Ord, J., Maclaren, J., and Palmer, S., MWH (UK) Ltd, UK
(free)With the introduction of the Thermal Hydrolysis Advanced Digestion systems and ROCs over recent years there has been a drive towards the installation of large highly electrically efficient gas engines to maximise electrical energy generation. With the incremental cost to go from 1.5MWe to 2.0MWe output seemingly being paid back in a matter of months by the additional power generation, the installation of the largest affordable engine at the design condition has seemed very logical. The answer is not so clear when you factor in the impact of small gas bags, running at average rather than design conditions, the trade-off between waste heat recovery and natural gas import for steam generation, the introduction of HRIs and the complication of Grandfathering rights on installed equipment.
This paper will compare and contrast the impact of these factors on gas engine selection and review selection options for a specific site. Introduction Identifying the potential power generation and natural gas consumption (and thus the operating income) for an advanced digestion installation at a high level is relatively straightforward. Realising the theoretical revenue, or revenue used to justify the capital investment, can be more challenging in practise as many variables impact on how the CHP plant operates.
In practise the ability of a site to realise the theoretical revenue values is a function of a number of variables, including:
Understanding the behaviour of the biogas storage system is critical to operation of any CHP system, since it is normally the level / pressure in the storage vessel which dictates when the gas engines operate. Gas engine operation not only impacts on power generation but also on the quantity of natural gas needed for the installation to provide the balance of steam needed by THP processes. Misalignment of the gas engine, waste heat recovery and the THP steam demand can have a significant impact on project viability. If engines do not operate continuously then not only will the plant generate less power, there will also be less steam generated from the waste heat recovery and this will increase significantly the quantity of natural gas needed. Hence the overall project finances will be hit by both reduced revenue from less power generation and increased expenditure due to high natural gas consumption. To understand how engine operation (starts and stops), natural gas consumption and net operating revenue are related, a dynamic model was generated to link all the above parameters in a single simulation. The diagram below outlines how the simulation models the interaction between biogas generation, biogas storage, power production and steam generation split.
Keywords: Gas engine, biogas storage, CHP, THP, OPEX, Operability
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