Biomass Combustion for District Heating –
Relevance and Design Requirements

Thomas Nussbaumer, Verenum Research, Zürich (Switzerland)
Jaap Koppejan, IEA Bioenergy Task 32, Enschede (The Netherlands)

District heating offers interesting opportunities to use biomass for heat and to replace decentralised fossil fuels for heating. Due to a relevant plant size of typically more than 1 MW, relatively high combustion efficiencies and low pollutant emissions can be achieved. Hence district heating and biomass combustion exhibit relevant synergies with a positive effect on climate and environment. However, district heating also induces additional costs and energy losses. Therefore a most economic and efficient design of the district heating system is an important pre-condition to further promote biomass combustion.

A study of the effect of the design on the economy and efficiency was performed by Task 32 [1] and supplemented by an international survey of existing biomass fired district heating networks [2], which assessed characteristic parameters such as the annual heat losses, the connection load, and linear heat density in MWh/year per meter.

The investigation showed, that a typical district heating system with a linear heat density of 2 MWh/year per meter reveals heat distribution costs of slightly more than 2 euro cent per kWh heat in the case of optimised pipe diameters (see below).

The total costs are dominated by the capital costs with fuel and electricity costs being of minor importance. Since decreasing linear heat density induces increasing capital costs, the linear heat density should exceed a minimum value to ensure economic district heating applications. For this purpose, a minimum value of 1.8 MWh/year per meter is proposed in mid-European countries and related to a target of less than 10 % heat losses in the heat distribution. The survey of existing district heating systems however reveals typical heat losses of 13 % at the pre-described linear heat density thus indicating a potential for design improvement (see below).

Although the linear heat density is confirmed to be an important parameter, the heat losses of existing district heating systems vary by more than a factor of three at the same linear heat density. Consequently, additional parameters also influence the heat distribution losses and costs according to the following trends:

The pipe diameter strongly affects the capital costs and the heat distribution losses. Application of pipes with significantly larger diameters than what is required to avoid cavitation pitting, leads to strongly increased capital costs and heat distribution losses. A design with the smallest allowable pipe diameter is therefore crucial to improve the economy and to reduce the heat losses of future district heating systems.

Other key parameters to minimise heat losses and costs are the temperature spread, the temperature level, the insulation class, and the ratio between the operation hours of the district heating system and the full-load hours of the heat consumers. Furthermore, a network layout based on the location of the heat production plant and the choice of the network type also play an important role.

While heat production plants exhibit strong economies of scale, the heat distribution is related to diseconomies of scale. Consequently, large district heating systems are only economically feasible thanks to strong economies of scale in the heat production, which is typically the case for combined heat and power applications. Large district heating systems however also exhibit higher distribution losses when compared to smaller networks.

A key factor in the optimum design of district heating systems is therefore the use of a minimum pipe diameter in the network, which can be achieved by enabling high temperature differences between feed and return temperatures, thanks to optimised design and operation of heat exchangers at the end users. By enabling automatic combustion systems equipped with state-of-the-art particle removal technology, biomass can be used to supply heat while safely avoiding a negative effect on the local air quality.

Background reports

VERENUM prepared two reports for Task 32 on the optimisation of biomass fired district heating networks to analyse where improvements can be made in both the design and operation.

The first report contains a theoretical assessment of the influence of system design parameters on heat distribution cost. The report shows what design optimisations can be made for pipe diameter, temperatures maintained, and insulation class based on given line heat density and the utilisation rate.

The second report contains an evaluation of the performance of actually operating district heating networks, and shows the difference between individual countries.