According to the National Renewable Energy Action Plans of the European Union member states bioenergy is the largest contributor to the renewable energy targets for 2020 with 54.5% and energy from solid biomass will contribute 36% [1
]. Hence, a strong increase in biomass utilization has been observed in the last two decades in many European countries. As modern society, beyond 2020, will be driven by a low-carbon economy, or rather by a bio-economy based on the principle of cascading, more and more biomass will be utilized for non-energy purposes. This requires the efficient recovery of residue biomass, for e.g. heat production, as high-value biomass will be mainly used for material purposes.
With a growing share of wind and solar power in the energy mix, bioenergy becomes a vital renewable energy source for compensating the fluctuating availability of wind and solar power dependent on weather conditions. A smart utilization of bioenergy will be integrated with the other renewable energies and bio-based products, acknowledging that wind, solar, and bioenergy need to be complementary in order to achieve a 100% renewable heat and power production by the end of the century [2
The provision of a large variety of non-woody-residue biomass for small-scale heat and power production will be made possible through adequate fuel-enhancement technologies in order to diminish disadvantageous fuel characteristics when combustion or gasification is the conversion of choice.
Two of the most frequently available residue biomass types in urban areas across central Europe are both grass clippings and foliage. For Germany the biomass potential of grass clippings from road-side verges alone was estimated to be as much as one million tonnes (fresh matter (FM)) in 2009 and their conversion to bioenergy has been suggested [3
]. Public parks, lawns, cemeteries, and road-side verges are commonly planted with grass and other non-woody species, while deciduous trees may be present in all of those areas, too. Currently, the periodically incurring amounts of foliage and grass-clippings are gathered during park maintenance activities, etc.
Often, parts of the biomass are stored for composting while, due to missing alternative utilization strategies, excess biomass has to be burned in waste combustion plants. In the case of composting, added value can be generated; however, no greenhouse-gas savings are to be expected [4
], which will be the case for combustion or gasification applications when fossil fuels are substituted.
Compared to wood as a raw material for fuel pellets, the main insufficiency of grass and foliage are their chemical and physical-mechanical characteristics [5
]. Especially the high ash content (up to 45% dry matter (DM) in foliage, due to soil-uptake during collection [7
]) and with it the non-organic matter are a challenge during combustion. The high concentrations of potassium (K), chlorine (Cl), nitrogen (N), sulfur (S), and the sum of elements highly involved in particulate matter (PM) formation (K, Na, Zn, Pb) are of great concern since they are known to be the main agents of high gaseous (HCl, SO2
) and PM emissions [8
]. These elements and their interactions during combustion can be summarized by fuel indexes [9
] to help foresee possible emission, corrosion, and slagging related issues that might reduce the performance of combustion plants. Furthermore, the low bulk density of loose grass and foliage, as well as their moisture content, is disadvantageous for transport and storage.
It is evident that residue biomass needs to be upgraded before application in combustion systems, especially small scale boilers, as has been suggested by several studies over the past 20 years [10
]. In many of those studies the term “leaching” [10
] was used to describe the underlying principle of fuel enhancement while the actual technology was referred to as a combination of hydrothermal conditioning [11
] and mechanical dewatering [11
]. In all cases the intention is to address the aforementioned problems of high non-organic matter content in residue biomass by washing and leaching those constituents. While earlier studies on straw and grass, and recent ones on foliage, have been on small amounts of material prepared in lab-scale setups, data from trials with grass in at least two different prototype plants with a capacity (input) of 300 kg FM (30% DM content) per day [14
] and 3000 kg FM/h (20% DM content) [15
] is available. The latter one is the prototype plant built according to the florafuel process which was introduced by the florafuel AG, Germany [15
]. In this particular concept the enhancement process includes up to ten different process steps (a detailed description can be found in 2.2 and Figure 1
) of which the washing and cutting of wet biomass and the mechanical dewatering are most crucial.
It is suggested that the press water extracted from the raw material during the process (containing particulate organic material and solved alkali and earth alkali salts) can be utilized for anaerobic digestion to produce biogas [15
] and the digestate may still be valuable as fertilizer [17
]. The solid fraction (press cake) will be used as fuel for combustion and investigations on the press cake of grass and foliage after application of the florafuel-process have shown reductions in Cl concentration by 90% and 77% DM, respectively [15
]. K concentrations in grass were reduced by 83% and N by 50% DM. Similar results were obtained across a whole range of grassland biomass types processed with the prototype presented by Hensgen et al.
In the literature the results of leaching and the respective change in fuel characteristics are well discussed amongst the studies published, in some cases going as far as estimating ash melting behavior (for foliage [6
] and grass [11
]) and potential performance during combustion (e.g. regarding emissions) by means of guiding values [14
]. However, no studies have been published which show actual combustion performance of leached, hydrothermally-conditioned, and/or mechanically-dewatered fuels. This seems to be a major research gap which the authors of this paper would like to address.
For standardized emission measurements and an investigation of fuel performance during combustion small-scale boilers are most suitable and they might also be the application of choice for a future market for leached biomass pellets. The performance of several types of small-scale boilers with pellets from agricultural residues have been studied previously [19
]. For that purpose the densification of upgraded raw material into either pellets or briquettes is desirable depending on the requirements of the boiler type. Other than that, densification is a common measure to improve handling, transport, and storage of the fuel. Extensive research has been conducted in recent years to fully understand and optimize the processes that occur during densification of biomass [22
]. While the global wood pellet production has grown from 4 million tons to 24 million tons in the last ten years [23
] pellet production from non-woody residue biomass is mainly limited to raw materials, such as straw or miscanthus. Grass pellet production is currently more a matter of animal fodder than a means for fuel production. Yet, recent studies are specifically dealing with pelletization of alternative biomass, such as grassland biomass for energy purposes [24
In Germany, the strict emission thresholds that exist for non-woody biomass when burnt in small-scale appliances and the respective lack of experience and trust in a rather new product, such as leached biomass fuels, might partly be the reason why they are currently not commercially available.
The goal of this study was to investigate the performance of solid biofuel pellets made from leached, non-woody raw material in available small-scale boilers by comparing the emissions with the given German legislation for boilers with a nominal thermal heat output of <100 kW (1. BImschV [26
]), which currently has the strictest PM thresholds within Europe and North America [27
]. Furthermore, the fuel and bottom ash characteristics of grass, foliage, and mixed (50% grass, 50% foliage) pellets were compared and discussed using fuel indexes, for example. Providing data from small-scale combustion trials with these novel fuels will help bridge the gap between the theoretical understanding of their fuel characteristics and the actual performance during combustion.