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In this paper different gas-steam combined-cycles fueled by syngas produced in a local downdraft gasifier, are analyzed. At first, the downdraft gasifier model is briefly described, where waste biomass is transformed into syngas, which can be used more efficiently than the original solid biomass to generate useful power, and can be transported much more easily. The gasifier model is able to estimate, with good approximation, the composition of the produced syngas, taking separately into account the biomass drying and the pyrolysis, oxidation and reduction processes. The gasifier operates at ambient pressure using air as gasification agent and biomass as input. Among others, pomace has been considered, since, in Italy (where the plant is supposed to be located) there are many regions, like Apulia, where this biomass is largely available. Three different plant configurations have been proposed and compared in terms of overall performance. The first two, named REXC (Regenerative cycle with EXternal Combustor) and CR (Conventional Regenerative cycle), burn the syngas in an internal combustor, whereas the third one, named SyEXC (Syngas External Combustion), considers an externally fired configuration for the syngas combustion. In the REXC cycle, a secondary external combustion system, fed by cellulosic biomass, is connected to a heat exchanger in order to increase the air temperature, as in a regenerative cycle. The combustion products pass through a primary heat exchanger placed in the external combustion system, heating the compressed air, which flows into the primary internal combustion chamber, where a defined quantity of syngas reacts with the compressed air. The turbine exhaust gas (TEG), before going back into the external combustion, partially transfers its enthalpy content to a Heat Recovery Steam Generator (HRSG) of a bottoming Rankine cycle. The second plant configuration (CR cycle) is characterized by a main internal combustor fueled by the produced syngas and implements a conventional regenerative process. The TEG is reheated by an externally fired combustion before flowing in the HRSG of a bottoming Rankine cycle. The last plant configuration (SyEXC) differs from the CR one only for the main externally fired gas turbine fueled by syngas, thus avoiding any costly cleanup operation and cooling.

The paper presents a mixed integer linear programming (MILP) approach to optimize multi-biomass and natural gas supply chain strategic design for heat and power generation in urban areas. The focus is on spatial and temporal allocation of biomass supply, storage, processing, transport and energy conversion (heat and CHP) to match the heat demand of residential end users. The main aim lies on the representation of the relationships between the biomass processing and biofuel energy conversion steps, and on the trade-offs between centralized district heating plants and local heat generation systems. After a description of state of the art and research trends in urban energy systems and bioenergy modelling, an application of the methodology to a generic case study is proposed. With the assumed techno-economic parameters, biomass based thermal energy generation results competitive with natural gas, while district heating network results the main option for urban areas with high thermal energy demand density. Potential further applications of this model are also described, together with main barriers for development of bioenergy routes for urban areas.

Abstract This paper compares different operating strategies for small scale (100 kWe) combined heat and power (CHP) plants fired by natural gas and solid biomass to serve a residential energy demand. The focus is on a dual fuel micro gas turbine (MGT) cycle. Various biomass/natural gas energy input ratios are modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions. On the basis of the results from thermodynamic assessment and simulation at partial load operation, CAPEX and OPEX estimates, and Italian energy policy scenario (incentives available for biomass electricity, on-site and high efficiency CHP), the maximum global energy efficiency, primary energy savings and investment profitability is found, as a function of biomass/natural gas ratio, plant operating strategy and energy demand typology. The thermal and electric conversion efficiency ranged respectively between 46 and 38% and 30 and 19% for the natural gas and biomass fired case studies. The IRR of the investment was highly influenced by the load/CHP thermal power ratio and by the operation mode. The availability of high heat demand levels was also a key factor, to avoid wasted cogenerated heat and maximize CHP sales revenues. BL operation presented the highest profitability because of the higher revenues from electricity sales. Climate area was another important factor, mainly in case of low load/CHP ratios. Moreover, at low load/CHP power ratio and for the BL operation mode, the dual fuel option presented the highest profitability. This is due to the lower cost of biomass fuel in comparison to natural gas and the high subsidies available for biomass electricity by feed-in tariffs. The results show that dual fuel MT can be an interesting option to increase efficiencies, flexibility and plant reliability at low cost in comparison to only biomass systems, facilitating an integration of renewable and fossil fuel systems.

The paper presents the application of a mixed integer linear programming (MILP) methodology to optimize multi-biomass and natural gas supply chain strategic design for heat and power generation in urban areas. The focus is on spatial and temporal allocation of biomass supply, storage, processing, transport and energy conversion (heat and CHP) to match the heat demand of residential end users. The main aim lies on the assessment of the trade-offs between centralized district heating plants and local heat generation systems, and on the decoupling of the biomass processing and biofuel energy conversion steps. After a brief description of the methodology, which is presented in detail in Part I of the research, an application to a generic urban area is proposed. Moreover, the influence of energy demand typologies (urban areas energy density, heat consumption patterns, buildings energy efficiency levels, baseline energy costs and available infrastructures) and specific constraints of urban areas (transport logistics, air emission levels, space availability) on the selection of optimal bioenergy pathways for heat and power is assessed, by means of sensitivity analysis. On the basis of these results, broad considerations about the key factors influencing the use of bioenergy into urban energy systems are proposed. Potential further applications of this model are also described, together with main barriers for development of bioenergy routes for urban areas.

The focus of this paper is on the part load performance of a small scale (100 kWe) combined heat and power (CHP) plant fired by natural gas (NG) and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative microgas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with NG takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for preheating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant. Different biomass/NG energy input ratios are also modeled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate and (ii) higher primary energy savings (PESs) and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD), and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average, and mild climate conditions.

The proposed work aims to assess the energy crop suitability of Puglia Region (Southern Italy), the techno-economic feasibility of small scale CHP plants fired by energy crops, and the environmental performances of the proposed CHP plants. In the first part of the work, a GIS model is defined and applied to evaluate the land suitability for energy crops in the Puglia region. In the second part, a financial appraisal of small scale CHP plants under Italian legislative framework (feed in tariffs) is proposed; the two case studies of bio-oil fired internal combustion engine (ICE) coupled to vegetable oil mill plant and fed by oil seeds (brassica carinata seeds) and syngas fired engine coupled to a pellet production unit and fed by herbaceous energy crops bales (fibre sorghum) are investigated. In the third part, the energy balance and the CO2 emissions of the whole bioenergy routes are assessed, in order to calculate the costs for the community (in terms of subsidies) to save a tons oil equivalent (TOE) of primary energy and to avoid a tCO2 in the atmosphere by these small scale routes. The results report a potential in Puglia Region of about 293 and 729 kt y−1 of brassica carinata seeds and fibre sorghum bales, respectively; the financial appraisal of the proposed chains, under the Italian legislative framework, reports an internal rate of return (IRR) of 38% and 17%, respectively, while the energy balance assessment reports an overall efficiency of the bioenergy routes of 2.72 and 2.95, respectively.

This paper describes ESCO approaches and business models for biomass heating and CHP generation. State of the art, policy measures and main barriers towards the implementation of such ESCO operations in Italy are discussed. Moreover, on the basis of the proposed framework, representative case studies in the Italian residential, tertiary and industrial market segments are compared. The case studies are referred to a 6 MWt wood chips fired plant. The case study of the industrial sector is based on a constant heat demand of a dairy firm, while in the tertiary and residential sectors the options to serve a concentrated heat demand (hospital) and a community housing by a district heating network are explored. The further option of coupling an organic Rankine cycle (ORC) for CHP is explored. The relevance of the research relies on the assessment of the main key factors towards the development of biomass-ESCO operations. The results of the techno-economic assessment show that the agro-industrial case study for heat generation is extremely profitable, because of the high baseline energy cost, the high load rate, the availability of incentives for biomass heating. The cogeneration option is also profitable, even if the higher investment cost determines a longer pay back time. The tertiary sector case study is also a profitable, for the presence of a concentrated load with high heat load rate and high energy cost. Finally, the residential sector case study is the least profitable, for the high district heating cost and the lower heat load rate, not compensated by the higher heat selling price. The higher investment cost of CHP, even if attracting further income from electricity sale, does not present higher profitability than the only heat generation plant. In addition, the heat load rate results a more influencing factor than the thermal energy selling price.

How large is the potential demand for bio-heat in the UK? Whilst most research has focused on the supply of biomass for energy production, an understanding of the potential demand is crucial to the uptake of heat from bioenergy. We have designed a systematic framework utilising market segmentation techniques to assess the potential demand for biomass heat in the UK. First, the heat market is divided into relevant segments, characterised in terms of their final energy consumption, technological and fuel supply options. Second, the key technical, economic and organisational factors that affect the uptake of bioenergy in each heat segment are identified, classified and then analysed to reveal which could be strong barriers, which could be surmounted easily, and for which bioenergy heat represents an improvement compared to alternatives. The defined framework is applied to the UK residential sector. We identify provisionally the most promising market segments for bioenergy heat, and their current levels of energy demand. We find that, depending on the assumptions, the present potential demand for bio-heat in the UK residential sector ranges between 3% (conservative estimate) and 31% (optimistic estimate) of the total energy consumed in the heat market.

Small-scale combined heat and power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on-site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small-scale micro-gas turbine for heat and power cogeneration, a bottoming organic Rankine cycle (ORC) system can be coupled to the cycle, however, this option reduces the temperature and the amount of cogenerated heat available to the thermal load. In this perspective, the paper presents the results of a thermo-economic analysis of small-scale CHP plants composed of a micro-gas turbine (MGT) and a bottoming ORC, serving a typical residential energy demand. For the topping cycle, three different configurations are examined: (1) a simple recuperative micro-gas turbine fueled by natural gas (NG); (2) a dual fuel externally fired gas turbine (EFGT) cycle, fueled by biomass and natural gas (50% share of energy input) (DF); and (3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming ORC is a simple saturated cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild, and hot climate conditions) and CHP plant operational strategies (baseload versus heat-driven versus electricity-driven operation mode) on the global energy efficiency and profitability of the following three configurations: (A) MGT with cogeneration; (B) MGT+ ORC without cogeneration; and (C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following trade-offs: (i) lower energy conversion efficiency and higher investment cost of biomass input with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration with the bottoming ORC; and (iii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.