Overview of Biomass Conversion and Generation Technologies

Mathias Loeser

University of Bath, UK m.loeser@bath.ac.uk

Miles Alexander RedfernUniversity of Bath, UK m.a.redfern@bath.ac.uk

Abstract-- The total energy stored in terrestrial biomass outnumbers the annual world energy consumption by a factor of more than fifty. Being highly available, renewable and geographically dispersed, biomass can form a substantial part of future energy sources and biomass-derived energy generation can result in both CO2-neutral and stable long-term energy supply for most areas in the world. Having a relatively low energy density, biomass processing in decentralised plants seems best suited to minimise transport cost of both the raw material and the products. To facilitate a wide-spread use of decentralised plants, their design has to be simple and they need to be easy-to-operate and flexible.

This paper covers the two sequential steps of biomass power: conversion technologies to transform the raw feedstock into suitable intermediate energy carriers, and generation technologies to gain energy in the form of heat and/or electric power. A broad number of conversion technologies currently exist for both wet and dry biomass, ranging from research-stage up to commercialisation.

In this paper the main ways of converting dry as well as wet feedstock will be discussed: combustion, gasification, pyrolysis and liquefaction for the further and fermentation and anaerobic digestion for the latter. Additionally, the common generation technologies will be analysed: internal combustion engines, stirling engines and internally- and externally fired microturbines.

Finally it will be recommended which technologies to use to meet a substantial part of the future energy demand on the basis of biomass in micro- or small-scale applications.

Keywords-- biomass, micro-scale applications, decentralised generation, stand-alone-systems

I. INTRODUCTION

Biomass, being defined as all organic matter such as wood and wood waste, agricultural residues and farming manure [1, 2], is one of the most wide-spread energy resources worldwide. Its high availability and dispersed location enable it to be used for decentralised power generation. Due to being renewable, a long-term energy supply on the basis of biomass can emerge. While its low energy density could be seen as a potential barrier for implementation, when using biomass in small- and micro-scale applications these shortfalls can be overcome and it can even substitute grid connection for remotely located customers with sufficient amounts of feedstock on site [3].

This paper discusses common biomass process technologies to be able to evaluate those worth for employment in micro-scale plants. Therefore, a short technology review is followed by a relative comparison and performance ranking to finally conclude with a recommendation about which technologies to use.

II. CONVERSION TECHNOLOGIES

Biomass in general is divided into wet and dry feedstock, the first with a moisture content of significantly less than 50%, and the latter with up to more than 90% for animal manures [4]. Wet biomass is normally treated biochemically, whereas dry biomass is processed thermochemically. In both ways, an intermediate fuel is produced to be used for generation purposes. Extensive and detailed conversion technology descriptions can be found in literature, thus this section will only provide a compact overview of suitable conversion technologies, divided into thermochemical and biochemical.

A. Thermochemical conversion Four main conversion technologies have emerged for

treating dry biomass: combustion to immediately release its thermal energy and gasification, pyrolysis and liquefaction to produce an intermediate liquid or gaseous energy carrier. Aside from the low efficiency of common combustion equipment, its immediate energy release results in low flexibility [5-7], so it is less suited for flexibly running energy systems.

Instead, gasification as the substochiometrical oxidation of biomass with air or steam as gasification agents seems more promising [2, 7]. Several reactor designs from simple fixed bed to fluidized bed or entrained flow reactors have been investigated and several commercial applications exist [8]. The process temperature level of around 800-1000°C can be achieved by combusting part of the feedstock or by applying internal process heat cycles [2, 6]. The main product of gasification is producer gas with a calorific value of 4-6MJ/Nm3 using air and/or steam. It consists mainly of CO, H2, CH4 and CO2 and can be stored to be used when needed [9].

Pyrolysis is the heating of biomass in the absence of oxygen and results in char, bio-oil and pyrolysis gas in varying yields, depending on a range of parameters such as

heating rate, temperature level, particle size and retention time [2, 7, 10]. The advantage of being able to receive a ‘tailor-made’ product range competes with the disadvantage of lower yields due to the normally lower temperature range of around 300-500°C [6]. Fast and flash pyrolysis achieve higher yields, however their requirements regarding heating rate and particle pre-treatment are advanced. In general, pyrolysis has not reached full commercial status yet and further needs for development are mentioned [2, 7].

Liquefaction is the low-temperature cracking of biomass molecules due to high pressure and results in a liquid diluted fuel. The advantage of this process, employing only low temperatures of around 200-400°C, has to compete with comparably low yields and extensive equipment prerequisites to provide the pressure levels needed (50-200bar) [6, 11]. Therefore, current interest in liquefaction is low and it is regarded as the least developed conversion technology [6, 11].

B. Biochemical conversion Wet biomass can be processed in two main ways: by

fermenting the feedstock, using yeasts to convert the contained sugar into ethanol. This produces a diluted alcohol which then needs to be distilled and thus suffers from a lower overall process performance and high plant cost [12, 13].

In comparison to that, anaerobic digestion (AD) employs bacteria to transform the organic matter into gaseous products. It shows better economics and numerous applications are operating [14, 15]. The biogas produced has a calorific value of around 20-25MJ/Nm3 [16] with a methane content varying between 45-75% and the remainder of CO2, the ratio depending on a number of factors such as retention time, the digester pH value and the temperature level, to name but a few [13, 17]. Three process temperature levels exist: around 15°C for psychrophilic, 35°C for mesophilic and 55°C for thermophilic bacteria. Those low temperatures and the comparably simple digester design, basically a plug-flow or steady-flow stirred tank, are further advantages of AD when compared to fermentation.

III. GENERATION TECHNOLOGY

The intermediate fuel produced in the conversion section of a biomass plant will be used to supply electrical energy in a generation engine. Four main types of engines can be used for the desired size range: internal combustion engines (ICE) and microturbines (MT) as fuel-fired technologies, as well as stirling engines and externally fired microturbines (EFGT) as indirectly fired engines, employing high temperature heat exchangers between the combustion chamber and the working medium. Fuel cells are still in an early stage of development and due to their very high cost seem to not be a viable option [18].

Important parameters for choosing an engine for a biomass plant are its electrical efficiency, as well as maintenance

efforts and investment costs. Especially for remotely located applications, robust and durable components are needed to maximise availability and minimise repair and maintenance time. Additionally, when desiring a stand-alone application, the generation part of the plant needs to have a fast response performance for varying loads.

In general, ICEs have the highest nominal (at full power) electrical efficiency of the engines covered in this paper, with around 30-40% [19-21], closely followed by microturbines which lay in the range of 25-35% [18, 21]. Stirling engines and EFGT applications, using external combustion and heat exchangers, can only provide lower efficiencies of around 20-25% [18, 21-23].

Especially when running in stand-alone mode, part-load behaviour becomes a key indicator for an engine’s suitability: the optimal engine should immediately correspond to load changes and remain a high efficiency. Several experiments on part-load behaviour have shown that ICE again provide a slightly better part load efficiency than microturbines [18, 24]. In comparison to that, stirling engine and EFGT part load efficiency seems to decrease more significantly, although only few results have been published so far [22, 25, 26].

Discussing maintenance efforts and interval cycles, it can be stated that microturbines and stirling engines are significantly easier to maintain. Both technologies can run up to 10,000-15,000hrs continuously and normally need only one day of maintenance per year [22, 27, 28]. In comparison to that, conventional reciprocal engines need significantly more maintenance, and especially in bio-applications their oil lubrication suffers from the solubility of H2S and they require frequent oil changes of up to every 500 hours of operation [18, 27, 28], which strongly limits their suitability for remote areas without skilled personnel.

Discussing investment costs of the different engines, a clear tendency towards ICE can be found, followed by microturbines and EFGT. Stirling engines are still significantly more expensive [18, 21]. However, it should be taken into consideration that the latter three are still relatively new technologies which cannot provide the economies of scales of several decades of ICE manufacturing yet, and that, at least for microturbines, the market just recently started to become more mature and decreasing prices seem likely.

The findings of the above discussion are summarised in Table I in a comparative way. This ranking shows a tendency towards microturbines, despite their higher investment cost and because of their significantly better maintenance and operation behaviour.

Additionally, more efficient heat cycles and by that a higher total plant efficiency can be implemented when using microturbines with their high exhaust gas temperature levels of around 300-500°C, compared to only around 100°C for reciprocating engines [16, 18, 26, 29].

TABLE I GENERATION TECHNOLOGY COMPARATIVE RANKING

Category Technology

Stirling EFGT MT ICE

Full efficiency -- -- + +++

Part efficiency -- -- + +++

Load flexibility - - ++ ++

Investment cost --- -- - +++

Maintenance efforts ++ ++ +++ ---

Emission levels ++ ++ +++ ---

Level of development + ++ ++ +++

IV. CURRENT PLANT DESIGNS

Before suggesting a small-scale plant design being able to cope with the stand-alone requirements, this chapter covers current market deployment of biomass energy plants and shows which applications are available at the desired scale.

For the treatment of dry biomass, predominantly gasification-based plants are employed. Most are connected to ICE [30, 31], whereas fewer use MT technology [31, 32] or stirling engines [3, 31]. Gasifier reactor designs are predominantly atmospheric co- and counter-current fixed bed, and only very rarely fluidised bed or pressurised reactors [30, 32, 33]. Broad varieties of feedstock in terms of size and moisture content have successfully been processed, however varying gas qualities and producer gas tar amounts are mentioned as problems causing downtime.

The producer gas is normally not stored, but directly burnt in the encompassing engine to generate electricity. The use of combined heat and power (CHP) designs is common, which means that the process heat is used for hot water supply, sometimes also to provide heating within the process; the electricity generated is regarded as a surplus and will either be used or exported to the grid. Most plants are thus run in a heat-driven operation mode, its output depending on the heat demand of the customer and the electricity needs supplied by either the plant or the grid [34].

For wet biomass treatment, AD plants are dominating the market. They are predominantly used on farms to process cattle, pig or chicken manure or vegetable wastes. Most plants are in the range of 50-200kWe and employ mesophilic temperature ranges with rather long retention times of around 15-20 days [14, 27]. Some advanced reactor designs are used as well, including filter technology [35] or a combination of thermophilic and mesophilic reactors [15, 36]. The generation engines coupled with the digester are predominantly ICE and microturbines, and similar to gasification plants, they are mostly run on steady state grid-connected mode to mainly supply heat to the farms and to cover their electricity demand with grid support.

V. CONCLUSION AND OUTLOOK

Gasification and anaerobic digestion as biomass conversion technologies seem probable for the intended scale of 5-50kWe. When considering potential customers for those small-scale applications, a major focus should be laid on farms. They typically can provide large amounts of waste which can be used as biomass feedstock, and due to their often remote location the development of grid-independent energy supply becomes a viable alternative to save installation and maintenance cost incurring with a grid connection.

A farm normally undertakes both livestock management and plant cultivation, so both wet and dry feedstock will be available. Thus a combination of thermochemical and biochemical treatment to be able to use both feedstocks is a way to significantly increase the fuel output. Additionally, designs including both high- and low-temperature conversion processes allow a more efficient internal handling of process heat and by that increase the total plant efficiency.

Our hybrid plant proposal, consisting of a co-current fixed-bed gasifier and a thermophilic anaerobic digester, will best be able to provide an efficient waste management system for the customer and to produce considerable amounts of biogas and producer gas. These gases are then used to provide electricity by employing a microturbine, which has been chosen due to its advantages of long maintenance cycles and good response to load changes. Additionally, a microturbine can be operated autonomously by remote or preset control and thus shows distinct advantages for deployment in remote areas over long periods of time.

The proposed plant will be required to continuously cover the electrical load demand of the customer, which is the electricity need of farm houses and adjacent buildings. As a result, a highly transient and fluctuating load demand curve is expected.

Existing plants are designed to operate as base load applications with a grid connection used to import or export the difference between the consumption and the production of power. Stand-alone or island applications employ large batteries to instantly cover the load change and to allow sufficient time to change the generation output of the engine, however resulting in high costs and losses.

Our proposed plant will be designed to run on a comparably steady load which will always cover the load demand and hence result in a surplus of electricity generation. An electricity sink in the form of an electric feedstock heater will be used, which in fact will result in better conversion efficiencies due to drier feedstock. Thus more intermediate fuel gas will be produced, which can then be stored. This design seems to be more effective than trying to mirror the load demand with the generation output and covering the transition interval with electricity storage. Additionally, this system control will allow the generation part of the plant to run on higher load levels and thus significantly higher efficiencies.

In general, heat demands of the customer are significantly lower than the process heat output. Most CHP applications thus try to enhance the use of excessive process heat by supplying hot water to the customer, however times of high electricity demand do not always cohere with times of high heat demand and thus not all heat can be used productively. Therefore, we aim for a high level of internal usage of the process heat.

Our hybrid plant proposal has been found to be a promising project to supply electricity to remote customers and overcome grid-dependency in the long term.

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