Spark Ignition CHP – Great performance, but lots to think about

Elson, O., Atkins, UK


‘Combined Heat and Power’ (CHP) systems have been commonplace on waste water treatment works for
decades. The latest engine technology however offers significant gains in terms of efficiency and
emissions. The ability to successfully integrate this type of engine, and hence access these benefits,
relies upon the careful consideration of some key aspects:

  • Sizing/utilisation – High capital cost makes redundancy uneconomic. These engines can ‘ramp’
    to a greater extent, but constraints still apply.
  • Gas quality – Spark ignition engines have increased sensitivity to contaminates in the biogas.
  • Integrated control – ensuring that the site systems and proprietary engine systems can
    communicate without compromising network security.
  • Load management – The low inertia of this type of engine makes them susceptible to trips in
    response to load steps. (A particular challenge if the CHPs are going to be used during power

Asset planners and accountants can sometimes be forgiven for being so excitedly seduced by the
prospect of step changes in OPEX opportunities that a high efficiency CHP installation can offer. When
assessed simplistically based on the typical engine efficiencies, the numbers speak for themselves:
Table 1: Indicative OPEX Improvements
(See .pdf)
Such a site would likely be at the mid to lower end of the spectrum in terms of CHP viability for municipal
wastewater treatment plant applications.

These theoretical improvements can indeed become a reality. In many cases, opportunities to improve
heat recovery system performance, and reduce maintenance down-time, means the benefits can be
extrapolated further still.

However, any project to upgrade or install a spark ignition CHP engine must be carefully conceived if it
is going to deliver successfully, and go on to return the anticipated ‘operational expenditure’ (OPEX)
benefits. Some of the influencing factors are obvious, but others are more subtle. This paper seeks to
explore some of the key aspects from a general perspective and is written in laymen’s terms as far as
is possible. The paper is written in the context of CHP at municipal wastewater treatment plants. This
is due to circumstance, with the author’s experience arising from projects in this sector. It is however
suggested that the same challenges apply to varying degrees in other applications such as landfill or
food waste processing.

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