A highly productive, waste-free society would be undeniably beneficial. Whether a genuinely circular economy is realized, if this goal reduces waste and limits the use of scarce resources, it must be recognized as a beneficial instrument. The same applies to the concept of sustainability (Geissdoerfer et al., 2017). A circular economy is defined by two guiding principles: maximize the service offered by the materials contained in goods and minimize service loss over time (Velenturf & Purnell, 2021).

The ROBINSON project uses diverse technologies, including biomass gasification, wastewater valorisation, and heat and oxygen reuse based on the industrial symbiosis concept. The high availability of RES and the generated green hydrogen will also be examined for other uses such as ship cargo loading/unloading electrification and biogas road transport. The AD+BES allows for the conversion of liquid waste from the fish business into biomethane, which can then be utilized as fuel driven by RES excess. This novel bioenergy-based technology is another source of dispatchable energy generation and storage applications, which is especially important for islands with biological waste. A CHP unit (mixed fuel), a steam boiler, an anaerobic digestion coupled with bio-electrochemical system (AD-BES) described in Figure 1, a biomass gasifier, and a gas fuel mixer are supposed to be installed on the islands, converting island organic waste streams into biomethane. In the notion of industrial symbiosis, for example, biomass and/or waste feedstock will be utilised. Industrial symbiosis is defined as a concept that "allows entities and companies that have traditionally been separated to cooperate among themselves in the sharing of resources, contributing to the increase of sustainability with environmental, economic, and social benefits (Neves et al., 2020), and should thus result in a more circular economy. Waste heat from industrial activities will be used to replace fossil-fuel based heating systems at Eigerøy, and waste steams containing organic material will be used to create bio-fuel - an industrial symbiosis idea with high replicability potential on other islands.

Figure 1 AD+BES Scheme

A circular bioeconomy necessitates the use of sustainable biomass to ensure that the restoration cycle is completed, and it might be continuously repeated, and that’s what ROBINSON will try to promote and incorporate. Biomass is a general term applicable to all plant and animal derived materials (Sherwood, 2020). Biomass differs from other renewable energy sources since it's a substance combusted to create heat, (before it can be transformed into electrical or mechanical power) or utilized as a precursor to goods. The most common forms of biomass and their Biomass Characteristics Index (BCI) are presented in Table 1.

Biomass types	Moisture	Moisture	Avg.	Bulk density	Bulk density	Avg. bulk	BCI
	(%, Min)	(%, Max)	(%)	(t/m3, Min)	(t/m3, Max)	(t/m3)
Air dry wood chips	20.00	25.00	22.50	0.190	0.290	0.240	18,600
Green wood chips	40.00	50.00	45.00	0.280	0.410	0.345	18,975
Kiln dry wood chips	10.00	15.00	12.50	0.190	0.250	0.220	19,250
Empty fruit bunch	15.00	65.00	40.00	0.160	0.550	0.355	21,300
Kiln dry wood chunks	10.00	15.00	12.50	0.200	0.310	0.255	22,313
Air dry wood chunks	20.00	25.00	22.50	0.240	0.370	0.305	23,638
Green wood chunks	40.00	50.00	45.00	0.350	0.530	0.440	24,200
Mesocarp oily fiber	30.00	–	30.00	–	–	0.305	21,350
Kiln dry sawdust	10.00	15.00	12.50	0.240	0.370	0.350	30,625
Fresh fruit bunch	40.00	–	40.00	–	–	0.480	28,800
Green sawdust	40.00	50.00	45.00	0.420	0.640	0.530	29,150
Straw bales	7.00	14.00	10.50	0.200	0.500	0.350	31,325
Green roundwood	40.00	50.00	45.00	0.510	0.720	0.615	33,825
Air dry roundwood	20.00	25.00	22.50	0.350	0.530	0.440	34,100
Ash	0.00	–	0.00	–	–	0.437	43,700
Sterilized fruit	30.00	–	30.00	–	–	0.660	46,200
Fruitlets	30.00	–	30.00	–	–	0.680	47,600
Wood pellets	7.00	14.00	10.50	0.500	0.700	0.600	53,700
Press expelled cake	12.00	–	12.00	–	–	0.650	57,200
Palm nuts	12.00	–	12.00	–	–	0.653	57,464
Cracked mixture	12.00	–	12.00	–	–	0.653	57,464
Dry EFB cut fiber	10.00	–	10.00	–	–	0.710	63,900
Shell	12.00	–	12.00	–	–	0.750	66,000
Coal	6.00	10.00	8.00	0.700	0.800	0.750	69,000
Wood briquettes	7.00	14.00	10.50	0.900	1.100	1.000	89,500

Table 1 Biomass characteristics (Sims, 2013; Tang et al., 2016)       

Anaerobic digestion (AD), at its most basic, takes mixed, low-value waste streams and transforms the carbohydrate within them into methane with a theoretical maximum carbon efficiency of 50%. Several products can be converted into waste-to-use materials, thereby enhancing the circular economy concept. As a result, the goal of anaerobic digestion should be to provide value-added products generated from carefully selected feedstocks, rather than just to eliminate undesired wastes. The principal product is methane, although utilising the digestate as a fertilizer is also a crucial issue (Behera et al., 2022; Fuentes-Grünewald et al., 2021; Velvizhi et al., 2022).

Biomass gasifiers are an excellent sustainable solution towards sustainable development and circular economy as past research have shown (Ribó-Pérez et al., 2021). An HRES equipped with a biogas generator has been evaluated for two European cities in (Tiwary et al., 2019). The study proved that a biogas generator can produce more than 60% of the total energy demand for both the cases, leading to an innovative and promising system (Tiwary et al., 2019). On this context, ROBINSON is suggesting to also exploit sustainably produced heat sources, such as biomass and (renewable) hydrogen in order to produce heat. Also, it is worth mentioning that a gas storage will be installed in Eigeroy in order to store the gas and manage the amount of gas going into the gas mixer. Finally, biomass will be kept as (waste) wood chips to be used in biomass gasification. The nature and size of these storage media are currently being considered.

Concluding, all the aforementioned will move the case studies one step closer to circular economy, taking full advantage of their wastes in order to either produce heat, or energy. In this case, otherwise wasted heat will be harvested, contributing to reduced demand of primary energy and also to energy efficiency. The crucial mentality shift that occurs with a circular economy is that waste management becomes a process of returning resources to use rather than just eliminating trash, which will be also beneficial for the energy transition agenda of each island that the tool is replicated to.