Technical, economic, and environmental feasibility of rice hull ash from electricity generation as a mineral additive to concrete

Economic feasibility assessment methods

In this study, the cost and environmental performance of RHA used as an SCM were examined based on the life cycle perspective. The life cycle of RHA used in concrete and the system boundary of this study are shown in Fig. 2.

Figure 2
figure 2

Life cycle of rice hull ash generation.

Rice hulls are the hard husks that cover rice kernels, and as such their availability is a function of annual rice production, which is driven by demand for food resources. Whole rice is harvested and transported to rice milling facilities where rice hulls are separated from the rice kernel. Rice hulls comprise approximately 20% of whole harvested rice by mass39. Annual estimates for rice hull generation in California are shown in Table 5 based on the assumptions above.

Table 5 Rice hull availability in California40.

Currently, about half of the rice hulls generated in California are used at the Wadham facility in Williams, California, to generate electricity41. The Wadham facility has 29.1 MW of maximum plant capacity. It generates about 200,000 MWh annually by combusting rice hulls at 857 °C, selling the electricity to Pacific Gas and Electric18.42. For the economic assessment of energy generation, the Wadham facility was used as a reference case.

In this study, there are two potential revenues streams from the power plant: net-generated electricity and RHA. To analyze the economic feasibility of the process, the price of electricity was calculated and then compared to the average retail price of electricity in California. By comparing this, the required revenue from RHA was determined. The price calculation method is described in Eq. (1). The average retail price of electricity was assumed at $0.15/kWh in California29.

$$Pric{e}_{target}=\frac{Required\, revenue{e}_{levelized\, annual}}{Total\, production},$$


$$Required \,revenu{e}_{levelized\, annual}=\left({\sum }_{year=1}^{Lifetime}\frac{{\left(Required\, revenue\right)}_ {year}}{{\left(1+Cost \,of \,money\right)}^{year}}\right)\times Capital \,recovery \,factor,$$

$$Capital\, recovery\, factor=Cost\, of\, mone{y}_{i}\times \frac{{\left(1+Cost \,of \,mone{y}_{i}\ right)}^{lifetime}}{({\left(1+Cost \,of \,mone{y}_{i}\right)}^{lifetime}-1)},$$

$$Cost\, of \,mone{y}_{i}\left(inflation \,adjusted\, cost\, of \,money\right)=\frac{1+Cost \,of \,money}{ 1+General\, inflation}-1.$$

The calculations for economic feasibility were based on assumptions from the literature and the Wadham facility reference case. Table 6 shows the assumptions for the economic analysis in this study. For the parameters related to the power plant operation, publicly available information on the characteristics of the Wadham facility were used18.42and the capital expense (CAPEX) and the operating expense (OPEX) values ​​were imported from a United States Energy Information Administration (US EIA) report for a biomass power plant with a capacity of 50 MW in the United States43. The US EIA report provides CAPEX for both Northern California and Southern California regions, and their average was used in this study. The report provides variable OPEX and fixed OPEX, and fuel cost was excluded from the variable OPEX. In this study, the fuel cost including the transportation cost may be assumed at or near zero depending on the location of a rice miller and a power plant. Rice hulls, the fuel feedstock in this study, are collected at milling sites regardless of their use or value, and thus, the transportation cost may be reduced by geographic location of a power plant. In addition, while market dynamics can fluctuate, rice hulls are not currently considered a high-value commodity and are used only in very low-value uses such as poultry bedding, which make rice hulls available at or near zero cost. The range of plausible costs were analyzed and discussed. As a cash flow method, discounted cash flow rate-of-return was used, and a straight line 30-year depreciation was assumed. For the cost of money, the weighted average cost of capital (WACC) for merchant-owned biomass facilities (7.21%) was used44. By using the WACC for biomass facilities as the cost of money for this study, the economic feasibility of this study compared to other biomass facilities can be determined.

Table 6 Assumptions for economic analysis.

Experimental testing methods

To understand the feasibility of using RHA to make cement-based materials, experimental testing was performed on concrete specimens to measure key engineering design parameters, namely 28-day compressive and flexural strengths. RHA was obtained from the Wadham bioenergy facility in Williams, California (CA) owned by the Enpower Corporation. Lehigh Southwest Cement Company in Stockton, CA was the source of PC (ASTM type II/V). Both fine aggregates (99.95% passing a No.4 sieve, 4.75 mm) and coarse aggregates (with a 100% passing a 1″ sieve, 25 mm) were locally sourced from Esparto, CA.

Two concrete mixtures were made: a control mixture containing no RHA and a mixture with RHA replacing 15% of the PC by mass (Table 7). No chemical admixtures were used. For compressive strength and chloride ingress tests, 100 mm × 200 mm cylinders were prepared. Prismatic beams of 100 mm × 100 mm × 300 mm were made for flexural strength testing. All specimens were demolded one day after casting and then cured in a curing chamber at 25 °C and \(\ge\) 80% of relative humidity until testing.

Table 7 Concrete mixture proportions.

Compressive strength and flexural strength were determined after 28 days of curing. Compression tests were conducted on a SoilTest CT-950 load frame following ASTM C3945 where cylinder specimens were capped on either end with a neoprene-padded aluminum cap. Five specimens were tested for each mixture. The compressive strength of the concrete mixtures was determined using the maximum load before softening or failure occurred.

Flexural strength was determined by performing three-point bending tests, at 28 days. Testing was performed on an MTS Testline Component load frame managed by an MTS TestStarIIs controller following ASTM C29346. Three specimens were tested from each mixture and the flexural strength was determined using the maximum load prior to failure.

In addition to mechanical performance, a rapid chloride ingress test was performed to provide an initial indicator of the potential durability and longevity of the material. Chloride permeability reflects the ability for chloride ions to permeate into concrete and is a critical durability property for concrete used in certain regions. Chloride ingress in steel-reinforced concrete is a large contributor to the corrosion of steel. This is of particular interest in California due to saltwater exposure in coastal regions and predominant use of reinforcing steel in structural concrete. In this test, the resistance of saturated concrete specimens, with and without RHA, to chloride diffusion were measured. Concrete cylinders were cured for 90 days and then cut into disks. Measurements were collected using a PROOVE-it control unit and testing cells with external cooling fins, following ASTM C120236.

Life cycle assessment methods

To assess the potential environmental benefits of RHA replacing conventional PC or common SCMs, this study applies LCA to each material, RHA, PC, and coal fly ash, with a functional unit of 1 kg. The life cycle inventory (LCI) dataset for PC was obtained from the Ecoinvent 3.5 database using the GaBi software tool. However, reference LCI data for coal fly ash, which RHA replaces, were not available, and only LCIs for cement blends with 5–15% pozzolana and fly ash and 15–40% pozzolana and fly ash were available from the database. Thus, the environmental impacts were compared between the RHA cement mixture from this study and the cement blends from the database, instead of between RHA and fly ash. Also, the environmental impacts of RHA and electricity were based on a reference LCI for electricity from solid biomass, and they were allocated to RHA and electricity based on their relative economic values. Table 8 reports all the reference LCIs used in this study. Environmental impact potentials were calculated using the life cycle impact assessment method CML 200147. The impact categories, characterization factors and indicator units are shown in Table 9.

Table 8 LCI data source (geographic region: United States) obtained from the GaBi software.
Table 9 Characterization factors and indicator units of the CML 2001 (August 2016)48.