Introducing polygeneration
To recover much of the energy contained in high temperature streams, electricity co-generation using a steam cycle or an organic-rankine cycle (ORC) is beneficial to almost any thermo-chemical conversion process. This way, in any case (parts of) the in-plant electricity consumption can be covered or green electricity can be sold. For low temperature heat there is the option of supplying district heat if an adequate infrastructure and sufficient demand exist. From this it already becomes clear that, more often than not, by-products besides the desired fuel exist.
Depending on the relative prioritization of these products, two operation strategies can be defined, “Fuel-orientation”, for maximized synfuel yields and “Polygeneration” for a best-fit product mix
Fuel orientation vs. polygeneration strategy for the production of synthetic fuels from biomass
Whenever only the synfuel is considered as a main product of the plant, while electricity and heat are regarded as mere by-products, the plant is operated at “fuel-orientation” mode. Hence, all synthesis gas is used for fuel production, and fuel efficiencies are maximized via producer gas reforming and off-gas recycling. This approach is common for large-scale plants.
If electricity and/or district heat are considered to be of equal importance as the biofuels, a “polygeneration strategy” is applied. In this case, some producer gas or the Fischer-Tropsch off-gas are combusted for electricity generation. As a result, highly flexible systems to meet specific heat or power demands can be designed. Polygeneration plants can benefit from green electricity tariffs and are likely to be the next step towards large scale production facilities.
Why polygeneration?
Several advantages exist with respect to polygeneration plants, the most important of which are briefly outlined below.
Flexibility
Benefits from green electricity tariffs and specific market needs
Depending on the relative advantages of synfuel production, electricity generation and district heat demands, the process can be optimized so as to match market demands.
“Energy center of the future”
Integration into other production facilities, e.g. wood industry
In many industries, e.g. in wood processing, significant amounts of heat and electricity are required for the manufacturing process. Typically these have to be supplied on-site, or energy services from outside must be bought. Applying the polygeneration approach to such systems, synergies can be taken advantage of and the co-processing can be optimized along with a complementary “energy centre” that also supplies high-value biofuels.
Reduction of risk
Reduced risk as power production is state-of-the-art technology and from diversification
Given the current state of synfuel production, risk is a crucial aspect in the realization of demonstration-scale facilities. By means of a polygeneration strategy risk is reduced in two ways:
Firstly, as the combined heat and power production can be regarded at as proven technology, it is only the synthesis part of the plant that is subject to high technological uncertainty. Secondly, through diversification of the products risk is reduced, and as a result of long-term green electricity tariffs, break-even points are low for the synthetic transport fuels.
Increased viability
Optimization of the product mix and maximum cost efficiency
Given the flexibility of the process, the plant can be designed to maximize earnings by the selection of the product mix. Additionally, costs can be kept low, as, for instance, FT-offgas recycling can be spared.
Summary
As a result of current concerns about both crude oil prices and CO2-accumulation in the atmosphere, biofuels play a major role in tomorrow’s energy supply. Synthetic biofuels that can be produced from biomass via gasification and subsequent catalytic conversion of the synthesis gas compounds CO and H2 are one promising option to meet the ambitious goals set by the European legislation.
While typically only the synfuel is regarded as the desired product and co-products such as electricity and district heat are of negligible interest, the polygeneration strategy brings in a different approach.
In polygeneration plants that purposively sacrifice some synfuel yield to the advantage of power production, a high degree of flexibility is obtained that allows to design the products mix to the specific needs of the market or of other production facilities. The latter may be especially valuable for the wood processing industry, as synergies with a complementary “energy centre” can be achieved. Furthermore, the use of low temperature heat for district heating which is possible in the small scale of up to 100 MW fuel power not only adds to the viability of the process, but significantly improves the overall efficiency and thus maximizes the amount of CO2-savings.
These effects were shown via two process analyses that were carried out with the aid of stationary computer simulation. For both BioSNG and Fischer-Tropsch liquids polygeneration plants were assessed. Not only did the results prove the energetic advantage of such trigeneration facilities, but equally promising break-even points were attained. Moreover both technologies have already be demonstrated at the CHP plant Güssing. Thus, the risk of the implementation of the technologies in a larger scale is reduced, as not only diversification applies, but also dependence on the yet developing synfuel technology is abated.
Further research and development in this field will be directed towards the demonstration of the technologies in the larger scale in order to build the bridge between laboratory research and eventual industrial application. Moreover, efficiency improvements and cost reduction potentials ought to be sought for, along with catalyst optimization, so as to efficiently integrate synthetic biofuel production plants from biomass into tomorrow’s fuel supply.
It is the aim of the ICPS 09 to push these approaches forward and to underline the advantages of the polygeneration strategy.