Comparative Technology | Economic Evaluation Of Four Biomass To Electricity Systems

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The following article analyses the different techniques involved in producing electricity from biomass systems. This article will outline 4 different biomass systems using 4 different scenarios for each system to calculate and determine the overall efficiency, specific capital cost, and the cost of electricity. Each scenario will be assessed for the individual systems over a specified range of rated power which will be 2.5MWe to 25MWe. PyrEng, GasEng, IGCC and Combust are the four different technologies outlined within this report.

Biomass systems are an alternative to conventional fossil fuels as energy sources due to the increasing scarcity of fossil fuels as well as being less environmentally damaging. With the introduction of pellets in the early 2000’s which have high energy densities and can be easily fed within heating systems, increasing government regulation and legislation requiring companies to reduce and even meet targets on greenhouse gas emissions coupled with increasing knowledge and information affecting public opinions on companies all leading to better Press for companies who are opting to ‘go green’ this which has led to a large increase in the popularity, use and development of biomass systems by companies as a viable long term option over fossil fuels as energy sources (ATech Electronics, 2019). For example in 2019 there are around 3800 active biomass power plants worldwide with an expected 5600 commissioned by the end of 2027 (ecoprog, 2019).

PyrEng: is the fast pyrolysis (fluidised bed) with compression-ignition engine, including intermediate liquids storage.
GasEng: atmospheric gasification (fluidised bed) with spark ignition engine, including Tar cracker and gas clean up.
Integrated Gasification Combined Cycle or IGCC: Pressurized gasification (fluidised bed) with gas turbine combined cycle, including hot as clean up.
Combust: Combustion (moving grate) with boiler and Rankine cycle.

Methodology

The overall efficiency is the annual net electricity delivered from the plant to the grid

Measured as a percentage and can be calculated using Equation 1.11 below:

$\frac{N e t p o w e r o u t p u t f r o m p l a n t (G J y^{- 1})}{T o t a l f u e l i n p u t t o t h e p l a n t (G J y^{- 1})} = O v e r a l l e f f i c i e n y (%)$

Equation 1.11 Equation to determine the overall efficiency of the plant used for each biomass system.

The specified range of rated power is split into 10 ranges. Rated power can be defined as the net power exported by the power plant if it is running at full load.

For each of the rated power ranges an initial biomass wet feed (m_B) is assumed which then allows the chemical energy in (E_c) to be calculated using the Heating value (H), which is of the biomass as shown in equation 1.12:

$E_{c} = m_{B} . H$

Where:

E_c = Chemical Energy in (GJ y^-1),

M_B= Initial Biomass wet feed (tonnes/ year),

H = Heating Value of biomass (GJ/ Tonne)

Equation 1.12 Equation to calculate the chemical energy in of the plant for each biomass system.

Once the chemical energy in has been determined the electrical energy out (E_E) can be calculated using the chemical energy in (E_c) and the overall electrical efficiency (_e) as shown in equation 1.13 below:

$E_{E} = E_{c} . η_{e}$

Where:

E_E = Electrical Energy out (GJ y^-1),

E_C= Chemical Energy in (GJ y^-1),

_e = Heating Value of biomass (GJ/ Tonne)

Equation 1.13 Equation to calculate the electrical energy out of the plant for each biomass system.

Conversion Efficiency is defined as efficiency in converting the raw biomass material energy to energy within the intermediary product for example with the PyrEng the energy this means the pyrolyser to the bio oil intermediary bio oil liquid storage. This Is calculated by using a function of the given values.

Generation efficiency is defined as the efficiency with which the conversion of energy within the intermediary product is converted into electrical energy which can be used and transported to the grid. This is also calculated by using a function of the given values.

Rated power (P_E) for each scenario and system can then be worked out using the electrical energy out (E_E) as seen in equation 1.14 below:

$P_{E} = C . E_{E} / f$

Where:

P_E = Rated Power (MWe),

C = Units constant,

E_E = Electrical Energy out (GJ y^-1),

f = Capacity Factor

Equation 1.14 Equation to calculate the Rated power for each biomass system.

Capacity factor is calculated using equation 1.15:

$\frac{A c t u a l a m o u n t o f e n e r g y o u t p e r y e a r}{A m o u n t o f e n e r g y o u r p e r y e a r i f p o w e r s t a t i o n r a n c o n t i n o u s l y a t r a t e d p o w e r} = C a p a c i t y F a c t o r (f)$

Equation 1.15 Equation to calculate capacity factor used in the calculation for equation 1.14.

Once the above has been worked out for each biomass systems and each of its 4 scenarios the economic measures can be calculated.

The specific capital costs (SCC) consist of four different elements known as preparation, drying, conversion and generation.

SCC can be worked out using equation 1.16 shown below:

$\frac{T o t a l C a p i t a l C o s t s (£)}{R a t e d P o w e r (k W e)} = S p e c i f i c C a p i t a l C os t s (£ / k W e)$

Equation 1.16 equation to calculate the specific capital costs at each rated power range for each biomass system.

Cost of Electricity (CoE) is then worked out following the determination of the specific capital costs (SCC) and can be calculated using the equations 1.17:

$\frac{\sum_{a l l s y s t e m o p e r a t i n g \cos t s p e r a n n u m (£)}}{a m o u n t o f e l e c t r i c i t y e x p o r t e d p e r a n n u m (£)} = \cos t o f e l e c t r i c i t y (£ / k W h)$

Equation 1.17 equation to determine the cost of electricity at each rated power range for each biomass system.

The total of all system annual operating costs consist of seven elements known as feed production, feed transport, labour, utilities, overheads, maintenance, and annual cost of capital.

Feed preparation includes the reception, handling, screening, grinding and storage.

Overheads are a percentage of the total capital cost.

Feed production is calculated using equation 1.18 below:

Price per tonne of Dry feed (£k) x annual supply (kt/year) = Feed production cost (£/kt/year)

Equation 1.18 equation to calculate the feed production costs per year.

When calculating the feed production cost, it is done on a dry basis. The feed rate is given as a wet basis. Wet basis is defined as the mass of moisture divided by the total mass on the product. To gain the dry basis mass the wet basis mass is divided by 2 as an assumption is made that the product is 50% wet basis.

Annual capital costs are the annual cost equivalent spread over the lifetime of the process. As stated above annual cost of capital (ACC) is used in the calculation to determine the sum of annual operating costs and can be evaluated using equation 1.19:

$\frac{C . r . {(1 + r)}^{n}}{{(1 + r)}^{n} - 1} = A n n u a l c a p i t a l \cos t s (£ k / y)$

Where:

C = Total capital cost (£k),

r = interest rate (%),

Equation 1.19 equation to calculate the annual capital costs included in the operating costs per annum outlined in equation 1.17.

Learning factors are the reflection in the learning process of new plants and can be defined as the amount capital cost reduces every time the number of plants in operation doubles.

The learning factors can be factored into calculation using equation 1.20 below:

$C_{n} = C_{m} . {(1 - l)}^{\frac{\ln (n) - \ln (m)}{\ln (2)}}$

where:

C_n = Capital cost for n amount of plants,

C_m = Capital cost for m amount of plants,

l = learning factor

Equation 1.20 equation to calculate the new capital cost including the learning factor.

Wherever a biomass system is described as ‘mature’ the learning factor, l, is set to 0.

Data Analysis and Discussion

Graph 1.11 Graph to show the overall efficiency compared to the rated power.

Graph 1.11 above compares the overall efficiency (%) between all four systems over the rated power range (MWe).

IGCC is the most efficient of all the biomass systems over all the rated power ranges. For example, at a rated Power range of 5 MWe the IGCC had an overall efficiency of 0.36% compared to the second-highest of 0.28% for GasEng followed by PyrEng at 0.25% and lastly the Combust at 0.18%. This trend does not change over the rated power range.

The IGCC system works by utilizing gasification followed by the use of gas clean-up and lastly a gas and steam turbine to create a combined system to allow for a more environmentally friendly, efficient and clean way of creating electricity especially compared to classic combustion systems as seen in graph 1.11 above (Wang, 2016).

Gasification is the process in which carbon-based material such as biomass is turned into fuel/ energy without the use of combustion. This is done by incomplete combustion of the biomass feed in an oxygen-deficient area. This leads to less heat being released as compared to a combustion method as gasification packs energy into the bonds whereas combustion releases the energy from the bonds by oxidizing the feed giving off heat (Basu, 2010).

In the gasification process, syngas and ash amongst other intermediary products are converted into a gas.

Once the Gasification process is done gas is cleaned up by removing contaminants including CO_2, if carbon capture is used before the fuel is combusted, in the gas turbine phase meaning there is greater heating value in the combustion of the fuel as compared to combustion biomass systems. Moreover, it also leads to less harmful and polluting gases such as mercury and sulphur being combusted with sulphur being one of the lead causes of acid rain. Due to IGCC plants being run at high pressure the efficiency of the removal of the contaminants increases with the capital costs being reduced due to the lower volumetric flow rate of the pre combusted fuel as compared to cleaning the gas at ambient temperature post combustion at a higher volumetric flow rate. This also contains more gases as compared to just fuel which is pre combusted (Wang, 2016).

Figure 1.11 Diagram demonstrating the IGCC biomass system (Mitsubishi Hitachi Power Systems, 2019).

Combust is the oldest of the four systems and the least efficient. Combustion systems such as the Rankine System work by utilizing an external heat source such as the combustion of biomass material to provide heat to a closed system containing an operating fluid.

A pump is used to pump high pressurized operating liquid into a boiler where it is heated using the external heat source, in this case combusted biomass material, where the operating liquid undergoes Isobaric heat transfer reaching its saturation temperature and further heated until it evaporates and is fully converted into saturated steam.

This saturated steam then travels to the turbine upstream of the boiler within the closed system and here it undergoes isentropic expansion. The saturated steam here expands, working on the surroundings producing electricity by spinning the turbine. This expansion, however, is limited by factors such as the temperature of the cooling medium, erosion of the turbine blades and the liquid entrainment when the medium reaches a two-phase region which all affect its efficiency.

This vapour liquid mixture leaves the turbine undergoing isobaric heat rejection in which the vapour liquid mixture enters a surface condenser. Here it is cooled and condensed using a cooling medium such as cooling water.

This condensed liquid is then recycled and sent back to the pump at the beginning where it begins the cycle once again (Muller-Steinhagen, 2011). NOx exhaust must be controlled more in combust than other systems.

There are though, inefficiencies in the combust process such as the inefficiencies of the boiler. The boiler does not convert 100% of the fuel energy into steam energy due to the high temperature difference between the combusting fuel and the vapour temperature. This high-temperature difference leads to greater entropy leading to greater energy dispersion which, compared to the IGCC system which does not fully combust its fuel, has less energy lost between the gas turbine and the incompletely combusted fuel (Muller-Steinhagen, 2011).

Moreover, there are losses of energy through the turbine due to a number of reasons such as erosion of the turbine blades and as the steam cools vapour entrainment occurs (Muller-Steinhagen, 2011). Vapour entrainment is when liquid droplets are trapped within vapour. This can cause mechanical damage to turbine blades as well as reducing the efficiency of separation in the evaporation stage (R K BAGUL, 2013). Furthermore, the build-up of dirt/fouling reduces the efficiency of the condensing within the condenser (Muller-Steinhagen, 2011).

Figure 1.12 Diagram to show the Rankine Cycle and example of combust (Muller-Steinhagen, 2011).

Fast Pyrolysis is used in the PyrEng biomass system. Fast pyrolysis involves the thermal decomposition of carbon-based material in this case biomass in the absence of oxygen at low pressure and around 500^oC. This results in the biomass being vaporized leaving a residue of char and ash. This fast pyrolysis in the PyrEng system produces bio oil as an intermediary liquid which can be compressed and ignited as it is an energy-dense liquid feedstock that can be utilized within a compression engine and used as a fuel to produce electricity (A.V. Bridgwater, 2002).

The bio-oil is created by first drying the feedstock making sure most of the moisture is released. It is then heated to further release moisture and some gas and char. This is done around 100 – 300^oC. The biomass is then cooled to form ash, char, permanent gases and the desired bio-oil intermediary liquid. The remaining vapours are further vaporized at a temperature above 600^oC to become secondary char and more permanent gases.

However, bio-oil if stored for too long can start to separate out into different phases. Moreover, it can become increasingly thick leading to complications in use as a fuel for compression engines. To combat the separation bio-oil must be kept in storage conditions with less than 30% water content to stop the bio-oil mixture from separating from an aqueous phase and a gummy-like phase as seen in figure 1.13. Furthermore, bio-oil can age leading to a change in properties reducing usability. To protect against ageing, it is important to keep low temperatures and reduce sheer stress and both can lead to accelerated ageing. This leads to storage, handling and transportation differences compared to other biomass systems.