شاخص کیفیت پلت چمن: ابزاری برای ارزیابی تناسب گلوله های چمن برای سیستم های احتراق در مقیاس کوچک

US renewable fuels policy strongly encourages biomass crop production, which should lead to expansion of biomass heating scenarios. Chemical composition of grass biomass can be extremely variable, depending on species, soil fertility, and harvest management. Biomass quality concerns have hindered the development of grass biomass for residential combustion, with no comprehensive evaluation system for grass pellet quality. Quality labeling will strengthen the fledgling grass biomass heating market, gain consumer confidence, and help to control combustion related emissions. The proposed system sums qualitatively different parameters into one Quality Index for relative evaluation and ranking of grass pellets for residential combustion potential. Parameters were selected and weighted for their relative importance based on available literature. Weighting was accomplished by adjusting the compositional working range for each parameter. A limit also was established for each parameter, beyond which the pellet lot was considered as unacceptable for residential combustion, regardless of the total Quality Index score. The model structure allows for effective evaluation and ranking of grass pellet lots regardless of the specific values ultimately chosen for acceptable limits and working ranges by the industry. Applying the Quality Index to a range of grass pellet types resulted in a reasonable ranking of pellets based on physical characteristics and composition.

Rural North America has a tremendous capacity for energy production [1], [2] and [3]. While the ongoing desire for energy security is the primary driving force for alternative energy development in the USA, environmental issues will sooner or later overshadow energy supply issues. An ideal alternative solid biomass feedstock should be nearly carbon neutral, without significant net increase in atmospheric carbon dioxide. It also needs to be seen as a valuable crop from the farmer’s perspective [4] and [5]. The Northeastern USA has significant space heating demand, and most states in the region have state regulations controlling combustion emissions [6]. New York and the New England states represent approximately 80% of the total heating oil demand in the USA. The Northeast USA has millions of acres of abandoned and underutilized land suitable for grass biomass, without interfering with traditional agricultural crops and without the requirement for establishment [7]. Existing mixed grass stands are appropriate for grass biomass production if the energy conversion process is combustion. The use of such lands for this purpose makes this biomass option relatively immune from the indirect land use change debate [8]. Energy conversion efficiency can be very high with grass combustion [9]. Grass energy farming is a small-scale, low-technology, closed-loop energy system that will result in rural jobs and economic diversification, absorbing excess production capacity. In general, the desired feedstock for biomass combustion is opposite of that required for ruminant animal forage, with a range in composition among herbaceous plant species [10] and [11]. While grass breeding for biomass will eventually lead to compositional changes in the feedstock [12], [13] and [14], major compositional changes can be achieved in the near-term through agronomic and harvest management [15]. 1.1. Biomass composition and combustion Soil type and inherent soil fertility can strongly impact mineral uptake [16]. Plant uptake of potassium (K) and chloride (Cl) are generally well correlated, due to luxury uptake of both elements in excess of plant requirements, and the common practice of fertilizing with KCl [17]. Potassium concentration has been reported as low as 0.6 g kg−1 to as high as 70 g kg−1 in cool-season grasses on a dry matter basis [18]. Concentration of most elements in grass decreases with plant age, making mature plants more desirable for combustion from a compositional standpoint. Harvest management can have a major impact on grass composition, particularly for water soluble components such as K and Cl [19] and [20]. Delayed baling following mowing, as well as overwintering grass in the field, also will reduce insoluble nitrogen (N) and silica (Si) in grass feedstock due to the preferential loss of inflorescence and leaf blade which are higher in N and Si content than other plant parts [21] and [22]. Dry matter losses in storage also are possible [23]. Biomass quality issues have hindered commercialization of grass pellets in residential combustion systems [24]. The most serious quality issues with grass feedstock are generally considered to be the alkali and chloride content [17], [25], [26] and [27]. Boiler corrosion and fouling are directly related to alkali and chloride content. Particulate emissions primarily consist of aerosol-forming elements like potassium and chloride, as well as sulfur (S). Chloride also acts as a catalyst, facilitating the movement of iron away from metal surfaces and the deposition of inorganic compounds [28] and [29]. Potassium is the primary alkali element present in grasses, with typically very low concentration of sodium [20]. Release of K can be minimized by controlling combustion temperature, but this does not prevent the release of Cl [25] and [26]. Nitrogen content of grass has little impact on combustion efficiency but is undesirable from an environmental standpoint. Nitrogen oxides are the second most important contributor to global acidification from human activities [25]. There are concerns that N release to the atmosphere from biofuel production negates any green house gas reduction benefits [30]. Nitrogen emissions are positively correlated with feedstock N content. Sulfur and Si, in combination with alkali, lead to reactions associated with fouling and slagging in boilers [28] and [31]. In general, approximately one half of the total ash content of grass is silica. Silica, in combination with K and other elements, affect the ash melting behavior in grasses [32]. The total amount of ash impacts the design of ash handling and storage systems for a given appliance, but ash content per se is not a major drawback to grass combustion within certain limits. Some residential heating appliances currently available are able to handle ash content up to 10%. Of course, total ash content is likely to be positively correlated with concentrations of undesirable elements, particularly Si and K. High total ash and Si content result in lower energy content. Other ash forming elements such as P, Ca, Mg, Al, Fe, and Mn have less impact on combustion, or have a relatively small range in concentration in grasses. Elements such as As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, V, or Zn might be a cause for concern, and German and Austrian wood pellet ENplus quality standards include analysis for heavy metals [33]. Such elements are normally present in grasses in very small quantities, significantly lower amounts than found in wood [34]. This is one of the few advantages that grass biomass has over woody biomass, as relatively slow growing woody species have the potential to accumulate significant quantities of heavy metals such as mercury. One exception might be with grass grown on fields treated with industrial sewage sludge [35], a relatively rare occurrence in the Northeast USA due to restrictions on such applications. Ash melting behavior [32] is an important parameter, but determination of melting temperatures is not feasible for routine sample analysis. A contributor to elemental contents of grass that is difficult to assess is that of surface soil contamination. Over 100 lots of mature hay bales were sampled in New York in the fall of 2011, and ash content ranged from 38 to 212 g kg−1 (Cherney, JH, unpublished). Hay lots with high ash values also had corresponding very high Al, Fe, and Ti concentrations, indicative of soil contamination. Elemental components of a grass sample analysis originating from surface soil contamination will vary depending on soil type, plant digestion technique, and level of contamination [36]. Plant digestion analysis techniques only partially release elements bound in soil. Level of soil contamination is a function of how rough the soil surface is, soil moisture content, the particular type of mowing, raking and baling equipment used, and whether the grass is baled in the fall, or left standing or windrowed over winter. Soil contamination is also typically highly variable from bale to bale. The plant chemical components most impacted by surface soil contamination are silica and total ash. Soil contamination negatively affects gross energy value of the feedstock on a dry matter basis through dilution effects. 1.2. Quality evaluation Quality standards for grass pellets would encourage their sustainable production and consumption for space heating. A survey of bioenergy experts in the EU showed that a majority of respondents agreed that there was a lack of European standards for bioenergy production, trade, and development [37]. The majority of respondents also agreed that a European-level standard would help to develop a sustainable bioenergy trade and encourage public acceptance of biomass energy, and that certification of bioenergy was necessary. Certification of biomass provides added value through product differentiation, enhancing market competitiveness. A multi-criteria assessment model was used recently to rank biomass pellets for suitability for use in large heat and power generation plants [38]. Technical, environmental and economic factors were assigned weights and evaluated for the quantitative and qualitative criteria. While this model is useful for energy planning, it would have limited value for evaluating specific lots of pellets for compositional parameters. Physical characteristics are the primary basis for evaluating wood pellet quality in North America [39]. Properties included in the fuel quality grade specifications include fines, bulk density, diameter, length, pellet durability index, moisture content, ash content, and chloride content (Table 1). Concentration of chlorine is generally expressed as chloride ion. Fines are the percentage of fuel that pass through a screen. Screen size is stated by the specific standard method, it may vary with pellet diameter [40]. Bulk density is the fuel mass per unit volume of fuel, a uniform density ensures steady combustion behavior [41]. Pellet durability index (PDI) is a measure of the ability of fuels to resist degradation due to shipping and handling [40]. Of the available standardized pellet tumbling devices the device with the most accuracy and precision was the tumbler described by ASAE S269.4 [42]. Pellet durability can be evaluated immediately after pelleting (green strength) or after pellets have cured (cured strength). Particle density is included in European standards and is related to bulk density [24]. Fines are rarely mentioned in European literature. The goal of biomass densification is to produce a strong and durable product that will resist breakdown during handling, transportation and storage [43]. Feedstock composition has a major effect on densification, and is also impacted by particle size, feedstock conditioning and densification equipment variables. Herbaceous biomass, particularly overwintered warm-season grasses, can be particularly difficult to pellet consistently. Temperature and chemical composition influenced bonding of particles of wood or straw [44]. Compressive force, particle size and moisture content significantly affected pellet density in switchgrass [45]. Optimum densification conditions for switchgrass were achieved by preheating to greater than 75 °C at a moisture content of 8–15% [46]. Currently in Northeastern USA and Canada, grass pellets are being produced using a wide range of pelleting equipment, some of this equipment is not well suited to grass pelleting. As most equipment currently used for grass pelleting is either designed for pelleting wood or animal feed materials, modifications to allow for grass pelleting are done by trial and error, and result in a range in pellet physical quality. Physical characteristics therefore are an important component of overall grass pellet quality, although chemical composition remains the critical component. Grass pellets are being used on a limited scale as a combustion fuel in Europe and North America. The major issues facing grass pellets for residential combustion are unlike those of wood pellets. Grass is not an ideal combustion fuel, with elevated levels of several problematic elements. The potential range in composition of grass biomass is tremendous. Mature grass harvested in NY in 2010 ranged from 0.1 to 13.4 g kg−1 Cl on a dry basis, depending on species, fertilization, and harvest management (Cherney JH, unpublished). Residential scale combustion equipment is now being designed to specifically address the compositional issues of grass pellets. A residential pellet boiler burning reed canary grass was within acceptable European emissions limits for CO and NOx, and emitted approximately one half of the particulates as wood pellets [47] and [48]. Also, small scale appliances can be fitted with equipment for exhaust gas after-treatment if necessary [49]. Fuel indexes that characterize a combustion feedstock can form a good basis for evaluation of combustion-related problems [50]. Wood pellet standards have been applied to grass pellets [51], but this system is inadequate for non-wood pellet fuels. Without any system for evaluating and ranking the suitability of grass biomass for residential combustion, the variability in grass composition will seriously hinder the development of a residential grass pellet combustion industry. The objective of this research is to develop a Quality Index for grass pellet evaluation system that weights the relative contributions of major physical and compositional parameters to generate a single value for evaluation and ranking of pellet quality for combustion.

The biomass heating industry in North America is relatively immature compared to Europe, and additional quality standards would encourage grass biomass producers to generate more consistent fuel quality, and help to gain consumer confidence in biomass fuels. Some method of evaluating and ranking grass pellets for both their physical and combustion properties is essential if these pellets are to be considered for possible residential combustion use. Any index needs to be flexible enough to allow inclusion of new parameters and adjustment of their relative ranking, as we move towards a better understanding of herbaceous biomass combustion. The proposed system roughly approximates a crude expert system. That is, the model was adjusted based on available facts and expert opinion, until it generated results for a given set of pellet parameters that might be deemed reasonable by an expert. Selection of parameters to include and their optimum values were based on available literature. Selection of acceptable limits and working ranges for parameters are based on the estimated relative importance of parameters to the residential combustion process. Although the values chosen could be debated by a group of experts, the model structure itself should allow for effective evaluation and ranking of grass pellet lots regardless of the specific values ultimately selected for acceptable limits and working ranges. Eleven different lots of grass pellets were evaluated and the relative ranking based on the Quality Index appears reasonable. While much more elaborate systems are possible, the proposed system here is a relatively straightforward method of summing qualitatively different parameters into one index for evaluating and ranking grass pellets for residential combustion potential. An extension of existing quality labeling systems to grass pellets would encourage production of a more consistent fuel, and help to win the trust of industry and small combustion appliance owners.