compared with those from a number of smaller plants.

Many Fuels can be Used

DH systems may vary in size from those serving a small group of houses to those covering an entire metropolitan area.

Any fuel or any source of waste heat may be utilised in a DH system, and systems over a certain size may utilise heat from more than one heating station, perhaps burning different kinds of fuel.

In DH plants fuels are coal, fuel oil. natural gas. biogas. waste, wood chips, and straw. CHP stations play an important role in generating the heat, and so also do waste incineration plants. In addition, small decentralised CHP systems are becoming increasingly important. Also, surplus heat from cogeneration in industry contributes to DH systems. With all kinds of fuel, emissions can be held clearly below the stipulated levels stated by the National Agency of Environmental Protection.

DH will, therefore, hold a strong position in all efforts to secure the most rational use of energy and hence contribute substantially to the reduction of air pollutants, including carbon dioxide.

Documentation

The example also considers emissions (A) without and (B) with flue gas cleaning.

The alternatives are compared for three fossil fuels:

- Coal

-Fuel oil (light oil for Individual heating)

- Natural gas

The examples may not correspond to real situations as the heating of a city would seldom be based on a single fuel. In order to demonstrate the effect of CHP idealised cases are described.

There are many different pollutants in the flue gases from the combustion of fossil fuels. Four have been calculated In the example:

Sulphur dioxide (SO₂ )contributing to the acidification of the environment.

Nitrogen oxide (NO_x) contributing to the acidification but also one of the greenhouse gases

Carbon dioxide (CO₂) the dominant greenhouse gas.

Particulates: contributing to local pollution.

Combined heating Combined electricity Individual heating Electricity

A: Without flue gas cleaning B' With flue gas cleaning

The examples illustrate a considerable reductionof pollution due to:

- lowered fuel consumption

- increased overall efficiency

- improved techniques for flue gas cleaning.

A considerable reduction of ground level concentrations of local pollutants will also result from substituting the controlled emission from tall chimneys expected with large units such as CHP plants for combustion in individual small units. The health risks will be reduced accordingly.

Other pollutants are also correspondingly reduced, e.g. ozone (0₃) - a local pollutant as well as a greenhouse gas - and the carcinogenic PAH (Polycyclic Aromatic Hydrocarbons).

Another greenhouse gas, methane (CH₄), mainly produced during mining and drilling operations and transportation of fossil fuels is also reduced proportionally due to the improved energy efficiency of combined production.

The effect of introducing CHP will depend on the kind of fuel used in the plant. on which fuel is substituted, requirements for emission control etc.

The figures will vary according to circumstances but the end result with the DH/CHP solution will always be a considerable reduction of emissions.

Financial viability depends on.

-Local fuel prices

-Availability of waste heat

-Construction costs for pipe networks. etc.

-Local climatic conditions and regulations

-National approach to energy conservation and the related taxation structure

CHP technologies are well proven and have been applied in many places. In Denmark CHP supplies more than 25% of the space heating demand (CHP + DH more than 45%). CHP with DH has a demonstrated record in reducing energy costs as well as environmental pollution.

Basic major assumptions for the example:

Total Heat Demand Including losses: 14.4110 TJ/year (equivalent to 175.000 homes)

Peak load: 800 MJ/sec.

Base load: 500 MJ/sec (6400 utility hours)

Extraction Power Plant: 350 MW (electricity) 500 MW (heat).

Power Plant efficiency: 0.85 (.83 with flue gas cleaning)

District Heating network efficiency: 0.85

Individual Heating efficiency: 0.75 - 0.80 (annual basis).

Flue gas cleaning requirements vary considerably according to national regulations and type of fuel used. for comparison purposes the same figures have been used for all fuels:

SO₂ : max. 120 g/GJ

NO₂ : max. 100 g/GJ

Particulates: max 18 g/GJ

Conclusion

The solution for the heat market is district heating and combined heat and power which has an enormous potential. The technology is proven, it would be relatively simple to satisfy a substantial part of energy demand. and its basic efficiency means reduced pollution and greenhouse gas emissions.